In DNA, the amount of adenine equals the amount of thymine. This relationship is known as part of Chargaff’s rules and follows from the standard base-pairing pattern of double-stranded DNA: adenine pairs with thymine, while guanine pairs with cytosine.
Adenine and thymine are held together by two hydrogen bonds. Guanine and cytosine form a stronger three-hydrogen-bond pair. Because every adenine on one strand is matched by a thymine on the opposite strand, the total quantities of A and T are equal in a typical double-stranded DNA molecule; likewise, G equals C.
The rule does not mean that every species has the same overall percentages of the four bases. Organisms can differ greatly in their AT-to-GC balance. For example, a genome may be relatively AT-rich or GC-rich while still maintaining approximately equal amounts of adenine and thymine across its complete double-stranded DNA.
Erwin Chargaff measured DNA base compositions in the late 1940s and early 1950s. His results challenged the idea that DNA was simply a repetitive molecule and helped Watson and Crick develop the double-helix model. Uracil is the comparable pairing base in RNA, not ordinary DNA.