BSC second year

Sex determination in animals



Sexually reproducing animals can be divided into monoecious or hermaphrodite or bisexual and dioecious or unisexual. Majority of animals are dioecious. The species split up into two sections from which the word sex is derived . it comes from Latin word sexus meaning sections or separation. The male section has testis or male gonad and female section has ovaries or female gonad. This kind of separation of sexes is known as gonachorium. The male and female animal differ in primary sex organ, secondary sex organ and tertiary sex characters. The phenomenon of molecular, morphological, physiological and behavioral differentiation between male and female sexes is sexual dimorphism.



The sexual dimorphism has been a biological riddle for the thinkers and Biologists of all time. People always tried to know those factors which determine the male and female sexes of species. Thousands of mistaken hypothesis and wild guesses were proposed before 1900 in vain attempts to find out solution to the problem of determination of sex. Geneticists have come up with different mechanisms of determination of sex.



Genetically controlled sex determining mechanism

a) Sex chromosome mechanism or Heterogamesis

Henking 1891 noted half of sperms contain an extra chromosome called by him as x body. McClung in 1902 suggested X body involved in determination of sex. Miss Stevens and Wilson studied oogenesis and spermatogenesis of insects and it was realized that X body was X chromosome. In male, chromosome homologous to X was smaller and was called as Y chromosome. There are two types of chromosomes.

i. Autosomes – they have no relation with sex and called as autosomes(A).

ii. Sex chromosomes- they are responsible for determination of sex.



Types of sex chromosomal mechanism



A) Heterogametic males

B) Heterogametic females



Heterogametic males

i) XX – XO type

All eggs produced are same having one X chromosome but about half of the sperms contain X chromosome and other half is without X chromosome. The sperm which fertilizes the egg determines the sex of new born. Example. Certain insects.

ii) XX – XY type

All eggs produced are same having one X chromosome but about half of the sperms contain X chromosome and other half is having Y chromosome. The Y containing sperm if happens to fertilize the egg, male baby is born otherwise female baby is born. Example. Man, Drosophila etc.



Heterogametic females

i) ZO – ZZ type

Eggs produced are not same here. Albout half of the eggs produced contain Z chromosome and other half are without Z. All the sperms are same having one Z chromosome. The egg containing Z if fertilized denotes the maleness. Example. Moths, butterflies and domestic chicken.

ii) ZW- ZZ type

All the sperms produced are of same type having one Z chromosome. Eggs are of different type. About half of them contain Z chromosome and other half contain W chromosome. The egg with Z chromosome is for male and egg with W chromosome is for female. Example insects, vertebrates like fishes, birds reptiles etc.



Genic Balance Mechanism



The experiments of Wilson 1909, Bridges 1921, Goldschmidt 1934 revealed that most organism have inherent potentialities for both sexes. There seems to exist delicate Genic balance. The first Genic balance mechanism was first studied by C B Bridges in 1921 in Drosophila.



Y chromosome is found essential for fertility an nothing to do with sex determination. The sex of an individual depends upon ratio of X chromosome to autosomes. The value of haploid set of autosomes is equal to 1 and value of X is one and one half ( 1 ½ ).



In female AAXX, the ratio is 2:3 which goes for female and in male AAXY the ratio is 2: 1 ½ which goes for the male.

BSC II Year

DNA Replication


One of the important features of DNA is that it can form its exact copy. This is known as DNA Replication. This essential feature of DNA occurs during synthesis phase of cell cycle. Each DNA molecule fives rise to two DNA molecules identical to each other as well as to the parent molecule. The precise replication without error is required for cell. If error rate of 0.01% occurs, then it can cause significant gene mutation.



Max Delbruck proposed those possible ways of DNA replication. They are:



1. Semi- conservative replication

2. Conservative replication

3. Dispersive replication



Semi-conservative replication



Watson and Crick suggested very simple mechanism of DNA replication on the basis of their own model. This is most widely accepted method of DNA replication. It occurs in the nucleus of cell. It requires a supply of free nucleotides which exist in nucleoplasm. To this, extra phosphate groups are added to activate them. Taylor 1956 first demonstrated the semi-conservative mode of replication and later Meselson and Stahl 1964 experimentally proved it in E coli.



Main features

1. A progressive separation of the two strands of a DNA molecule or DNA molecule unwinds and two strands are separated by breakage of Hydrogen bonds between bases.

2. Each strand acts as a template of the synthesis of new strand.

3. Nucleotides with appropriate complimentary bases then slot into place opposite the exposed bases on each strand by base paring that is A with T and C with G.

4. Hydrogen bonds between the complimentary bases hold them in place. The sugar of one nucleotide then joined to the phosphate of the next nucleotide to form polynucleotide chain.

5. The result is that two new DNA molecules are formed from a DNA molecule. These processes are dependent on a number of enzymes including DNA polymerase.

6. Each new DNA molecule contains one old polynucleotide strand and one new strand. During replication half of the original DNA molecule is conserved and the complementary half is newly synthesized by base pairing. So, this method is called semi-conservative replication.



Mechanism of DNA replication





DNA replication is a complex multi enzymatic process involving several enzymes and factors. The replicating units of chromosomes are called replicons. Replication starts with an incision or a cut at a specific point generally referred to as nicking. An incision enzyme called endonuclease brings this about. Bacterial and viral DNA has a single origin of replication but in eukaryotes there are many origins of replication due to a large size and association with proteins.



It includes following steps:

a) Activation of nucleotide

The deixtribibyckeotide monophosphates such as deoxyriboadenosine monophosphate(dAMP), deoxyriboguanosine monophosphate(dGMP),

deoxyribocytosine monophosphate(dCMP), deoxyribothymine monophosphate(dTMP) occur in nucleoplasm. These are activated into deoxyribonucleoside triphosphate. This process is called phosphorylation.



dAMP→dADP→dATP



dGMP→dGDP→dGTP



dCMP→dCDP→dCTP



dTMP→dTDP→dTTP



b) Unwinding of DNA

Replication of DNA begins at a specific point called initiation point. The unwinding proteins bind to the initiation point on DNA helix and initiate unwinding of its strand. Thus two strands of DNA start separating by breakdown of Hydrogen bonds between paired nucleotide.

c) Base pairing

Deoxyribonucleoside triphosphates pair with appropriate nitrogenous base of trmplate DNA strand according to base pairing tht is A with T and C with G.

d) Conversion of Triphosphate to Monophosphate

The deoxyribonucleoside triphospahate molecule on pairing with nitrogenous bases of template strand set free pyrophosphate molecule and change into dexyribonucleotide monophosphate. This occurs in the presence of enzyme pyrophosphatase during which a large amount of energy is released.

e) Formation of new DNA chain

The energy so released is utilized in joining the adjacent nucleotide to form polynucleotide chain. This process is catalyzed by DNA polymerase (enzyme).



The new strand of DNA is synthesized in 5’→ 3’ direction on 3’ →5’ template is continuous and known s leading strand while the synthesis of 3’→ 5’ strand on 5’ →3’ strand is discontinuous and lagging strand. After sometime of replication of 3’ →5’ strand, primase again initiates the replication of 5’ →3’ (lagging) strand. The replication of lagging strand generates smaller polynucleotide fragments called Okazaki fragments. The Okazaki fragments are 1000 – 2000 nucleotides long in prokaryotic and only 100 – 200 nucleotides long in eukaryotes. The Okazaki fragments are joined by enzyme polynucleotide ligase.



Conservative replication

In this, completely new DNA molecule is synthesized from old DNA. In new DNA molecule, both strands are new and in old DNA, both strands are old.



Dispersive replication

In this, original DNA chain breaks and recombines in a random fashion. The newly

formed DNA molecules contain partly new and partly old strands.

for BSC Second year

Structure of DNA and its function



Chromosomes are made up of nucleoprotein. The nucleoprotein contains basic protein and nucleic acid. There are two types of nucleic acids. One is Deoxyribose nucleic acid (DNA) and other is Ribose nucleic acid (RNA). The nucleic acids were first reported by Friedrich Miescher (1871) from nuclei of pus cells. He called them nuclein . They are found in nucleus and cytoplasm. In cytoplasm, they have been found to be associated with protein synthesis.



Feulgen in 1924 developed fuchsin staining technique. With this technique, DNA was noticed in chromosome. DNA is storehouse of genetic material. It transmits information to RNA. DNA is a macromolecule formed by end to end polymerization of a large number of repeated units called deoxyronucleotide or simply nucleotide. Each nucleotide is made up of deoxyribose sugar, nitrogen base and phosphoric acid.



1. Deoxyribose sugar

It is a pentose sugar with five carbon atoms.



2. Nitrogen bases

They are of two types. A) Purines B) Pyrimedines

Purines are Adenine(A) and Guanine(G) with double ringed structure or dicyclic in nature. Pyrimedines are Cytosine(C), Thymine (T) and Uracil (U). In RNA, instead of Thymine, Uracil is present. Pyrimedines are single ringed or monocyclic in nature.

