About a symmetry of the genetic code.

Considering the three codonic positions as independent, it is possible to explain the grouping of the 64 trinucleotides (without AAA, TTT, CCC and GGG) into three equal sets T0, T1 and T2 according to their preferable reading frame (0, 1 or 2) in coding sequences. Supposing that the two complementarity strands of DNA are coding, it is demonstrated that the complementary of a codon is classified in to the same set, which has been observed statistically (with a few exceptions) in the coding sequences by Arquès & Michel [(1996) J. theor. Biol. 182, 45-58] and Arquès et al. [(1997) J. theor. Biol. 185, 241-253]. Finally, the circular property of the code pointed out by these authors is demonstrated, and a direct consequence to biological considerations of these properties is discussed. Copyright 1997 Academic Press Limited.