From Mendel’s experiment on hybridization it is understand that genes are hereditary units transmitted from one generation to other generation through the germ cells and associated with carrying the characters. Mendel used the term factor for these hereditary units.
The term gene was first used by johansen 1909. After him several workers experimentally proved that genes are the fractions or part of DNA molecule which regarded as the genetic material. Suton and boveri independently suggested that chromosome is the container of hereditary units. T.M Morgan proposed the gene theory which state that:
i) Chromosomes are bearers of hereditary units and each chromosome carries hundreds or thousands of genes.
ii) The genes are arranged on the chromosomes in the linear order and on the special regions or locus.
Classical concept of gene:
Classical concept of gene was introduced by sutton (1902) and was elaborated by morgan (1913). Bidge (1923), Muller (1927) and others which outlined as follows.
i) Genes are descrete particles inherited in mendelian fashion that occupies a definite locus in the chromosome and responsible for expression of specific phenotypic character.
ii) Number of genes in each organism is more than the number of chromosomes; hence several genes are located on each chromosome.
iii) The genes are arranged in a single linear order like beads on a string.
iv) Each gene occupies specific position called locus.
v) If the position of gene changes, character changes.
vi) Genes can be transmitted from parent to off springs.
vii) Genes may exist in several alternate formed called alleles.
viii) Genes are capable of combined together or can be replicated once during a cell division.
ix) Genes may under for sudden changes in position and composition called mutation.
x) Genes are capable of self duplication producing their own exact copies.
Modern concept of gene:
S. Benzer (1957) coined different terms for different nature of gene and genetic material in relation to the chromosome on the basis of genetic phenomena to which they involve.
i) Genes as unit of transmission or cistron :
The part of DNA specifying a single polypeptide chain is termed as cistron. A cistron can have 100 nucleotide pairs in length to 30,000 nucleotide pairs. It transmits characters from one generation to other as unit of transmission.
ii) Genes as unit of recombination or recon :
The smallest segment of DNA capable of being separated and exchange with other chromosome is called recon. A recon consists of not more than two pairs of uncleotides.
iii) Gene as unit of mutation or muton :
Muton is the smallest unit of genetic material which when changed or mutated produce a phenotypic trait. Thus muton is delimited to a single nucleotide.
On the basis of their behavior the genes may be categorized into the following types.
i) Basic genes: These are the fundamental genes that bring about expression of particular character.
ii) Lethal genes: These bring about the death their possessor.
iii) Multiple gene: When two or more pairs of independent genes act together to produce a single phenotypic trait.
iv) Cumulative gene: Some genes have additive effects on the action of other genes. These are called cumulative genes.
v) Pleiotropic genes: The genes which produce changes in more than one character is called pleiotropic fene.
vi) Modifying gene: The gene which cannot produce a character by itself but interacts with other to produce a modified effect is called modifier gene.
vii) Inhibitory gene: The gene which suppresses or inhibits the expression of another gene is called inhibitory gene.
Gene action: (One gene one enzyme concept):
The influence of gene resulting in the expression of a genetic character is called gene action. The genes are generally associated with the production of enzymes, which they synthesize from chemical substance available in the body cells through a process of autocatalysis. As a rule one gene affects one enzyme. The various actions of genes are expressed in their development of pigments. Colors, hormones, size and form, production of proteins, antigen and antibody production decisive effect on human disease like albinism, tyrosinosis etc. This idea was given by beadle and tatum (1941)
Genes in protein synthesis and genetic code:
Different types of aminoacids in jinear sequence forms a polypeptid resulting production of specific protein. Protein syntheis is under the direct control of genes located on the chromosome of that cell. From chromosome at nucleus specific m-RNA are transcribed with series of triplet bases called codon. There are 64 codons assigned for 20 different aminoacids codon series on m-RNA is specified by the genes and correspondign aminoacids brought together in chains in contrast to the condons. “AUG” is the codon on M-RNA asigns the aminoacid methoionine in the polypeptide series. Jacob and monod propsed “Operon model” To explain the mechanism of gene action in protein synthesis is regulated by three specific genes located on chromosomes.
i) Structural genes:
It regulates to produce specific m-RNA and determine the kind of protein to be synthesized.
ii) Operator genes:
These genes act as switches to turn on or turn off the activities of structural genes, regulating the elongation and termination of polypeptide chain.
iii) Regulator genes:
These genes produce certain proteinaccous substance called repressures which prevent the operator genes from their action.
The above concepts of gene have been proposed on the basis of experimental evidences after identification of genetic material and studying the behavior of genes and interaction during hybridization. Identification of genes and interaction duting hydridzation. Identification of gene and genetic material had done by different a geneticist which includes:
i) Transformation experiment in bacteria by Frederick Griffith (1928) on diplococcus pneumoniae.
ii) Bacteriophage multiplication studies by hershey and chase (1952)
iii) Tobacco mosaic virus (TMV) multiplication studies by H.F. Frankel (1955).