8. A cistron is a single stretch of DNA

1.8 A cistron is a single stretch of DNA

Key terms defined in this section
Cistron is the genetic unit defined by the cis/trans test; equivalent to gene.
Complementation group is a series of mutations unable to complement when tested in pairwise combinations in trans; defines a genetic unit (the cistron).
Gene (cistron) is the segment of DNA involved in producing a polypeptide chain; it includes regions preceding and following the coding region (leader and trailer) as well as intervening sequences (introns) between individual coding segments (exons).
One gene : one enzyme hypothesis is the basis of modern genetics: that a gene is a stretch of DNA coding for a single polypeptide chain.

The first systematic attempt to associate genes with enzymes showed that each stage in a metabolic pathway is catalyzed by a single enzyme and can be blocked by mutation in a different gene. This led to the one gene : one enzyme hypothesis. Each metabolic step is catalyzed by a particular enzyme, whose production is the responsibility of a single gene. A mutation in the gene alters the activity of the protein for which it is responsible.


Identifying which protein represents a particular gene can be a protracted task. The mutation responsible for creating Mendel’s wrinkled-pea mutant was identified only in 1990 as an alteration that inactivates the gene for a starch branching enzyme!




Figure 1.19 Genes code for proteins; dominance is explained by the properties of mutant proteins. A recessive allele does not contribute to the phenotype because it produces no protein (or protein that is nonfunctional).

A mutation is a random event with regard to the structure of the gene, so the greatest probability is that it will damage or even abolish gene function. This explains the nature of recessive mutations: they represent an absence of function, because the mutant gene has been prevented from producing its usual enzyme. Figure 1.19 illustrates the basis for the dominance relationship between recessive and wild-type alleles. When a heterozygote contains one wild-type allele and one mutant allele, the wild-type allele is able to direct production of the enzyme. The wild-type allele is therefore dominant. (This assumes that an adequate amount of protein is made by the single wild-type allele. When this is not true, the smaller amount made by one allele as compared to two alleles results in the intermediate phenotype of a partially dominant allele in a heterozygote.)


A modification in the hypothesis is needed to accommodate proteins that consist of more than one subunit. If the subunits are all the same, the protein is a homomultimer, represented by a single gene. If the subunits are different, the protein is a heteromultimer. Stated as a more general rule applicable to any heteromultimeric protein, the one gene : one enzyme hypothesis becomes more precisely expressed as one gene : one polypeptide chain.


How do we determine whether two mutations that cause a similar phenotype lie in the same gene? If they map close together, they may be alleles. However, they could also represent mutations in two different genes whose proteins are involved in the same function. The complementation test is used to determine whether two mutations lie in the same gene or in different genes. The test consists of making a heterozygote for the two mutations (by mating parents homozygous for each mutation).


If the mutations lie in the same gene, the parental genotypes can be represented as:



The first parent provides an m1 mutant allele and the second parent provides an m2 allele, so that the heterozygote has the constitution:



No wild-type gene is present, so the heterozygote has mutant phenotype.


If the mutations lie in different genes, the parental genotypes can be represented as:



Each chromosome has a wild-type copy of one gene (represented by the plus sign) and a mutant copy of the other. Then the heterozygote has the constitution:



in which the two parents between them have provided a wild-type copy of each gene. The heterozygote has wild phenotype; the two genes are said to complement.




Figure 1.20 The cistron is defined by the complementation test. Genes are represented by bars; red stars identify sites of mutation.

The complementation test is shown in more detail in Figure 1.20. The basic test consists of the comparison shown in the top part of the figure. If two mutations lie in the same gene, we see a difference in the phenotypes of the trans configuration and the cis configuration. The trans configuration is mutant, because each allele has a (different) mutation. But the cis configuration is wild-type, because one allele has two mutations but the other allele has no mutations. However, if the two mutations lie in different genes, we always see a wild phenotype. There is always one wild-type and one mutant allele of each gene, and the configuration is irrelevant. The basic test and some exceptions to it are discussed in Complementation.


Failure to complement means that two mutations are part of the same genetic unit. Mutations that do not complement one another are said to comprise part of the same complementation group. Another term that is used to describe the unit defined by the complementation test is the cistron. This is the same as the gene. Basically these three terms all describe a stretch of DNA that functions as a unit to give rise to an RNA or protein product.




Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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