Questions: G to A and C to T substitutions, is this a rule ?

Chris Noren noren at
Tue Apr 19 16:54:01 EST 1994

In article <zxiong.48.000BFC6A at>,
zxiong at (Zhongguo Xiong) wrote:

> What was surprising was a consistent rule of sequence variaions. G is 
> always subsituted with a A, or vice versa. C is always substituted with a T, 
> or vice versa. But there is never a G to (C, T) change or vice versa.

> Is there a theory describing the rule  of nucleotide substitution during 
> evolution? I feel very ignorant and hope someone can give me a pointer to how 
> to explain my observation.

There is a chemical explanation. Both C and A are known to spontaneously
deaminate (at very low frequency) to give U and I (inosine), respectively,
which replicate as T and G. Normally there are repair enzymes (e.g. ung
protein uracil-DNA glycosylase) that correct the resulting mismatches, but
if replication precedes repair (as might be expected during viral
replication), the C and A would be effectvely substituted with T and G, as
you observed.

Christopher Noren
New England Biolabs
(800)632-5227 ext 261
noren at

"Once in a while you get shown the light
In the strangest of places if you look at it right"

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