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

Olav Hungnes ohungnes at
Tue Apr 12 09:51:54 EST 1994

Zhongguo Xiong (zxiong at wrote:
: Dear Colleagues:

: Being not familiar with molecular evolution. I have been troubled with some of 
: my data in RNA virus sequence. We are working on a small RNA virus and nearly 
: complete the sequence of the viral RNA genome from cDNA clones. RNA viruses 
: are known to be heterogenous (quasi-species), so it was not surprising to see 
: nucleotide sequence variations when sequences are obtained from different 
: clones. 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.

In RNA, U is able to base pair with G to some extent, as well as to A. See?
T-C substitutions would appear on the complementary strand.

: Let me try to explain it a little better. We have found 16 nucleotide 
: substitutions in about 1500 nucleotide of overlapping sequences. There are 11 
: C to T or T to C substitutions and 5 G to A or A to G substitutions. We have 
: not found any other possible substituions.

: 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.

: Any comments or suggestions are welcome.

: Xiong
: Zxiong at

Olav Hungnes                     ohungnes at
National Institute               Phone  (+47)22042200
of Public Health                 FAX    (+47)22353605

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