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.