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.
I