There is such a strong connection between UV and
nucleotides, that I can't help but think that UV was
instrumental in the origin of life.
(this based on info from Intro to Research in UV
Photobiology, John Jagger)
1. all bases of nucleic acids are good UV absorbers.
2. but paired bases are less likely to absorb UV "The bases
of nucleic acids are often so close together that they
'stick' in parallel planes, permitting overlap of
electron orbits, which leads to decreased absorption
(hypochromic effect)".
3. 'phosphate or sugar moieties... are poor UV
absorbers.... One does not expect chain breakage"
4. 'Pyrimidines are roughly ten times as sensitive to UV as
the purines. Therefore the only important targets for
many effects on nucleic acids are thymine and cytosine
in DNA, and uracil and cytosine in RNA" (This suggests
why I think there may have been a purine world (A,G)
that preceded the RNA world, There may not have been
need for Watson Crick type base pairing at this very
very early stage.)
5. Absorption spectra at pH 7 of purines and pyrimidines
shows a strange difference between the two purines that
may be important. Adenine, like a single mountain shape,
peaks at 260 nm, but Guanine is very different and has
two peaks - one to each side of Adenine. The largest at
240 nm, and the other at 280. Chemical selection from UV
would behave differently on each of these.
6. "Only pyrimidines adjacent in a chain are likely to
dimerize..." Thus I would think that coding would be
chemically selected that didn't have adjacent
pyrimidines.
All this would suggest to me that what would be chemically
selected is:
7. nucleotides that are paired, base stacking
8. phosphate and sugar are not absorbers and would be
little effected by UV.
9. Adenine and Guanine, the purines are 10 x less likely to
be damaged than C,T, U the pyrimidines, and would be
more prevalent.
10. The two purines (A,G) have vastly different
absorption spectra.
11. Adjacent pyrimidines (T,C,U) would not be chemically
selected for
What seems to best survive UV also seems to represent what
is most life-like. I don't think that is an accident.
Also I note this line: "Hydration and dimer formation by far
UV in pyrimidines are of great interest, because their
production at low doses and their REVERSIBILITY apply to
pyrimidines in some polynucleotides and may therefore also
apply to DNA and RNA."
Does that mean dimer destruction is reversible? If so does
anyone know anything about that
Comment on any of the above?