Nucleosides are formed by Purine or Pyrimedine nitrogen base and pentose sugar. DNA nucleosides are deoxyribose nucleosides.



3. Phosphate group

It is present as phosphoric acid H3PO4. Each phosphate group is joined to carbon atom 3 of one deoxyribose sugar and carbon 5 of another deoxyribose sugar.



James Watson and Francis Crick established a double helix or double stranded structure of DNA molecule in 1953 on the basis of X-ray diffraction method. They were awarded Nobel Prize in 1962. DNA is composed of 2 anti parallel poly nucleotide that form double helix around axis.



Each strand of DNA is made up of alternate bond of deoxyribose sugar and phosphate group joined by phosphodiester linkages. Each deoxyribose sugar in the strand has one nitrogenous base horizontally attached to it at carbon 1. The four nitrogen bases can occur in any possible sequence along the length of strand. The nitrogen base, deoxyribose sugar + phosphate group together form one unit of deoxyribose nucleotide. Many deoxyribonucleotide are linked with each other in a linear fashion. Therefore resulting strand is described as polynucleotide strand and DNA molecule as polynucleotide molecule.



Two polynucleotides are held together by Hydrogen bonds between specific pairs of Purine and Pyrimedine. Purine of one polynucleotide strand pairs with Pyrimedine of another strand.



A with T and C with G



A=T



C≡ G



Two strands of helix are of opposite polarity. One strand runs in 3→ 5 direction and another runs in 5→3 direction. The distance between two turns of helix is 34 Ȧ. There are 10 base pairs in a single turn with distance of 3.4 Ȧ between 2 base pairs. Single turn means distance between two shallow groups.



Functions



1. As genetic material – it carries hereditary information from one generation to another generation.

2. It has unique property of formation of exact copies which is essential for transfer of genetic information.

3. DNA forms RNA through Transcription process.

4. DNA plays role in protein synthesis.

5. Any change in sequence of nitrogen bases of DNA causes mutation.

for BSC second year

Giant chromosomes




Some cells at particular stages contain large nucleus with giant or large sized chromosomes. The giant chromosomes are Polytene and lamp brush chromosomes.



Polytene chromosome



It occurs in the tissues of salivary gland, gut, trachea, fat body cells and Malpighian tubules of many insects of the order Diptera. It was first reported by E G Balbiani in 1881. The name polytene was suggested by Kollar due the occurrence of many chromonemata.



The polytene chromosomes are larger in size. The polytene chromosome of Drosophila melanogaster has total length of 200 microns in comparison to the 7.5 microns of somatic chromosome. The enormous size of the polytene chromosome is achieved by the duplication of chromonema for nine or ten times and duplicated chromonemata are not separated.



The polytene chromosome consists of closely coiled or associated homologus pairs of chromosomes. This association is called somatic pairing.



The polytene chromosome contains about 1000 times more DNA material than the somatic chromosomes.



Dark and light bands



Polytene chromosome bears alternating dark and light bands. The dark bands take deep stain with basic chromosomal stain and contains large amount of DNA and small amount of RNA. These are mainly composed of euchromatin. The light bands take light chromosomal stain and contains small amount of DNA and large amount of RNA. They are mainly composed of heterochromatin.



The polytene chromosome is caused by the process of endomitosis. In endomitosis, chromonemata become duplicated and resulted chromonemata do not separate to form new chromosome but remain closely associated. Also in endomitosis, nuclear membrane does not rupture and no spindle fibre formation takes place.



During development stages, polytene chromosome of larva of Dipterous insect, develop swelling at particular point of dark and light band. Such swelling is called puffs or bulbs. The puffing process involves accumulation of acidic protein, synthesis of RNA and storage of synthesized mRNA.



The chromonema of polytene chromosome gives out series of loops laterally. These rings are known as Balbiani rings. They are rich in DNA and mRNA. The formation of Balbiani rings are similar to puffs.



Lampbrush chromosome



The yolk rich oocytes of many vertebrates such as fishes, amphibians, reptiles and birds, contain exceptionally large sized chromosomes known as lamp brush chromosomes. They were discovered by Ruckert.



The lamp brush chromosomes are much longer. The length of lamp brush chromosome in Urodele oocyte may reach up to 5900 microns.



The lamp bush chromosomes are composed of the main axis and lateral loops. The main axis is composed of 4 chromatids or two bivalent chromosomes. The chromonemata of these chromaitd give out fine loops at lateral sides giving appearance of lamp brush or test tube brush.



The loops of lamp brush chromosomes are rich in RNA and protein. The RNA appears to be mRNA. The synthesis of protein and yolk take place near the loop.



Cytogenetical functions of chromosomes



Chromosomes are the most significant component of cell. They control most of the biological and genetical activities of cell. They contain genetical material, DNA which ultimately influences the biological phenomena atg molecular, physiological and gross morphological level.



Crossing over



The linked genes may remain together during the process of inheritance showing complete linkage or many be separated or segregated during gametogenesis displaying incomplete linkage. The incomplete linkage takes place due to new combination or recombination of liked genes . this recombination in tis turn accomplished through a process of crossing over.



In crossing over, the non sister chromatids exchange chromosomal parts or segments. Crossing over is a process of formation of new combinations( recombinations) of genes by interchanging of corresponding segments between non sister chromatids of homologous chromosomes, the chromatins resulted from interchange of chromosomal parts are cross over.

Types

1. Somatic or mitotic crossing over

It occurs during mitotic cell division, it is rare and of no genetical significance. The somatic crossing over reported in Drosophila by Curt Stern.

2. Germinal or meiotic crossing over

It occurs during gamete formation in germinal cells. It is universal occurring and of great genetic significance.

Mechanism of crossing over

The process of crossing over includes four stages

1. Synapsis

2. Duplication of chromosome

3. Crossing over

4. Terminalization

Synapsis- during Zygotene stage of Prophase I of Meiosis, homologous chromosomes come close to each other and pairing of them takes place. The phenomenon of pairing is called synapsis. It is of prime importance because is provides mechanical basis of heredity and variation.

Duplication of chromosome- each homologous chromosome of bivalent splits longitudinally and form two identical sister chromatids. These are held together by unsplitted centomere. There are four chromatids and this stage is called tetrad.



Crossing over- crossing over occurs in the homologous chromosome during four stranded or tetrad stage. First, two non sister chromatids break at the corresponding points due to activity of endonuclease ( Stern and Hotta,1969). A segment of one side of break connects with a segment of opposite side of the break so that two non sister chromatids cross each other. The fusion of chromosomal segments with that of opposite one takes place due to action of ligase. The crossing of tow chromatids is called chiasma formation, the crossing overf includes breaking, their transposition and fusion.



Chiasma frequency

Crossing over may take place at several points making several chiasmata formation, the number of chiasmata depends upon length of chromosome. Longer the chromosome, greater the number of chiasmata formation. In a species, each chromosome has characteristic number of chiasmata. When two genes are located closer, there are lesser chances of chiasma formation between them and vice versa.



Terminalization

After crossing over, non sister chromatida start to repel cach other. The force of attraction between them decreases. The chromatids separate progressively form centromere towards chiasma. The movement of chiasma is known as terminalization . due to terminalization the homologous chromosomes are separated completely.



Types

1. Single crossing over

When chiasma is formed at one point only it is single crossing over. It produces two cross over chromatids and two non cross over chromatids.

2. Double crossing over

It is when chiasma occurs at two points in the same chromosome;. In this each chiasma is independent of other chiasma. There are 2 sub types

I. Reciprocal chiasma

The same two chromaids are involved in the second chiasma as in the first. The second chiasma restores the order which was changed by first. It produces two non cross over chromatids.

II. Complementary chiasma

When both chromatids in the second chiasma are different from those involved in the first chiasma. It is known as complementary chiasma. The complementary chiasm occurs when 3 or 4 chromatids of tetrad take part in the crossing over.

3. Multiple crossing over

It is when crossing over takes place at more than two places in the same chromosome pair. Such is known as multiple crossing over. It occurs rarely.



Significance

The phenomenon of crossing over is universal in its occurrence and found in Viruses, Bacteria, plants and animals. It is necessary for Natural Selection as the chances of variation increases.

for BSC second year

Gene and gene concept




Shortly after rediscovery of Mendel’s Laws, Sutton and others suggested that hereditary factors are located on the chromosomes. In a cell, chromosomes occur in pairs because the homologous chromosomes are derived from each of the two parents. A chromosome contains two threads of chromatin called chromatids or chromonemata lying closely applied to each other. Each chromatid (chromonema) is a fine thread with a linear series of darkly staining particles of different sizes called chromomeres. Chromomeres are in pairs and lie opposite to each other in the two chromatids. The chromomeres contain ultra microscopic nucleoprotein particles called genes. The term gene was given by Johannsen in 1909.



The old definition of gene was that it is a stable chemical unit of heredity which is transmitted in a gamete and it controls the development of a single character in the adult. Genes are symbolized in letters. Dominant genes are represented by capital letter ( as T for tallness) and recessive genes (as t for dwarfness). This is a convenient method to follow up the transmission and distribution of genes in cross experiment. Thus T and t are allelic genes or alleles.



