IF as I suggest the code is basically a split
the more pyrimidine in the codon = the more phobic the more
purine = philic
Then dimers may be important. (that is the destruction of
adjacent base pairs on a single strand by UV)
The major difference between a codon of purine and one of
pyrimidine is that IF the codon with pyrimidine had two
pyrimidines together on the strand, they would be likely to
have UV damage through a dimer. That would be if the 3 base
codon was: pyr, pyr , pur or pur, pyr, pyr or pyr, pyr, pyr.
(pyr = U/C, pur = A/G)
Thus it is unlikely to think that pyrimidines would code
outside of the safety of water in a wet dry heat cycle.
These codons would be damaged by UV if they were on the
anticodon position, or the 3 base long acceptor stem
position on the 3' end. Those other bases would be base
paired and probably safer from UV damage.
Could it be that the split between pyrimidine phobic, and
purine philic, is that pyrimidine in the sun = UV damage
through dimers, and would not survive?
Thus we are left with two groups -
purine codons in sun or water pyrimidine codons only in
water.
Now to go a bit further. You would see that most coding with
pyrimidines would be destroyed by dimers (any pyr, pyr, pur,
or pur pyr pyr or pyr, pyr, pyr combination). While most
coding with purines would not be.
That suggests that most coding will be for purine amino
acids or philic amino acids. Thus first you would have a
mostly A/G or purine world that would well interact with
water and not be destroyed by dimers when it is subjected to
UV (during that part of the heat cycle when it is outside of
water in the sun/hot/dry part.
It also suggests that the phobic/pyrimidine coded amino
acids came second, or less often. (Perhaps they would bring
in the membrane/cell world to prebiotic life)
The phobic/philic split is not exact. Yet it probably
started out inexact with only two results, most amino acids
would be water loving, some few would be water hating.
Comment?