Fundamentals of Microbiology

3.4.2 Mutation

Mutation is an inherited change in the base sequence of the nucleic acid comprising the genome of an organism. A strain carrying such a change is called a mutant. A mutant may differ from its parent strain in genotype (sequence of nucleotides in the DNA of the genome) and sometimes in phenotype (observable properties from its parent) also. A nutritional mutant that has a requirement for a growth factor is called an auxotroph and the wild-type parent from which the auxotroph was derived is called a prototroph.

Mutation can be either spontaneous or induced. Spontaneous mutation occurs naturally (natural radiation or due to error in pairing of bases during replication). Mutation involving one or a very few base pairs are referred to as point mutations. Mutation involving change in base pairs without causing change in the amino acid that code for is called silent mutation. (For eg. Change in UAC to UAU would not account for change as both code for tyrosine). Mutation involving change in base pair which codes for a different amino acid is called missense mutation (UAC - Tyrosin; AAC– asparagine). Some times a mutation may result in premature termination of translation (as the base pair alteration contribute to stop codon TAG ® UAG (stop codon) resulting in incomplete protein – such is called non-sense mutation.

Agents that induce mutations are called mutagens which may be chemical or physical agents. Eg. chemical mutagens – Nitrous acid (HNO3), Hydroxylamine (NH₂OH), alkylating agents. Physical mutagens – UV and ionizing radiation (x-rays)