Mutations and its types

109

Mutations are the changes in primary sequence of genomic DNA.

Mutations that emerge in the absence of a certain mutagen are called spontaneous, whereas the mutations occuring after the exposure to definite mutagenic factor (radiation, temperature, chemical and other agents) are termed induced.

Mutagens

Mutagens are the substances, agents and factors, causing mutations.

The frequency of mutations accelerates greatly by exposure of bacterial cells to mutagens.

Mutagens can be divided into three broad categories: chemical agents, physical factors (most important is radiation) and biological mutagens (e.g., genetic transposable elements).

Ultraviolet (UV) light is a potent physical mutagen that alters DNA sequence by conjugation of thymine bases resulting in thymine dimer formation. Two nucleotides produce dimers, which impair normal replication of DNA. Photoreactivation of DNA structure by visible light is performed by special enzyme, photolyase, which breaks down thymine dimers. Sequence recovery is not completely precise; therefore, various mutations can arise.

Chemical mutagens exert mutations by changing either the chemical structure or folding of DNA molecules. Various chemical substances directly modify the bases within DNA. For instance, nitrous acid (HNO2) reacts with hydroxyl groups resulting in formation of amino groups. That leads to incorrect DNA replication during cell life cycle.

Mutations, affecting reparation enzymes, change their specifity and activity. In that case these enzymes play a role of biological mutagens. Other biological mutagens are transposons, IS elements and temperate bacteriophages that alter DNA sequence after their incorporation into a new site of bacterial genome.

A great number of versatile microbial traits may be affected by the mutations – auxotrophy to various growth factors (vitamins, nucleotides, or amino acids), antimicrobial resistance, sensitivity to bacteriophages, enzyme expression, etc.

Types of Mutations

Spontaneous mutations affecting a certain gene can occur with a frequency of 10-6-10-8 in microbial population generated from a single bacterial cell.

Mutations comprise base insertions, deletions, duplications, substitutions, inversions, translocations and some others.

Deletions eliminate the genetic sequences from the genome. They can affect large genetic regions and usually don’t revert to the initial state.

Insertions occur after the addition of a new genetic material into primary DNA sequence; duplication presumes the addition of the same or closely related DNA fragment. The latter mutations are largely unstable resulting in spontaneous reversions. Some other mutations invert the sequences of DNA (inversions) or deliver DNA fragment to another location (translocations).

Substitutions result from the mispairing of complementary bases in the process of replication. The frequency of substitutions is about one base for 1010 nucleotides incorporated into DNA molecule during replication.

The deletion or insertion of one nucleotide into DNA molecule leads to the shift of the triplet sequence of DNA. This creates new codons resulting in translation of altered protein sequence because of incorporation of incorrect amino acids. This type of genetic alterations is known as frame shift mutations.

According to their size there are large (e.g., gene), and small (point) mutations. The large rearrangements rest on the deletion or insertion of a considerable portion of the gene. Point mutation are located within the gene itself, usually resulting in deletion, inclusion, or replacement of one nucleotide pair within DNA molecule. Large mutations in bacteria are commonly lethal, albeit point mutations more easily repaired.

Many point mutations are not detected readily at the phenotypic level, as they don’t alter significantly the biological function of the translated enzyme or protein. It concerns the missense mutations, followed by the substitution of one amino acid for another in final protein sequence. Nevertheless, phenotypic results of such a mutation might be negligible. Nonsense mutations terminate the process of protein translation in mutational site. This usually leads to substantial misfunction of final protein molecule.

The recovery of normal biological activity of the protein after mutation is denoted as phenotypic reversion. It may happen due to the restoration of primary DNA sequence in the mutated site (genotypic reversion). Sometimes phenotypic reversion arises from the mutation within another genetic site that shuts down the effects of primary mutation (so-called suppressor mutation). Intragenic suppression is the secondary mutation within the same gene that was initially affected by primary mutation. In some cases it may normalize the function of defective protein. Extragenic suppression occurs after next mutations in genes beyond the affected one.

For detection of mutant phenotypes, various permissive media supplemented with antimicrobial agents are used. The mutant bacterial strain can be resistant to the antibiotic and hence propagates, but the growth of wild type bacterial populations is inhibited.

Mutations affecting the biochemical properties of bacteria could be detected by their cultivation on minimal nutrient media containing a very limited number of carbohydrates or other essential substances. For instance, as the result of mutations the switching from prototrophic to auxotrophic type of nutrition may arise.