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As used in genetics, a mutation that causes a sequence such that the reading frame groups of three bases in mRNA become out of register; the insertion or deletion of one or two bases, for example, would lead to an altered grouping of three bases causing incorrect amino acid residues to be incorporated into growing polypeptide chains, or would signal premature chain termination.
Farlex Partner Medical Dictionary © Farlex 2012
frameshiftan altered reading of the GENETIC CODE during TRANSLATION in PROTEIN SYNTHESIS, caused by a change in the nucleotide base sequence of DNA and RNA due to the insertion or deletion of one or more bases (except three bases or multiples of three bases). Fig. 165 shows an example of a DNA base insertion between bases 4' and 5'. When the sequence is read from left to right the first amino acid is unchanged, but the second and subsequent amino acids will be altered, producing a mutant protein.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005