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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). The ability to form
hydrogen bonds makes the base pairs
more stable
structurally.
These base pair
relationships are often called Chargaff's
rules of DNA base pairing, named after his
observations that there are equal molar concentration of
A & T, as well as G & C in most DNA molecules.
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 (Chargaff's rule).
Similarly, whatever the amount of guanine (G), the
amount of cytosine (C) is the same.
The C+G
: A+T ratio varies from
organism to organism among the prokaryotes), but within
(particularly the limits of experimental error), A = T and C = G.
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