Mutation: Change in the Genetic Material

A mutation is a change in the nitrogenous-base sequence of DNA; that change may cause a change in the product coded for by the mutated gene ( a silent mutation changes a base pair in the DNA but after transcription the resulting codon specifies the same amino acid that was coded for before the mutation).

Many mutations are neutral, some are disadvantageous, and others are beneficial.

Types of Mutations

A point mutation or base substitution occurs when one base pair in DNA is replaced with a different base pair.

Alterations in DNA can result in missense mutations (which cause amino acid substitutions) or nonsense mutations (which create stop codons).

In a frameshift mutation, one or a few base pairs are deleted or added to DNA.

Mutagens are agents in the environment that cause permanent changes in DNA.

Spontaneous mutations occur without the presence of a mutagen.

Mutagens

Chemical mutagens include base-pair mutagens (for example, nitrous acid), nucleoside analogs (for example, 2-aminopurine and 5-bromouracil), and frameshift mutagens (for example, benzpyrene).

Nitrous acid, a base pair mutagen

Nucleoside Analogs

Ionizing radiation causes the formation of ions and free radicals that react with DNA; base substitutions or breakage of the sugar-phosphate backbone result.

Ultraviolet radiation is nonionizing; it causes bonding between adjacent thymines.   Damage to DNA caused by ultraviolet radiation can be repaired by enzymes that cut out and replace the damaged portion of DNA.   Photoreactivation enzymes repair thymine dimers in the presence of visible light.

Frequency of Mutation

Mutation rate is the probability that a gene will mutate when a cell divides; the rate is expressed as 10 to a negative power.

Mutations usually occur randomly along a chromosome.

A low rate of spontaneous mutations is beneficial in providing the genetic diversity needed for evolution.

Identifying Mutants

Mutants can be detected by selecting or testing for an altered phenotype.

Positive selection involves the selection of mutant cells and rejection of nonmutated cells.

Replica plating is used for negative selection - to detect, for example, auxotrophs that have nutritional requirements not possessed by the parent (nonmutated) cell.

Identifying Chemical Carcinogens

The Ames test is a relatively inexpensive and rapid test for identifying possible chemical carcinogens.

The test assumes that a mutant cell can revert to a normal cell in the presence of a mutagen and that many mutagens are carcinogens.

Histidine auxotrophs (require histidine in the media) of Salmonella are exposed to an enzymatically treated potential carcinogen, and reversions to the nonmutant state are selected.