Frameshift Mutations and the Genetic Code

The genetic message is translated sequentially, three bases at a time, starting from a defined point at the beginning of a cistron (Crick et al., 1961; Terzaghi et al., 1966). The message can thus be separated into groups of three bases (codons) by a "reading frame" which is set in register at the beginning of the message (Crick et al., 1961). Deletions or insertions of a base (or bases) into the genome cause a shift in the reading frame and are thus called frameshift mutations; they result in a grossly different translation of the message beyond the site of the mutation. If the deletion of a base is followed by the insertion of a base, the reading frame, and the amino acid sequence of the protein product, might be altered only in the region between the mutations. Such double mutant strains may produce active protein and thus be pseudowild.