Degeneracy in the genetic code and its symmetries by base substitutions.

Degeneracy in the genetic code is known to minimise the deleterious effects of the most frequent base substitutions: transitions at the third base of codons are generally synonymous substitutions. Transversions that alter degeneracy were reported by Rumer. Here the other transversions are shown to leave invariant degeneracy when applied to the first base of codons. As a summary, degeneracy is considered with respect to all three types of base substitutions, the transitions and the two types of transversions. The symmetries of degeneracy by base substitutions are independent of the representation of the genetic code and discussed with respect to the quasi-universality of the genetic code.