Genes are not considered today as ‘unit determiners’ for a single trait in the Mendelian way. They act in conjuction with other genes upon organism as a whole.



According to a modern idea, a gene is a fixed area or unit of heredity and function. It is made up of a series of joined nucleotides located on the DNA molecule of a chromosome. Genes are made up of nucleoproteins. A gene is responsible for the synthesis and structure of a specific enzyme or protein in a cell.



Genes are located in nucleus, hence they do not act directly on the activities going on the cytoplasm. There is a messenger carrying instructions from genes in the nucleus to enzyme in the cytoplasm. Every gene serves as the source of information which is transferred to a molecule of RNA. The RNA molecule then migrates from nucleus to cytoplasm as a messenger carrying code information to synthesize a specific protein which will be responsible for some reaction in a cell. Generally genes are very stable and pass exact replicas of themselves( by replication of DNA molecule ) to all the cells which descent from zygote. Genes also control each and every step of development of an organism. Consequently the offspring resemble their parents.



Genes are no longer regarded as stable and indivisible units. They consists of smaller functional sub units. Recombination of sub units may occur to produce new gene. Benzer has proposed some new terms. The genes which is the smallest functional unit on a chromosome is Cistron. The gene which is a crossing over unit is and is inter changeable is Recon. The smallest mutable unit which can produce altered character is Muton. The action of gene is mainly synthesis of protein. Cistron controls formation of polypeptide chain of amino acid while recon and muton may control the synthesis of particular amino acid.



Castle has defined a gene as the smallest part of chromosome capable of varying by itself.







Functions of gene



1. A gene is self replicating unit of heredity and pass exact replicas to daughter cells.

2. It controls and guides development of zygote. If this were not so, off springs would not resemble parents.

3. Genes are capable of undergoing mutation. The altered gene breeds true. By undergoing mutation, genes provide material for evolution and adaptation.







Gene interaction

It is the influence of alleles and non alleles on the phenotypic expression of gene. It is of two types.

• Intragenic (intra allelic)

• Intergenic ( non allelic)



Intragenic (intra allelic) gene interaction



In this case two alleles located on the same locus (gene locus) on two homologous chromosomes interact to produce phenotypic expression. Out of two alleles (allelomorph) one allele might show simple (complete) dominance over the action of other which is recessive or both alleles might have partial or incomplete dominant relationship or both alleles might have expression which is equal or codominant relationship. These kinds of gene interaction occur in between two alleles of a single gene type and usually referred to as intra allelic or allelic genetic interaction. These kinds of gene interaction give the classical ratio of 3:1 or 9:3:3:1.



Codominance



When dominant character is not able to suppress even incompletely the recessive character and both the characters appear side by side in F1 hybrids, the phenomenon is called Codominance. For example in cattle, if a cattle with black coat color is crossed to a cattle with white coat color, the F1 hybrids possess roan coat color. In roan coat color, both black and white patches appear separately. So, the alleles which are able to express themselves independently when present together are called codominant.



Multiple Alleles



Mendelian hereditary characters are concerned with genes with two alternating forms or alleles. Now it has been noticed that for many characters a gene for a character exist in many alternative forms or alleles. These forms of a gene are due to mutation of a single wild type. When more than two allelic forms of wild type are located on the same locus in a given pair chromosome they are known to compose the series of multiple alleles. Actually, grouping of all different alleles that may be present in a gene pair is defined as a system of multiple alleles.





Multiple alleles possess following characters



1. Multiple alleles are located at the same locus in the chromosome.

2. Multiple alleles regulate a particular character.

3. Process of crossing over not exhibited due to location on same locus.



Blood group system in human beings



Blood group depends on presence or absence of specific substance on RBC. There are two substances Antigen A and Antigen B.



Blood group A presence of Antigen A



Blood group B presence of Antigen B



Blood group O absence of both Antigen A and Antigen B



Blood group AB presence of both Antigen A and Antigen B



Gene for absence of both antigens are represented by i or Io. Gene for presence of antigen A is represented by IA. Gene for presence of antigen B represented is by IB gene for presence of both antigen A and B are represented by IAIB. IA and IB are both dominant over Io or i.















































Genetic code

DNA is genetic material, it carries genetic information. A DNA molecule is composed of three kinds of moieties.

• Phosphoric acid

• Deoxyribose sugar

• Nitrogenous bases ( Purine – Adenine and Guanine, Pyrimidine – Cytosine and Thymine or Uracil, double bond between A and T and triple bond between C and G)



The genetic information may be written in any of one of three moieties of DNA. The poly sugar phosphate backbone is always same. So, it is unlikely that poly phosphate sugar backbone carry genetic information. The nitrogenous bases however vary from one segment to DNA to another segment. So, information might depend upon their sign sequence.



The specific arrangement of four nitrogenous bases like ATGC determines the sequence of aminoacids in a protein molecules as discovered by Sarabhai et al in 1964 in Bacteriophase T4 and Yanofski et al in 1964 in Escherichia coli. All genetic information therefore should be written by these four alphabets of DNA. The genetic information were existed in DNA molecule in the form of special language of code words which might utilize four nitrogenous bases of DNA for its symbols. Any coded message is commonly called cryptograms.



Basis of cryptoanalysis

The group of nucleotides that specify one amino acid is a code word or codon. The simplest possible code is a singlet code. In this, one nucleotide codes for one amino acid. Such code is inadequate because only four amino acids could be specified. A doublet code ( a code of two letters is also inadequate because it could specify 16 ( 4x4) amino acids only. A triplet code ( a code of three letters) would specify 64( 4x4x4) amino acids. It is likely that there may be 64 triplets for 20 aminoacids.



A

G

C

U









AA AG AC AU

GA GG GC GU

CA CG CC CU

UA UG UC UU

















AAA AAG AAC AAU

AGA AGG AGC AGU

ACA ACG ACC ACU

AUA AUG AUC AUU

GAA GAG GAC GAU

GGA GGG GGC GGU

GCA GCG GCC GCU

GUA GUG GUC GCU

CAA CAG CAC CAU

CGA CGG CGC CGU

CCA CCG CCC CCU

CUA CUG CUC CUU

UAA UAG UAC UAU

UGA UGG UGC UAU

UCA UCG UCC UCU

UUA UUG UUC UUU





The possible singlet, doublet and triplet codes customarily represented in terms of mRNA which copies genetic information(cryptograms of DNA).





The first experimental evidence in support to the concept of triplet is provided by Crick and coworkers in 1961.



Characteristic features of triplet code

a) Code is triplet – some do not code called non sense codon.

b) Code is degenerate – for particular aminoacid more than one codon can be used. Phenyl alanine can have two codons UUU and UUC

c) Code non overlapping- CAT, GAT represent two codons when not overlapped. But when overlapped four codons like CAT, ATG, TGA and GAT achieved.

d) Code is commaless – no punctuation

e) No ambiguous – a particular code is always for one amino acid except GGA which codes for Glycine as well as Glutamine

f) Code is universal – it is same for prokaryotes as well as eukaryotes.

























Gene Mutation

Mutation was studied by Hugo de Vries in 1901 on evening primerose Oenothera lamarckiana. During experiment, he grew plants for many generations. He observed several plants of Oenothera differing in size of leave and flowers. He called these plants as mutants and the phenomenon as mutation. Such plants were true breeding type. He considered mutants as new species. Later on, it was discovered that O. gigas mutant was tetraploid woth 28 chromosomes.



Mutation occurs naturally called spontaneous mutation some mutations occur artificially by chemicals or physical agents called induced mutation. The agents that induced mutations are mutagens. Chemical mutagenes are mustard gas, Ethyl Methyl sulphonate(EMS), nitrous acid, Hydroxyl amine, Hydrazine etc. the physical mutagenes are x-ray or radiation etc.



Types of Mutations



• Macromutation or Chromosomal Mutation

• Micromutaion or Gene or Point Mutation



Gene Mutation



Gene Mutations are sudden, stable changes in genes i.e. small section of a chromosome or DNA chain. A change of only one or few nitrogenous bases can amount to a Mutation. The gene Mutation, though minor change in DNA may have far reaching consequences for the cell or organism. For example, a change of a single nitrogenous base causes sickle cell anemia that may prove fatal by age 20.



In this way, a gene mutation can be defined as a sudden stable, inheritable alteration in the base sequence of a gene capable of changing the phenotype of an organism.



Detection of gene mutation



Unlike chromosomal mutation, gene mutations are not observable under microscope. They are detected when they cause noticeable change in phenotype of organism. They remain undetected if they do not produce any phenotypic variation. Most gene mutations are recessive, so no detected immediately. They must become homozygous before they can express themselves. Dominant mutations are rare. A dominant mutation causes disease aniridia, lack or defect of iris in human.



Muton is the smallest portion of a gene that can mutate is called muton. The smallest muton in a gene is a single nitrogenous base. A gene thus, consists of numerous mutons in a linear series.



