Looking at a tRNA I saw that there were
two keys spots where there are
always single strands
and always two pyrimidines.
(I'd seen the one at the acceptor stem before.
But just yesterday I discovered the 
one before the anticodon 

IF UV dimers were a major part of
selection in the genetic code (and 
nothing could base pair from
a UV caused dimer) 
THEN every tRNA has two pyrimidine sets
or 'dimer' positions.

1. The first is in the acceptor stem which
is always CCA in the 74,75,76 positions.
The two C's are pyrimidines

2 The other precedes the anitcodon in
positions 34,35,and 36:

Position #32 always has a pyrimidine 
(either C or U)
Position #33 always has a U

Thus it seems that the tRNA molecule is
folded such that it takes advantage of
the pyrimidine dimers to
a. keep the acceptor stem from base pairing
b. keep bases 32 and 33 from base pairing
which would close the anticodon loop.

BUT also notice this - how it effects wobble
in 3rd position of the anticodon #34

Because position 32 and 33 are both pyrimidines
There is chance for a dimer in those positions,
but if the last base in the anticodon #34, or
the wobble position, is a pyrimidine, there is
some danger of a dimer between
not only 32 and 33, but 33 and 34 too.

Perhaps that in some way explains
why there is wobble (dimers would
hurt position  3 in the anticodon
if it was a  pyrimidine) 

and
why the curve of the anticodon loop
is between position 33 and 34 -
to prevent the dimer problem if
34 was a pyrimidine too.
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 * RgateImp.MoonDog.BBS at 6/18/04 9:49:31 PM