No life form is a perfect machine, and from generation to generation, a genome may be subject to change – mutation. A mutation is any newly arisen, stably inherited alteration of a life form's genome. A mutation is not a transitory change, and it is not the same as damage, although damage to DNA may lead to mutation. A mutation may not impact the functioning or even the structure of gene products, but if it does, the effects can range from beneficial to fully lethal.
Micromutations are small, affecting at most a single gene and usually involving fewer than ~103 nucleotides. In this chapter, a classification of micromutations is followed by descriptions of their causes and effects. Large mutations – chromosomal abnormalities – are sufficiently different from micromutations that they are taken up later (Chapter 21).
Types of Micromutation
The five kinds of small mutation are substitution, deletion, insertion, duplication, and inversion. Most mutations are point mutations (single nucleotide change), and the most frequent kind of point mutation is a nucleotide substitution, also commonly called a base substitution. Micromutations can occur anywhere in a chromosome.
Substitution is nucleotide replacement (Figure 15.1). There are two types of nucleotide substitution, transition and transversion. A transition is substitution of a purine nucleotide for a purine nucleotide or substitution of a pyrimidine nucleotide for a pyrimidine nucleotide. A transversion is a substitution of a purine nucleotide for a pyrimidine nucleotide, or vice versa. Substitutions occur throughout chromosomes.