Frequency of gene mutation

Generally only one gene mutates at a time. Mass mutations are rare. It occurs in somatic as well as germ cell.







Types:

1. Substitutions or Replacement mutation

These are gene mutation where one or more nitrogenous bases are changed with others.

a) Transitions – these are substitutions of gene mutation in which Purine (say Adenine) is replaced be another Purine (say Guanine). Or Pyrimedine (say Thymine) is replaced by another Pyrimedien (say Cytosine).

b) Transversion – these are substitution gene mutation in which Purine ( Adenine or Guanne) is replaced by Pyrimedine(Thymine or Cytosine) or vice versa.

2. Frame shift mutation

a) Deletion- these are gene mutation in which one or more nitrogenous bases are lost from a segment of DNA that constitute gene.

b) Insertion(Addition) – these are gene mutation in which one or more nitrogenous bases are introduced into a segment of DNA that constitute a gene.



Back ground

The earliest record of point mutation dates back to 1791. Seth Wright noticed a lamb with exceptionally short legs in his flock of sheep. Visualizing the economic significance of short legged sheep he produced a flock of sheep by employing artificial breeding technique. The short legged breed of sheep was known as Ancon breed. Later on the trait of short legged individual was found to be homozygous recessive.

for class XI

Sub code 114


HSEB GRADE XI

2069(2013)

SUPPLEMENTARY EXAMINATION

Biology(Botany+Zoology)





Candidates are required to give their answer in their own words as far as practicable. The figures in the margin indicate full marks.

Time 3 hrs. Full marks – 75

Pass marks – 27



Note This question paper contains Botany portion in Group “A” and Zoology portion in Group “B”. So use separate answer sheets for Group “A” and Group “B” . First use answer for Group “A”.



Group “A”

Botany

Attempt all questions.

1. Answer in short : (any seven) 7x1=7

a. What is somatic cell?

b. Define heterosporous.

c. What is Oogamous?

d. Give and name one example of hypogynous flower.

e. What is the function of rhizome?

f. Give two examples of gymnosperm.

g. What is the function of Yeast?

h. Define parasitism.

i. What is molecular biology?

j. Define acid rain.

2. Answer in brief: (any five) 5x3=15

a. Describe the structure of nucleus.

b. Explain Metaphase and Anaphase stage of mitotic cell division.

c. What is Mitochondria? Explain.

d. Describe the structure and function of potonema.

e. Describe the scalariform conjugation.

f. What is binomial nomenclature system with example.

g. Discuss the function of carbohydrate.

3. Describe the family cruciferae in semi technical terms with distinguishing characters, floral diagram and floral formula with two examples of its economic importance. 7.5

Or

Explain life cycle of pteridophyte with emphasis on the alternation of generation.

4. Define ecosystem, describe in detail about the aquatic system. 8.





Group “B”

Zoology

Attempt all questions.

1. Answer in short : (any seven) 7x1=7

a. Define histology.

b. Name any two life processes.

c. Who gave the most widely accepted theory of “Origin of life”?

d. Who proposed the first theory of evolution?

e. Who is the father of taxonomy?

f. What do you understand by the term spawn?

g. What was the Hugo de Vries view regarding origin of species?

h. Name the hunting reserve of Nepal.

i. What do you mean by leadership?

j. Define pesticide.

2. Describe in brief : (any five) 5x3=15

a. The significance of conjugation in Paramecium.

b. Draw a well labeled diagram of T.S. of earthworm through gizzard.

c. Write down the main distinguishing feature of class mammalian.

d. The cutaneous respiration of frog.

e. Write a short note on vestigial organs.

f. The control measure of air pollution.

g. Flight adaptation in birds.

3. Discuss the process of food digestion in frog. 7.5

Or

Explain life cycle of Plasmodium vivax in human host.



4. Describe the various stages in the evolution of human being. 8.

M Ed biology

Cow and its life cycle




Domestication of cattle has in fact, been in practice since the new stone age in both Europe and Asia. It seems probable that primitive man first used members of the family Bovidae as a source of food. Domestication perhaps began when these animals were used as draft animals probably as the first step in the tillage of soil.



Life stock is a group of domesticated vertebrates of great economic importance to man for food (animal products milk meat and eggs etc, agriculture and commerce( wool, skin, cheese, ghee, khua etc). These domestic animals generally include cattle, sheep, goats, yaks, chouries and pigs, all coming under the family Bovidae with exception of pigs.



Nepal is also one the countries in the world where cattle are domesticated in large number since time immemorial. The other countries which possess a large number of buffaloes and cows are India, Pakistan, Bangladesh, China, Malaya, Egypt etc. Some of the Indian common breeds like Hariyana, Sindhi are however domesticated in Nepal also.



The distribution of cows in Nepal is as follows. Salyan, Doti, Saptari, Morang- Sunsari, Mahottari, Pyuthan and Terhathum are richer in bullocks and cows than in other districts. Cows are herbivorous, quite sensitive and fully terrestrial forms. Hindus worship the cows as Goddess Laxmi and the bull as the vehicle of Lord Shiva.



Breeds in Nepal



In Nepal, we have very limited number of indigenous as well as exogenous breeds of cows.

• Jersey – Jersey is the synthetic English breed. It is a cross of several breeds like Cettie shorthorn, Terentaise, Parthenaise, Bezadaise and Dun. It varies in color from light, red to black and from white spotted to solid in marking. The switch may be black or white. The muzzle is black with a light encircling ring. It is a high milking cow. It has straight top lines level rumps and sharp withers. The udder is large to produce more milk. It is weaker than Brown Swiss and the most suitable to domesticate the Jersey cross in the hills and Terai regions.



• Hariana – Hariana is a medium sized heavy type of cow for milk and bullock for transport and ploughing the land. It is mainly found in Terai regions up to 700 feet. It has small head with long, narrow face from which emerge the short and somewhat horizontal horns which grow longer and curve upwards and inwards in bullocks. The color is generally white or light grey. Ears are small and short and udder has prominent teats.





• Local Siri cow – its origin is supposed to be in Bhutan and is mostly distributed in Bhutan, Darjeeling hill tracts, Sikkim and most hilly region of Nepal. Its color is black and white or reddish and white. They are poor milkers. It has a massive rigid body, small head square cut, thick coat to be able to survive in cold climates. Horns are sharp, ears relatively small, the hump covered with tuft of hair at the top. The legs and tail are short. The dewlap is not prominent. The massive bullocks are good at pulling heavy loads with great ease especially during ploughing.



Feed of cattle

A good pasture is essential for all types of live stocks including dairy cattle. A good quality of pasture crops is able to maintain high production at relatively low cost. Animals breed in this natural way of feeding grow at a faster speed, develop bodies stout and are seldom become sick. Young pasture grasses and legumes are high in proteins and moisture but low in fiber. Pasture copes are usually rich in minerals although the mineral content varies with the fertility of soil. Phosphorus is most likely to be lacking in pastures. Vitamins are a problem for cattle although vitamin A is found in green forage and vitamin D in sunlight. The problem in Nepal is that good pasture lands are available only during the monsoon months, after which the availability of grazing ground becomes gradually poorer and finally nil in winter. During winter in Nepal, when the grass lands are scarce, the farm animals are supported with roughages that include straws, legume or non legume hays, maize or millet silages etc. The straw cut into chaffs, now mixed with watered cakes is popularly known as bhusa. In alpines, people use to store foliage or grass , silage for the drought climates of winter.

Life cycle

The age of maturity depends to a large extent on diet and management. The large majority calve first between 30 and 48 months. At Khumaltar, Jersey cow has first calved at the age of 2 years and 21 days. The local breed of cow is found to calve at the age of 3 years generally.



Heat period

The time period from one heat period to next one is known as oestrus cycle. This cycle is of about 21 days. Sometimes it may vary from 17 to 26 days. The heat period at which cows permit to mate lasts for 6 to 36 hours with an average duration of 18 hours or cows and 15 hours for heifers. During heat period, cows show signs of restlessness. There is slight increase in the body temperature. A flow of transparent mucous can be seen from the swollen genital organ. There is a sign of smelling and congestion. In case of bull, no such symptoms can be seen except the restlessness. The bull is able to give service at the age of 2 years. Well grown bull may serve slightly earlier. The optimum time of service is mid heat to the end of heat. There is some evidence of the breeding of the cow once in every 14 months.



Breeding

The age of maturity in cattle depends on various factors such as the breed, nutrition level, state of health and environmental condition. Ovulation does not take place until about 14 hours after the end of the heat period. In the temperate climate, cattle generally mature at the age of about 3 years for natural or artificial breeding.



Gestation period

The time period from the time a cow conceives until she gives birth to a calf is known as gestation period. It varies with individual animals and with breeds. The gestation period in the first calf in heifers will be in average of 2 days less than older cows of the same period. The average gestation period for a cow is 280 days.



