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In double-stranded DNA, the molar amounts of adenine and thymine are equal, as are those of guanine and cytosine. Because purines (adenine and guanine) pair with pyrimidines (thymine and cytosine), the total purine content equals the total pyrimidine content; each group makes up half of the bases. The overall ratio of A + T to G + C varies between species, while base composition remains constant across tissues and age within a species. These relationships do not necessarily hold in single-stranded DNA, RNA, or some viruses. When measured DNA departs substantially from the expected equalities, possible explanations include single-stranded regions, sequencing errors, or contamination.