A key part of the Watson-Crick model came when Watson realized that adenine could form hydrogen bonds with thymine and guanine could form hydrogen bonds with cytosine. This explains why A = T and G = C in Chargaff's rules. Also, these two hydrogen-bonded nucleotide pairs had the exact same width, so they could form the rungs of the DNA ladder. The fact that these pairs could match up only in this way meant that the sequence of bases in one strand could determine the sequence of bases in a second strand created from the first. The second strand is said to be complementary to the first strand. Individual bases are paired so that the identity of any base determines the identity of the base paired with it; that is, the complementary base.

This table lists the base abbreviations for bases in a sample of single-stranded DNA. Fill in the second column with the base abbreviations that are complementary to the given bases.

A
T
T
C
A
G
G
C
T
A