Cow is viviparous. The development of their young ones is intra uterine. It means inside the uterus of the mother. The minute egg ( alecithal ) contain so little yolk that they could never develop beyond the very early stages unless additional nourishment is somehow provided by the mother. During the development of all higher vertebrates or cow, certain membranous structures are produced which do not enter into the formation of the embryo itself. These are known as extra embryonic membranes. They include amnion, chorion, allantois and yolk sac. These membranes serve for nutrition, respiration, excretion and protection of developing embryo. In most mammals, allantois gives rise to a placenta. Plancenta is made by some part of the embryo and some part of the mother’s body. It is structure through which the fetus or developing embryo gets nourishment for the maternal uterine blood. The term placentation may be defined as an intimate relation between a portion of maternal uterine wall and a part or whole of the chorionic membrane or trophoblast of embryo for the purpose of nutrition, respiration and excretion.



Fetal and maternal bloods in placenta do not mix up with each other. The two blood streams are separated by barrier or membrane. The type of barrier found in cattle and sheep is called Syndesmo-chorial. In this type only uterine epithelium is eroded so that chorionic epithelium (or trophoblastic ectoderm) comes in contact with uterine connective tissue.



Artificial insemination

With advancement of animal husbandry nowadays, artificial insemination is in practice. This can ensure the birth of a good variety of calves. The sperms from a better progeny are preserved in the sperm bank at about the low temperature of – 18o Centigrade. During heat period, the preserved sperms are introduced into the vaginal passage of cow for the fertilization inside the body.

M Ed sanothimi

Minor phyla




Ctenophora



The ctenophore are biradially symmetrical organisms, lacking nematocysts, possessing ciliary plates in eight rows and possessing a gelatinous ectomesoderm containing mesenchymal muscle fibres.



Hermiphora plumosa



Systematic position

Phylum Ctenophora

Class Tentaculata - with two long aboral tentacles

Order Cydippikda - body oral or rounded, tentacles branched and retractile into

pouch

Genus Hermiphora

Species plumosa





position in animal kingdom - Ctenophores have certain characteristics in common with coelenterates. They are believed to have diverged very early from ancestral medusoid coelenterates which was a spherical animal with concentration of cilia along 8 meridional rows which later developed into comb plates, so known as comb jellies. However they represent a blind offshoot which gave rise to no higher forms. Ctenophores also present certain advancements over Radiata such as prominence of apical region, musculature derived from mesoderm, presence of gonoducts and determinate type of cleavage. This implies that Ctenophora are intermediate between Radiata and Bilataeria and have undergone considerable specialization with many striking characteristics of their own. Therefore, it seems quite logical to treat that Ctenophores should be included into a separate phylum rather than a class or subphylum of phylum of Coelenterata.



Hermiphora plumosa is slightly smaller and pear shaped organism about 5 to 20 mm in diameter. It is of glassy transparency. The species H plumosa is found in the Mediterranian sea .



External surface bears 8 equally spaced paddle plates or comb plates each made up of transverse rows of long cilia. So, they are also known as comb jellies. Near aboral end, on opposite sides, are attached two very long ( 15 cm) solid and highly extensible tentacles provided with lateral branches. Each tentacle can be completely withdrawn into a deep cavity or tentacle sheath or pouch. At the aboral end is a shallow depression with a sense organ or statocyst. A slit like mouth is present at the oral end. It leads into a long narrow pharynx or stomodaeum opening into the stomach. The stomach is connected to a system of gastrovascular canal. Hermiphora is strictly carnivore. Food is captured by tentacles with the help of colloblast. Colloblasts are adhesive cells also known as lasso cells. Undigested materials come out of mouth. The digestion is both extracellular and intracellular.



The body is covered externally by a delicate ectodermal epithelium. The epithelium of the stomodaeum is found by development to be ectodermal. The interval between the external epidermis and the canal system is filled by a soft, jelly like mesogloea. The tentacle sheath is is an invagination of the ectoderm and the tentacle itself is covered by a layer of ectoderm with which a is a core or axis formed by a strong bundle of longitudinal muscular fibres which are of mesodermal origin which serve to retract the tentacle into tits sheath.



The feeble development of the muscular system of course correlated with the fact that the swimming plates are the main organs of progression. A further striking difference between our present type and the Cnidaria is the absence of stinging capsules (nematocysts). This causes Ctenophora to be referred to as Acnidaria in older system. The place of stinging capsules is taken in a sense by the peculiar adhesive cell with branches of tentacles are covered. Adhesive cell has a convex surface produced into a small papillae which readily adheres to any object with which it comes in contact and is with difficulty separated.



The Hermiphora is hermaphrodite. Gonads develop in the meridional gastrovascular canal. Each of which has and ovary extending along the whole length of one side and testis along the whole length of opposite side. When ripe the ova and sperms are discharged into the canals, make their way to the infundibulum then into stomodaeum an finally escape by the mouth. Fertilization takes place in sea water.



Life cycle

The egg consists of and outer layer or protoplasm containing nucleus and of an internal mass of a frothy or vacuolated nature. The vacuoles contain homogenous substance which serve as a nutrient store to the growing embryo which corresponds with the yolk which occurs in a large proportion of animal eggs. Enclosing the egg is a thin vitelline membrane separated from the protoplasm by a considerable space filled with a clear jelly. Development includes a free swimming cydippid larva.

M Ed sanothimi

Life cycle of Balanoglossus




Life cycle

Sexes are separate. Male and female Balanoglossus live in separate burrows. The male and female are indistinguishable externally. The gonads occur in one or more longitudinal rows along the alimentary canal within genital pleurae in the anterior part of the trunk. Each gonad is a sac which continues into a ductile and opens to the exterior through a gonopore. The saccular gonads are lined with germinal epithelium. By proliferation of cells from the germinal epithelium sperms or ova are produced. They shed gametes in sea water. The egg is microlecithal. The eggs are of two types. The small ovum measures about 0.06 mm in diameter and undergoes indirect development with a pelagic larva known as tornaria larva. The larger one is about 0.4 mm in diameter and undergoes direct development without larval stage. The sperm has a rounded head and a flagella like tail. The mature sperms and eggs are shed into the burrows where fertilization takes place. The sperm fuses with egg in sea water. So, the fertilization is external. The fertilized egg or zygote undergoes cleavage. The cleavage is complete and is almost equal. A blastula is formed and later as a result of invagination, a gastrula is developed.



The development is indirect with a tornaria larva in Balanoglossus clavigerus or direct without a tornaria larva in Saccoglossus kowalevskii. After about 18 hours the animal –vegetal axis begins to elongate again and a larva is formed which is known as tornaria larva. It was first of all discovered by J Muller(1850) and was considered by his as the larva of echinoderms. It was Metschenikoff (1869) who established that is a larva of Saccoglossus. Tornaria larva is a free swimming larval stage of Balanoglossus.



Structure of Tornaria larva

The fully grown tornaria larva is usually ovoid in shape and is transparent. The size of tornaria larva varies from below one mm to 9 mm. It is provided with tow ciliated bands. At its anterior end a pair of pigmented eye spots. The mouth is situated on the ventral surface while the anus lies at the posterior terminal end. The alimentary canal is very simple having a mouth, oesophagus, stomach and intestine. A water sac is also found in the body of the larva which opens outside through dorsal pore the hydropore. There are three ciliated band which encircle the body of the larva. A pre-oral ciliated band lies anterior to mouth, post oral ciliated band lies behind mouth and a circum anal ring or posterior ciliated band lies around the anus. The oral bands help in the nutrition by directing water current towards the mouth. The circum anal ring regarded by some as a telotroch develops especially long and powerful cilia and is the chief locomotory organs of the tornaria. The interior of the tornaria contains a complete digestive tract like that of echinoderm larvae and one more coelomic vesicles. The heart vesicle develops in the later stages of tornaria and its pulsation were noticed by early observers.







Metamorphosis



After swimming for some time the tornaria larva sinks down the bottom. Its transparency is lost and the ciliated bands are disintegrated. Eyes also lost. The body elongates and is distinguished into the proboscis, collar and trunk and simultaneously the notochord, gill slits and coelomic sacs are also formed. Thus the larva gradually changes into the adult.



The metamorphosis of tornaria is accompanied by a great diminution in size probably due to loss of water. By this and thickening of skin, larva loses its transparency. Numerous gill slits are developed as outgrowth of the alimentary canal. Reproductive organs make appearance probably from mesoderm, the trunk meanwhile elongates so that the proportions of the adult are acquired.





There is no asexual reproduction. The fragile body of Balanoglossus may get broken easily and they have considerable power of regeneration.










Aves (flight and perching mechanism)



Flight is a highly spontaneous action. Larger birds either run or swim rapidly to gather enough forward momentum for a takeoff. Smaller birds s\usually take a quick jump by means of their legs followed by the beating of their wings. A bird flies on the principle of an aero plane, or heavier - than - air machine.



A bird flies on the principle of indirect movement. It moves the air, which by its displacement moves the bird. Air, displaced by the beating of wings, sets up current that keep the animal aloft and move forward, resulting in flight. According to Newton’s third law, the force of reaction of air is equal and opposite to that exerted by the moving wing on the air.



