Frameshift Mutation

The genetic code is made up of ‘letters’ called bases. Every three bases makes up a ‘word’ called a codon which codes for an amino acid. So during translation, codons are read by cellular ‘machines’ to make a chain of amino acids (a polypeptide, later to become a protein).

A frameshift is a very radical type of mutation in that alters how the codons are read (i.e. it alters the reading frame). If a base is added to a gene, all codons thereafter will be changed. This is because the added base forces all following bases to move one place forward. So the end base of a codon will become the start base of another codon.

This is the DNA sequence before a frameshift.

This is the DNA sequence before a frameshift.

The sequence after a frameshift (by a single base insertion).

The sequence after a frameshift (by a single base insertion). Notice how every base after the insertion (the red ‘A’) is moved one place to the right. This produces codons to be read differently with changed amino acids resulting.

Three bases are inserted. Notice how the reading frame is not changed. Because of this, the change is not a frameshift.

Three bases are inserted. Notice how the reading frame is not changed. Because of this, the change is not a frameshift.

Frameshifts and Nylonase

Frameshifts can produce a number of stop codons, and especially so when the sequence of bases is long. This fact shows why the nylonase adaption in bacteria did not arise from a frameshift — the gene lacks stop codons in 1,535 bases.[1]

Reference

  1. Yomo, T., Urabe, I. and Okada, H., No stop codons in the antisense strands of the genes for nylon oligomer degradation, Proceedings of the National Academy of Sciences USA89:3780–3784, 1992.
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