Nucleic acids are polymers of nucleotides. The two types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Nucleotide serves as the building block of nucleic acid.
The nitrogenous base molecule is joined to the sugar molecule by a glycosidic bond and forms a nucleoside structure. The nucleoside combines with a phosphate group by an ester bond to create a nucleotide.
There are four different bases found in DNA: adenine (A), guanine (G), thymine (T) and cytosine (C). RNA also contains adenine, guanine and cytosine, but instead of thymine, it has uracil (U). Adenine and guanine are double-ring bases called purines. Cytosine, thymine and uracil are single-ring bases called pyrimidines.
In DNA, the base pairing is specific. Adenine always pairs with thymine, and guanine always pairs with cytosine. Thus, all base pairs consist of one purine and one pyrimidine. Once the sequences of bases in one strand of a DNA double helix is known, the sequence of bases in the other strand is also known because of the specific base pairing.
Chargaff's rules state that DNA from any cell of any organism should have a 1:1 stoichiometric ratio of pyrimidine and purine bases. Specifically, the amount of guanine should be equal to cytosine, and the amount of adenine should be equal to thymine.
Thus, the ratio of A+G/T+C is not specific to any particular species.