The wing is not a simple airfoil or plane. It functions both as an airfoil (lifting surface) and as a propeller for forward motion. It is thick in font, thin and tapering behind and presenting profile a convex streamlined upper and a flat or slightly concave lower surface. As the air flows across the somewhat tilted wing, the air stream moves faster along its upper convex surface that the concave lower surface. In accordance with Bernoulli’s law in physics, this differential in air speed results in a decrease in air pressure above the wing surface relative to the underside. Bernoulli’s law states that in fluid stream the pressure is least where the velocity is greatest. This basic physical principle involved in flight was first worked out by the Swiss mathematician Bernoulli in 1738. The two forces thus generated, suction above and upward thrust below the wing tend to life, keeping the bird aloft and moving forward and upwards. The air also pushes the wing horizontally backwards and tends to drag or slow the bird down. Thus the force of air on the wing can be resolved into a vertical life component perpendicular to the air steam and a backward drag component parallel to the air stream. For the bird to fly, the lift force must equal the force of gravity on the bird that is the weight of the bird. Various factors increase the lift force such as increase in the surface area of the wing and in the speed of air flow across the wing. If the angle of wing becomes too great that is when the wings become tilted sharply air swirls into the low pressure area above causing turbulence (induced drag). As a result lift is lost and wings stall movement slows down and the plummets toward the earth.



Flight muscles

Birds fly by flapping the wings which are the modified forelimbs. The flight muscles are many and belong to three main categories. They are pectoral, accessory and tensor.

• Pectoral muscles- the most important flight muscles are pectoralis major, pectoralis minor. Pectoralis major is depressor muscle causing downward stroke. When it contracts, the wing is pulled downward and forward so that the body is lifted and propels itself through air. Pectoralis minor is an elevator muscle causing upstroke of the wing. During flight, pectoralis major and minor contract and relax alternately in rapid succession.

• Accessory muscle- several small accessory muscle also help in elevating and depressing wing. They mainly help to rotate the wing in the glenoid cavity.

• Tensor muscle- these deltoid muscle keep the prepatagium fully stretched when the wing is spread out during flight.

Perching mechanism

The muscles of the legs are enlarged and strong. The shank and feet have few muscles and look slender and delicate. But certain muscles in the upper part of the legs have a special arrangement and long tendons for moving the toes. As a result, when a bird sits on a perch ( branch of tree, wire or rod) and squats, its toes are mechanically flexed and firmly grasp the perch. The muscles involved are known as perching mechanism. This is quite automatic and its enables the bird even to sleep on a twig without any risk of falling down. Perching muscles are characteristic of all birds. They comprise of two sets of muscles flexor and extensor.



Flexor muscles

Gripping of the toes is chiefly accomplished by the action of 8 flexor muscles, 6 to the anterior toes and 2 to the hind toe or hallux. The important flexor muscles are

• Ambiens - small but characteristic muscle. Its tendon join upper end of the flexor muscle of the second and third toe. It has minor role in perching.

• Peroneus medius – this muscle is present on the anterior aspect of shank. Its tendon trifurcates to supply the three anterior toes.

• Gastrocnemius – it is big calf muscle present on the back of tibiotarsus. Its tendon joind those of peroneous muscle to supply to anterior toes.

• Flexor perforans – this muscle is also attached to upper part of the tibiotarsus. Its tendon passes to the hind toe. It is joined by a slip with the peroneus medius so that a pull on any tendon will flex all the toes.



Extensor muscle

Several extensor muscles are found at the front of the tibiotarsus. They become attached to the upper part of the phalanges. The contraction serves to open the toes when the bird raises the shank while taking off the perch.

M Ed Biolgoy

Adaptive radiation in reptiles




As competition for food and living space prevails, a single ancestral species evolves into different forms which occupy different habitats. This is called adaptive radiations or divergent evolution. Reptiles have shown the greatest evolutionary diversity and adaptive radiation of all vertebrates. Their adaptive radiation took place twice, first in the Paleozoic and secondly in the Mesozoic.



Paleozoic radiation

During Paleozoic, with no competitors on land, the ancestral reptiles or cotylosaurs multiplied rapidly occupying all ecological niches available to them. Their radiation involved adaptations to different methods of locomotion and feeding. Distinct anapsid and synapsid forms dominated.



Mesozoic radiation

By the end of Paleozoic , the ancestral colylosaurs had disappeared. Their descendants produced a second and bigger radiation during Mesozoic. They dominated not only land but also sea and the air. This Mesozoic era is called as Age of Reptiles. This lasted over a great span of time about 130 million years. By comparison man is no older than 2 to 4 million years. The Mesozoic reptiles are represented by as many as 16 Orders. Of these one led to birds, one led to mammals and four to the modern reptiles. The rest disappeared. We will discuss only notable lines based on the morphology of the skull such as anapsid, synapsid, euryapsid, parapsid and diapsid .



Anapsid line

The modern Chelonia (turtles and turtoises) represent a direct and an early offshoot of cotylosaurs retaining anapsid skull. They have remained unchanged since Triassic some 160 million years ago.



Synapsid line

The mammal like reptiles or Synapsid had a single temporal cavity in skull ventral to postorbital and squamosal. Early Pelycosauria or Theromorpha were similar to cotylosaurs. Later Therapsida with differentiated dentition and improved locomotion were more mammal like. Before disappearing in Jurassic they gave rise to ancestral mammals.



Euryapsid line

The euryapsid or plesiosaurs had a single temporal fossa in skull above the joint of postorbital and squamosal. They were large marine turtle like heavy bodied and logn necked creatures. They were obviously fish eaters. All became extinct towards end of Cretaceous.



Parapsid line

There was another marine blind alley like Euryapsida represented by fish like or purpoise like ichthyosaurs. They also became extinct.



Diapsid line

Most of the today’s reptiles are diapsid with tow temporal openings on either side of skull separated by squamosal and postorbital bones. The earliest diapsids divide into two branches like Lepidosauria and Achosauria. The Lepidosauria were probably the ancestral to modern Squamata ( snakes and lizards) and Sphenodon. The Archosauria were the ruling reptiles dominating the Mesozoic Era. They represented the extinct Pterosauria, the extinct Dinosaurs and modern Crocodilia. They also gave rise to the modern birds.



A) Pterosauria

The extinct flying reptiles called Pterosauria or Pterodactyla were of light built. Their fore limbs evolved into membranous wings or patagia. Rhamphorhynchus of late Jurassic was a primitive pterosaur with 1 meter wingspan, a long balancing tail and toothed jaw. Pteranodon of Cretaceous had a 9 meter wingspan but no tail .



B) Dinosaur

At the end of Triassic, Thecodontia, the early descendents of Archosauria gave rise to the most fantastic Mesozoic reptiles the dinosaurs ( Gr. Dinos, terrible+saurus , lizard). They sub divided early into two orders Saurischia and Ornithischia.



Saurischia means “reptiles hip” they possessed a tri radiate pelvis with pubis entirely separate and anterior ischium. Sub order Thorapoda included flesh eating and bipedal carnivores. Tyrannosaurus rex from Cretaceous was 15 meters long and stood 6 meters high. Branchiosaurus was more than 25 m long and weighed over 50 tons.



Ornithischia means “ bird hip” they had a typical tetraradiate bird like pelvic girdle with pubis directed backwards parallel to ischium. They were all herbivores. Stegosaurus measures 8 meters and weight 10 tons. Fore limbs were much shorter than hind limbs.



Causes of extinction

After thriving and dominating the earth for 130 million years, the great dinosaurs and their contemporaries became suddenly extinct by the end of Cretaceous period. Various factors have been suggested for their total extinction like catastrophism, epidemic, food poisoning, racial senescence, climate change, overspecialization, intra specific warfare. Probably a combination of several factors was responsible for their extinction.

M Ed Biology

 

Anatomical development of mammal from fertilization to organogenesis

The egg  or ovum of rabbit is 0.1 m in diameter while that of human being is 0.15 mm in diameter. It is oval in shape and on its outer surface, there is a thin vitelline membrane. The egg contains cytoplasm. In the cytoplasm, there is  eccentric  nucleus. The nucleus lies in the upper half or animal hemisphere. The egg of mammal is alecithal  or micro lecithal with almost no yolk or lower half or vegetal hemisphere has some particles of yolk distributed evenly in the cytoplasm.  The whole egg is surrounded by a striated secondary membrane called zona pellucida secreted by follicular cells. Outside zona pellucida, there are follicle cells forming corona radiata which is soon dissolved away.

The secondary oocyte is released from the Graafian follicle into the peritoneal cavity outside ovary. The wall of ovary ruptures to release the egg which is known as ovulation. In human beings, every month or every 28 days, one ovum generally is released either from right or left ovary alternating with one another. The development of ovary and development of egg or ovum within the  ovary is under the influence of Follicle Stimulating Hormone FSH produced from the Pituitary gland. The developing egg or ovum starts secreting female hormone called Estrogen.

