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.
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.
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