<P> The second rule holds that both% A ~% T and% G ~% C are valid for each of the two DNA strands . This describes only a global feature of the base composition in a single DNA strand . </P> <P> The second of Chargaff's rules (or "Chargaff's second parity rule") is that the composition of DNA varies from one species to another; in particular in the relative amounts of A, G, T, and C bases . Such evidence of molecular diversity, which had been presumed absent from DNA, made DNA a more credible candidate for the genetic material than protein . </P> <P> In 2006, it was shown that this rule applies to four of the five types of double stranded genomes; specifically it applies to the eukaryotic chromosomes, the bacterial chromosomes, the double stranded DNA viral genomes, and the archaeal chromosomes . It does not apply to organellar genomes (mitochondria and plastids) smaller than ~ 20 - 30 kbp, nor does it apply to single stranded DNA (viral) genomes or any type of RNA genome . The basis for this rule is still under investigation, although genome size may play a role . </P> <P> The rule itself has consequences . In most bacterial genomes (which are generally 80 - 90% coding) genes are arranged in such a fashion that approximately 50% of the coding sequence lies on either strand . Wacław Szybalski, in the 1960s, showed that in bacteriophage coding sequences purines (A and G) exceed pyrimidines (C and T). This rule has since been confirmed in other organisms and should probably be now termed "Szybalski's rule". While Szybalski's rule generally holds, exceptions are known to exist . The biological basis for Szybalski's rule, like Chargaff's, is not yet known . </P>

An organism with dna containing 30 adenine nucleotides must have