Mutation

DNA is composed of a double helix, each side of which is a long string of four types of nucleotides. Each nucleotide possesses identical sugar-phosphate groups that contribute to the DNA backbone but differs in the structure of the base suspended between the two backbones. The bases are adenine, thymine, cytosine, and guanine (A, T, C, G). Because of their structure, A pairs only with T across the double helix, and C only with G.

Within genes, the sequence of DNA encodes a sequence of amino acids used to build a protein. The DNA is read in triplets of bases, with each triplet coding for an amino acid. With the recognition that the genetic information lies in the sequence of bases in the DNA, it became possible to understand the chemical nature of gene mutations and how these could be as stable as the original allele of the gene.

Consideration of the genetic code linking DNA and amino acids reveals how mutations can either alter a protein, have no effect, or prevent it from being produced entirely. Mutations fall into four broad categories (point mutations, structural chromosomal aberrations, numerical chromosomal Figure 2. Frameshift mutations are deletions or insertions of bases in the DNA. Unless a multiple of three is involved in the deletion, all of the subsequent codons are altered, as illustrated by the deletion of T above. Hence, different amino acids will appear in the mutant protein, and its function will be destroyed. aberrations, and transposon-induced mutations), each of which may be subdivided further.