Hemoglobin and the genetic code. Evolution of protection against somatic mutation.

One-half of the twenty amino acids of the genetic code are just one mutational step away from the chain-terminator codons UAA, UAG, and UGA. It is postulated that somatic mutation to terminator is a hazard to which the organism has and to respond by adjusting certain proteins in the direction of fewer mutable residues. This view is supported by calculations based on the primary structure of five of the human hemoglobin chains. Each chain is scored for mutability to terminator in accord with the numbers and kinds of amino acids present. Among the adult chains, the most essential one, the alpha, has lowest mutability. The beta and delta follow, and in order of the presumed harm to the organism of a shortage of chain copies. Ante-natal chains tend to have higher mutabilities, supporting the view that cumulative mutational change in DNA can do little if the gene ceases to transcribe early in life. Two other predicitons based on the supposition of effective selection against mutability to terminator are also met: chain length of polypeptides is negatively correlated with their scores for mutability to terminator, and examination of the recently determined sequence of beta messenger RNA shows preferential use of codons that are not readily mutable to terminator.