Soon after the release of ovum, the follicular cells and blood clot forms the Corpus Luteum.  The Corpus Luteum acts as endocrine gland and produces important hormone known as Progesterone.  Both female hormones are responsible for development of secondary sex organs like the oviducts, uterus, vagina  and mammary gland etc. and tertiary sex characters like different pattern of distribution of hairs, high pitched voice and little bit of shyness  in case of  human beings and deposition of fat at different parts of body like thigh and broad hip in case of man and other  mammals.

The egg is fertilized in the upper part of oviduct called ampulla which is slightly swollen part after the fimbriated funnel. For fertilization, entire spermatozoon penetrates the egg but soon the tail is degenerated.  The penetration by sperm is brought about by an enzyme hyaluronidase present in the semen.  The haploid set of chromosome present in egg and sperm arranged into homologous pairs and diploid chromosomes is restored.  The restoration of diploid chromosome is called fertilization. With fertilization, the egg is changed into zygote.

The zygote undergoes second maturation division  and becomes mature ovum. Two polar bodies lie within the zona pellucida near the animal pole. The fertilization ensures the division of zygote and further development of it.

Cleavage

Cleave is complete and holoblastic but unequal. The first cleavage is vertical diving the zygote completely into two unequal blastomeres, one being smaller and more opaque than the other. The second cleavage is also vertical and at right angle to the first one.  Four celled structure is formed. After the third cleavage which is horizontal little above the equator, eight cells are formed. The further cleavages are irregular diving zygote vertically and horizontally at the same time. Then a solid ball of cells is formed called Morula.  The Morula is covered by dense coat of albumen. It passes down the oviduct and comes in contact with uterus of mother and absorbs liquid food from mother.

Blastocyst

A fully formd morula shows an outer or superficial layer of cells, the trophoderm or trophoblast, surrounding an inner cell mass of larger polyhedral cells. The Morula stage passes down the oviduct and implanted into the wall of uterus.

Morula absorbs fluid secreted by the uterine mucous membrane and swells up. The liquid food is collected in a cavity which separate an outer layer of smaller trophoblast cells from a solid inner mass of cells. The embryo is now called a blastocyst which appears like a blastula but is not comparable to it. Those trophoblast cells which lie above the inner cells mass are called cells of Rauber. The inner cell mass in sonly attached to the trophoblast at animal pole like a knob. It is called embryonal knob because all parts of embryo will be derived from it. Embryonal knob is comparable to the area pellucid of chick embryo. The embryo now forms a vesicular structure called blastocyst or blastosphere.

As cavity enlarges the knob like inner cell mass becomes flattened and known as germinal or embryonal disc, which gives rise to embryo proper. In case of rabbit and other mammals, the trophoblast cells (cells of Rauber) overlying embryonal disc disintegrate. As a result the embryonal disc comes to lie at the surface of blastodermic vesicle or blastoderm and its edge becomes continuous with trophoblast around it. The trophoblast is equivalent to extra embryonic ectoderm. The cavity of embryo is equivalent to sub germinal  cavity of chick. However blastocyst stage is  peculiar to mammalian development.

Implantation

The trophoblat cells opposite to the embryonal knob form small papillae of trophoblastic villi which penetrate into depression or crypts  in the uterine wall so the blastocyst becomes attached to or implanted in the uterus of the mother, the villi absorb food from uterus.

Gastrulation

Some cells separate from lower surface of the embryonal knob, these cells migrate and form a lining of the trophoblast, the new layer of  cells is endoderm. The blastocyst is now converted into a two layered gastrula. Its fluid filled cavity is a functionless yolk sac. However there is no yolk.

In the centre of embryonal disc, cells collect to form a primitive streak, movement of the cells of primitive streak on both sides result in the separation of mesoderm which at first are un segmented  and have no cavity. Cells move forward from the primitive streak  to form notochord. Formation of mesoderm and notochord is the same as in chick. Formation of mesodermal somites and splitting of lateral plate mesoderm with appearance of splanchnocoel proceed in the same manner as in chick. However in rabbit, the lateral plate mesoderm extends as far as the equator, so that the lower wall of yolk sac consists of only ectoderm. The primitive streak retreats towards hind end. After giving up the  chorda mesoderm cells of the notochord and mesoderm, the remaining cells of the embryonal disc are ectoderm. The ectoderm cells form neural plate and neural folds  which become a neural tube as in chick. The mammalian embryo in its later stages is called foetus.

 

Balanoglossus (tongue worm)

Hemichordata ( Gr.  Hemi = half, chorde = cord) has been treated as subphylum of Phylum Chordata ( or Protochordata).  Some authors regarded it to be independent phylum close to phylum Echinodermata.  But for convenience of comparative study of protochordates, hemichordate has been retained in the subphylum of Phylum choradata.  Sub phylum Hemichordata includes a group of soft, vermiform, marine and primitive chordates. The most popular hemichordate genus is Balanoglossus commonly known as tongue worm or acorn worm. Other genus is Saccoglossus. They belong to class Enteropneusta (Gr. Enteron, gut; pheustos, breathed), some characteristics in the animals of this class are  body large and worm like, gill slits numerous, intestine straight.

Systematic position

Phylum            Chordata

Group              Protochordata ( Acraniata)

Subphylum      Hemichordata

Class                Enteroneusta

Family             Ptychoderidae

Type                Balanoglossus ( tongue worm)

Delie Chaije in 1829 named and recorded Balanoglossus clavigerus.  The genus Balanoglossus was derived from two Greek words, Balanos and glossus. The term balanos means an “acorn” ( fruit of oak) and refers to the proboscis projecting from collar looking like acorn nut, therefore the common name acorn worm is given. The other term glossus means tongue which  refers to the shape of its proboscis, collar and genital wings bearing a close resemblance to an ox tongue so, tongue worm is given to it. The fisher men call Balanoglossus  by the name of ox tongue.

Geographical distribution

Balanoglossus  and other hemichordates are marine animal. It is cosmopolitan in distribution. Balanoglossus is found in tropical and sub tropical seas of world. Some other species of Balanoglossus are  B australiensis ( Australia), B carnosus ( Indo Pacific) B  misakiensis ( Japan), B jamaicensis (West Indies), B gigas (Brazil), B capensis ( South Africa),  B clavigerus ( Mediterrannean and British isles).

Habits and Habitat

Balanoglossus is a marine, tubiculous or burrowing hemichordates. It inhabits shallow waters of intertidal zone and few occur in deep water also.

Burrow

The animal may hide under stones, or sea weeds or excavate its own burrow in bottom sand or mud.  B clavegerus  makes a U shaped tube or burrow having two vertical limbs of  about  50 to 70 cm deep and two openings are 10 to 30 cm apart.  In Saccoglossus, the body of the animal and tube are twisted but the anterior and posterior extremities are   straight. The anterior opening of the burrow is funnel shaped and wide.  The anterior vertical limb may give out branches each having funnel like opening. The posterior opening  of the burrow is round and concealed below the spirally coiled faecal matter of the animal.

Protective device

The inner wall of the fragile burrow is smoothly lined by sand particles cemented together into a tough cast with sticky mucus secreted by the mucous gland present in the skin of the animal. Due to this the walls of the burrow are not collapsed and protects the delicate body of animal from burial in loose sand. They also secret foul smelling odour like iodoform for protection.

Movements

The tongue worm is sluggish in movement. It is little affected by the external stimuli.  It can move inside burrow with the help of cilia covering its body surface. The proboscis is the most active part of its body. The proboscis can be elongated or shortened by the action of muscle. The proboscis can help in burrowing.  When the tide is over, Balanoglossus protrudes its anterior end out of the burrow to explore the surrounding. The worm can extend the posterior end out of the burrow to cast the faecal material.

Feeding

Acorn worm takes a lot of sand or mud to obtain diatoms, protozoans and other microorganisms and organic detritus present in it. Sand or mud is not the food but as it cannot separate  food and sand or  mud they consume both. They take lot of sand or mud because  the sand or mud contains little amount of food.

External structure

• Shape – the shape of its body is cylindrical, worm like and bilaterally symmetrical.
•  Size – the body measures about 10 to 50 cm in length but it may differ according to species. B gigas attains about 1.8 m in its body length according to Sawaya 1951 or 2.5 m according to Spengel 1893.
• Color – color of its body is bright or drab with reddish or orange tint.
• Locomotory organ – the body is uniformly covered by short hair like cilia. Exoskeleton and external appendages are absent.

Division of body

The body of acorn worm is unsegmented.  It is divisible into three distinct regions or parts like proboscis, collar and trunk.

Proboscis

Proboscis is also known as protosome which  is the anteriormost part the body.  Proboscis is club shaped or conical.  It is circular in cross section.  It has thick muscurlar wall and cavity inside is proboscis coelom. The proboscis coelom  communicates to the exterior through minute proboscis pore situated mid dorsally near its base.  In some proboscis pore ends blindly or there are two pores. Posteriorly the proboscis narrows into a slender neck or proboscis stalk which is attached to the collar. Below the stalk, the base of proboscis  bears U shaped ciliated epidermal depression called the preoral ciliary organ which tests the quality or food and water entering the mouth.

