  Chargaff's second parity rule (CSPR) asserts that the frequencies of short
polynucleotide chains are the same as those of the complementary reversed
chains. Up to now, this hypothesis has only been observed empirically and there
is currently no explanation for its presence in DNA strands. Here we argue that
CSPR is a probabilistic consequence of the reverse complementarity between
paired strands, because the Gibbs distribution associated with the chemical
energy between the bonds satisfies CSPR. We develop a statistical test to study
the validity of CSPR under the Gibbsian assumption and we apply it to a large
set of bacterial genomes taken from the GenBank repository.
