Volume 1

Gene expression / Benjamin Lewin.

  • Lewin, Benjamin.
Date:
1974-1977
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Gene expression / Benjamin Lewin. Source: Wellcome Collection.

    26/668 (page 4)
    in which both copies of the gene are mutant. But crossing mutants located in different genes generates an organism which contains one wild type (+) copy and one mutant copy of each gene : a+ +b a + a+ X +b~^+b The gene can therefore be defined as a genetic unit by the results of a com plementation test; alleles do not show complementation when crossed with each other, whereas mutants in different genes do so. Metabolic processes in living organisms are mediated and controlled by enzymes and the idea that heredity might be exercised by a connection between genes and enzymes is long established. (At one time, indeed, fashionable opinion was that genes were enzymes). The research of Beadle and Tatum in 1948 on nutritional mutants of the fungus Neurospora crassa placed the idea that genes specify enzymes on a firm basis. When the fungus is grown on a substrate which is metabolized through an ordered pathway of reactions, mutation can block the process at any step of the pathway. Each metabolic step is catalysed by a particular enzyme ; since each of the inactivating mutations behaves as a single gene in inheritance, Beadle and Tatum proposed the one gene—one enzyme hypothesis: each gene is responsible for the synthesis of one particular enzyme. Direct evidence that genes code for proteins was first provided by Ingram in 1957 when he showed that the trait of sickle cell anaemia, caused by mutation in a single gene, can be accounted for at the molecular level by an alteration in the amino acid composition of the protein haemoglobin. Because some proteins comprise more than one type of polypeptide chain, the one gene— one enzyme theory has been rephrased since its original formulation. The functional unit of haemoglobin, for example, consists of two a-chains and two ^-chains. Genetic studies of mutant haemoglobins have shown that synthesis of the a-chains and ß-chains is governed by different genetic loci. This has led to the more precise definition that one gene is a segment of genetic material which specifies one polypeptide chain. One early concept visualised the gene as the unit of genetic recombination, but in 1945 Lewis demonstrated that mutants defined by the complementation test as alleles affecting the size of the eye of Drosophila can recombine with each other. Recombination between alleles generates the reciprocal products of a gene which carries both mutations and a wild type gene which carries neither mutation. A heterozygote in which one gene carries both mutations and its allele is wild type differs in phenotype from a heterozygote in which each allele carries one of the two mutations. The only difference between these two genotypes lies in the molecular arrangement of the mutant sites, for in both the cell has one wild type and one