question: What states the second parity rule (PR2)?
answer: The second parity rule (PR2), also known as Chargaff' s second parity rule, is an intra-strand rule which states that, when there are no biases between the two complementary strands of DNA in mutation and selection rates (substitution rates), complementary nucleotides are expected to have almost equal frequencies within single stranded DNA, namely A = T and G = C at equilibrium, without regard to the G + C content of the DNA.