This is consistent with there not being enough space (20 Å) for
two purines to fit within the helix and too much space for two
pyrimidines to get close enough to each other to form hydrogen
bonds between them. But why not A with C and G with T? The
answer: only with A & T and with C & G are there opportunities
to establish hydrogen bonds (shown here as dotted lines) between
them (two between A & T; three between C & G). These
relationships are often called the rules of Watson-Crick base
pairing, named after the two scientists who discovered their
structural basis. The rules of base pairing tell us that if we
can "read" the sequence of nucleotides on one strand of DNA, we
can immediately deduce the complementary sequence on the other
strand. The rules of base pairing explain the phenomenon that
whatever the amount of adenine (A) in the DNA of an organism,
the amount of thymine (T) is the same (called Chargaff's rule).
Similarly, whatever the amount of guanine (G), the amount of
cytosine (C) is the same. |Relat