Collar

Collar is also known as mesosome. It is middle, short and cylindrical part. The flap like or funnel like anterior margin is called as collarette. It completely surrounds and conceals the proboscis stalk and posterior part of proboscis. Ventrally, below the proboscis stalk, the collarette encloses a permanently open wide aperture called as mouth.  The mouth opens into a cavity called buccal cavity which lies inside collar. The collar is well demarcated  from the trunk by circular constriction. The wall of the collar is highly muscular. There is a cavity called collar coelom. The collar coelom opens to the outside through a pair of collar pores into the first  pair of gill pouches behind.

Trunk

Trunk is also called as metasome. It is the posteriormost and the largest part of the body. The trunk is flat and appears annulated due to circular constrictions on the surface.  The trunk bears a mid dorsal ridge and mid ventral ridge which correspond the nerve cord and blood vessel. The trunk is further divisible into three regions.

            Branchiogenital region

Branchiogenital region is marked  by a pair of lateral, thin, flat and longitudinal flaps called the genital wings which contains gonads. The gonopores are microscopic and cannot be seen by unaided eye. The anterior half of the branchiogental region bears two  longitudinal rows of branchial apertures or gill pores. The number of gill pores increases with the age of animal. Two genital wings can be curved to meet mid dorsally so as to conceal the gill pores.

Hepatic region

 The middle hepatic region of the body is somewhat smaller than  branchiogenital region. It is greenish in color and its dorsal surface is marked by presence of numerous irregular intestinal sacculations or hepatic caeca.

Posthepatic region

This is the posteriormost and  the longest part of the trunk. It is also called abdomen or caudal region. It is more or less of uniform diameter. The posterior end tapers somewhat and bears  terminal anus.

Life cycle

Sexes are separate. Male and female Balanoglossus live in separate burrows. When fully matured, they shed gametes in sea water.  The egg is microlecithal. The sperm fuses with egg in sea water. So, the fertilization is external. The fertilized egg or zygote undergoes cleavage. Embryo is formed. Embryo hatches into a free swimming planktonic larva called tornaria. It is ciliated larva. The development is indirect.

There is no asexual reproduction. The fragile body of Balanoglossus may get broken easily and they have considerable power of regeneration.

 

Pisces (osmoregulation and scales

Every organism in natute has a close relationship with its environment for water and salts. An optimum concentrations of water and salts is essential in the body for  proper metabolism. The maintainance of such a concentration is called osmoregulation. There may be three possible conditions of the concentration of waer and salts in the body of an organism in comparision to the concentration of the same substances in the environment.

1. isotonic  or iso osmotic- the cell or organism kept in it experiences no change in its volume. In this situation the osmotic pressure is same both inside and outside the cell or organism.
2. Hypotonic or hypo osmotic – which is more diluted as compared to the other. The solvent or water enters into the body of organism from surrounding environment.
3. Hypertonic or hyper osmotic – which is more concentrated as compared to the other. The solvent or water  comes out of the body of the organism into the surrounding environment.

Animals can be put in two categories according to their tolerance for osmotic changes.

• Stenohaline  - this includes the animals which can tolerate the narrow range of salinity. They change the osmotic pressure of their internal fluids according to the changes in their  external medium. This includes crustaceans and several mollusks.
• Euryhaline – this includes the animals which can tolerate the broad range of salinity. They do not change the osmotic pressure of their internal fluids according to the changes in their external medium. They can maintain the concentration of their body fluids at the same level in all situations. 

Marine elasmobranchs like sharks,  rays and skates have a special urea retention device. The migratory fishes have their own peculiarity of osmoregulation. For instance, when the fresh water eel reaches  the sea for spawning, there is a tendency for the fish to lose water and to increase the salt concentration in its body. The excess salts are removed to the outside by the active transport through the chloride –secreting cells of  the gills. The kidneys keep on reabsorbing water.  When they return to the rivers and grow into adults, they have to face the reverse problem where they remove the excess water along with the urine and reabsorb salts into the blood. These changes in the kidneys are regulated by the endocrine secretions.

Scales of fishes

In vertebrates, the exoskeletal covering of body is made on two types scales. They are epidermal and dermal. The epidermal scales are cornified derivatives of malpighian laer of epidermis and developed in terrestrial animals like reptiles , birds  and mammals. Dermal scales are mesenchymal in origin and especially developed in fishes. They are small, thin, calcareous or bony plates which fit closely together or overlap.  Primitive fossil fishes of Silurian and Devonian periods, possessed exoskeleton in the form of plates and scales which consist of three district layers. The innermost layer consisted of a compact bone, isopedine, the intermediate or middle layer of spongy vascular bone and the outer layer is of dentine.

According to the mode of origin, there are tow types of scales. (1) those which are formed due to the secretary activity of both epidermis  as placoid scales  of elasmobranch (ii) non placoid scales that are derived from dermis only as the scales of teleosts.. structurally the scales are classified as cosmoid, genoid or rhomboid, placoid, cycloid and ctenoid. The last two are called bony ridge scales.   

Cosmoid scales – the cosmoid scales are found in extinct Crossopterygii and Dipnoi.

Ganoid scales – ganoid scales are characteristic of the primitive acrinopterygians called ganoid fishes. These scales are heavy and have an outer layer of hard inorganic, enamel like material called ganoine. The middle layer  is cosmine containing numerous branching tubules. The innermost layer is thickest and is made up of lamellar bone called isopedine. 

Placoid scales – Placoid scales are characteristic of elasmobranch fishes( sharks). Each scale has a disc like basal plate embedded in the dermis and a spine projecting out through the epidermis. Placoid scale resembles the tooth.  The spine has an external covering of enamel like hard transparent material called vitrodentine. This is followed by a layer of dentine enclosing a pulp cavity from  which several branching dentine tubules radiate in different directions. Placoid scales are closely set in skin but do not overlap each other and fiving a sand paper like quality to skin.

Cycloid scales – cycloid   scales are thin flexible, translucent plates, rather circular in outline, thicker in the centre and marked with several concentric line of growth which can be used for determine the age of the fish. They are found in a large number of teleostean fishes having soft rayed fins, such as Labeo, Catla. The central part of the scale is called focus and is the first part to develop.  They form protective covering over the skin and project diagonally in an imbricating pattern.

Ctenoid scales -  Ctenoid scales are also circular and can be distinguished from the cycloid scales by having a more or less serrated free edge. Several spines are present on the surface of the posterior area of the scale.  These scales are characteristic of modern higher teleosteans such as perch, sunfish etc. They are firmly attached and their exposed free hind parts are not overlapped and bear numerous small comb like teeth or spines.

Economic  importance of Amphibia

Most of the amphibians are beneficial to mankind.  They are of great economic importance.  Specially  frogs and toads are of economic importance.

• Scientific study

Frogs and toads are included the syllabus of Proficiency certificate level of  different universities of world including  Nepal  from the past several years.  Specially in Nepal, the frogs are dissected in the laboratory to study the  general anatomy,  alimentary canal, reproductive organs, arterial system, venous system and brain etc. Amphibian developmental study is also included in the syllabus of University of Nepal.  The amphibians are studies mainly because they represent both lower and higher animals in many respects.  In USA, mud puppies( Necturus) are dissected  for the laboratory study.  Newt Diemictylus viridescens is widely used in research.

• As food

Frogs are mainly consumed as food by gourmets in USA and Japan and many other countries of the world. In Nepal also, frogs  (paha as local name) are eaten by the terrain people. Usually the fleshy  hindlimbs are eaten.  Other edible amphibians esteemed as food are Necturus and axolotls in America and giant salamander in Japan.

• Predation

Frogs and toads are destroyers of noxious insects. Toads are of great value because they live in gardens where insects are most injurious.  The French gardeners even buy toads to control harmful insects . Bufo marinus has been introduced in tropical sugarcane fields to destroy injurious insects. . in Nepal, farmers gave great resects fir   the frogs as they help to increase their production.

• Medicinal value

Toads are used in Chinese medicines. Skins of toad secretes a digitalis like substance that increases blood pressure when injected into humans.

• Trade, art and recreation

Skins of frogs are used for glue, book binding and making delicate purses. Amphibians and their larvae provide motifs in the art of American Indians, toads have played a role in religious beliefs of primitive people since ancient  times. Aesop 560 BC included frogs among his fables. Tribal magicians in America used parts of frog and toads in their magic. Bull frogs area said to participate in a jumping frog contest held every year in USA frogs and toads are kept as pet in household aquaria.

• Poisonous Amphibia

A wide variety of irritating toxic compounds Is produced by amphibians. Poison glands are located dorsally in  skin and defense postures of anurans and urodel present the dorsal glandular surface to their predators. Poisonous secretions (bufotoxins) of Bufo marinus are fatal to dogs and cats. Poison of Dendrobates a South American frog is used by tribal people to poison the points of their arrows.  Some poisonous amphibians such as Salamandra salamandra are warningly colored. Helbenders (Cyptobranchus ) are said to be poisonous but not dangerous to men.