It is the second important function of genetic material (DNA). first function : Transfer of character to the next generation. It is the molecular mechanism by which the heredity information present in the gene comes out and produce a character.
A . Garrod : 1 Mutant Gene =1 Metabolic Block
*Beadle and Tatum
1 Gene = 1 Enzyme HypothesisWork on Neurospora (Drosophila of plant kingdom).A wild type Neurospora can grow easily on minimal medium ( Sugar, salt, biotin and ammonium compound )
A wild type Neurospora exposed to x-ray they induces mutation.
they obtain many form which are now unable to grow in minimal medium.
They will grow when come addition nutrient are added in the mutation .
Such nutritional mutant are know as Auxotrophs.
They are three type of Auxotrophs
Cross I : They will grow when either ornithine / citrulline / arginine is added in the medium (enzyme a gets mutated )
Cross II : They will grow when either citrulline / arginine is added in the medium (Enzyme B gets mutated ).
Cross III : They will grow when arginine is added in the medium.( enzyme C gets mutated)
From this , they concluded an arginine biosynthesis pathway in Neurospora.
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Auxotrophs |
Thus, the concluded that each gene forms a special enzyme.
One gene =one enzyme hypothesis , one gene = one polypeptide (by Yanofsky), and on cistron = one polypeptide hypothesis.
TYPE OF GENE
1. CONSTITUTIVE GENE : (HOUSEKEEPING GENE)
Gene which always remains active and their protein product are continuously required in the body. These gene need not be regulated ON
example ; Gene of respiratory enzyme.
2.NON CONSTITUTIVE GENE : ( LUXARY/ SMART GENE )
Gene which do always remain active. These gene required an ON-OFF mechanism that is regulation mechanism.
They are two type-
(i) Inducible gene- off to on involved in catabolism, inducer substrate
(ii) repressible gene- on to off involved in anabolism. If the end products is available in the cell (ready made) then the synthesis stops.
* Poly cis - composed of many genes , split gene -composed of 1 gene
3.OVERLAPPING GENE
Very special and rare by t. sanger.
Gene which can produce more than 1 type of polypeptide.
It is due to overlapping genetic code.
It is found in some virus
example : x174, sv-40
4.HOMEOTIC GENE :
Special gene which regulate organ differentiation in organism. There activity is regulated by homeobox. Due to mutation -either organ is not form or they will form abnormally at wrong places
5. JUMPING GENE ( TRANSPOSONES )
Gene which can move from one locus to another locus on a some / different chromosome. discovered by : B. McClintok ( lady mendel ) in activator and dissociation present on chromosome 9 and move to other chromosome.
Few active gene are
(Copia like element ,fold back element , P and I element ) Drosophila
(Ty element ) yeast,
(Alu element )
Transposones used as vector in gene transfer experiment in genetic engineering
6.PSEUDOALLELES
They are identified by some rare crossing over.
example star (external) and asteroid (internal ) eye shape in drosophila
7.ISOALLELES
Different forms of an allele which produce similar phenotype.
Example : eye colour of drosophila
It produce similar phenotype (almost similar expression)
It is not counted as new allele but isoallele