<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> <P> The combined effect of Chargaff's second rule and Szybalski's rule can be seen in bacterial genomes where the coding sequences are not equally distributed . The genetic code has 64 codons of which 3 function as termination codons: there are only 20 amino acids normally present in proteins . (There are two uncommon amino acids--selenocysteine and pyrrolysine--found in a limited number of proteins and encoded by the stop codons--TGA and TAG respectively .) The mismatch between the number of codons and amino acids allows several codons to code for a single amino acid - such codons normally differ only at the third codon base position . </P> <P> Multivariate statistical analysis of codon use within genomes with unequal quantities of coding sequences on the two strands has shown that codon use in the third position depends on the strand on which the gene is located . This seems likely to be the result of Szybalski's and Chargaff's rules . Because of the asymmetry in pyrimidine and purine use in coding sequences, the strand with the greater coding content will tend to have the greater number of purine bases (Szybalski's rule). Because the number of purine bases will, to a very good approximation, equal the number of their complementary pyrimidines within the same strand and, because the coding sequences occupy 80 - 90% of the strand, there appears to be (1) a selective pressure on the third base to minimize the number of purine bases in the strand with the greater coding content; and (2) that this pressure is proportional to the mismatch in the length of the coding sequences between the two strands . </P>

Ratio of a to t and g to c in dna