Q. What is the numeric relation between purine and pyrimidine bases in the DNA molecule? Is that relation valid in the RNA molecules?
The DNA molecule is made of two bound polynucleotide chains that to form a helical structure (the double helix). The binding of the two chains is between their nitrogen-containing bases and it always obeys the following rules: a pyrimidine base, and guanine (G), adenine (A), a purine base, binds with thymine (T), a purine base, and binds to cytosine (C), a pyrimidine base. Therefore in one molecule of the DNA there will be the same number of adenine (A) and thymine (T) and same number of cytosine (C) and guanine (G). The quantities of purine and of pyrimidine bases then will also be the same in a 50% proportion for each type. The relation A = T and C = G, or A/T = C/G = 1, is called Chargaff's relation and the pairing rules described above are known as Chargaff's rules.
In the RNA there are not two nucleotide chains. The RNA is a simple chain molecule proportionality of nitrogen-containing bases to form it.