The response of amino acid frequencies to directional mutation pressure in mitochondrial genome sequences is related to the physical properties of the amino acids and to the structure of the genetic code.

Abstract excerpt

The frequencies of A, C, G, and T in mitochondrial DNA vary among species due to unequal rates of mutation between the bases. The frequencies of bases at fourfold degenerate sites respond directly to mutation pressure. At first and second positions, selection reduces the degree of frequency variation. Using a simple evolutionary model, we show that first position sites are less constrained by selection than...

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