GENERAL MICROBIOLOGY - T.P. Pirog - 2004

19. BACTERIAL GENETICS: STABILITY, VARIATION, AND HEREDITY OF TRAITS

19.2. MUTATIONS AND THEIR ORIGIN

19.2.1. Spontaneous mutations

Mutations that occur in Cells without any experimental intervention are called spontaneous mutations. Cells in which spontaneous mutations have arisen are referred to as spontaneous mutants. The fraction of spontaneous mutants in a Cell population is 10 *-10 ^и. Mutation frequency is defined as the number of mutations per cell or per generation.

"Silent" mutations. While a mutation is generally defined as a genotype change that manifests in the phenotype, at THE MOLECULAR LEVEL any stable hereditary change in DNA can be considered a mutation. However, due to the degeneracy of METABOLISM/28.html">The Genetic Code, not every mutation of this kind will be expressed phenotypically. In many triplets, A change in the third base has no consequences ("silent" mutations). Even the substitution of the second or first base in a triplet does not always lead to severe consequences. For instance, the mutation AUC → GUC results in the replacement of isoleucine with valine, meaning the substitution of one lipophilic amino acid for another. Conversely, the mutation CUU → CCU leads to the replacement of leucine with Proline. The consequence of such a substitution will be a deviation from the normal spatial configuration of the polypeptide chain, which in turn can significantly alter higher-order Structure.

Back mutations and reversions. A mutant may also undergo a reverse mutation, resulting in the restoration of wild-type properties. A true back mutation is defined strictly as a mutation that precisely restores the original genotype—that is, when the triplet altered by the initial mutation codes once again for the exact same amino acid as before. If, however, the original phenotype is restored, it is referred to as a reversion or suppressor mutation. Accordingly, cells in which reversions have occurred are called revertants.