"TomHendricks474"  wrote in message
news:ce4oha$1mk3$1{at}darwin.ediacara.org...
> My suggestion that dimer impact was important
> in setting up the genetic code would suggest that
> there must be a lot of UV.
> But I also note that if the following is true, the
> excessive UV may well have also made the RNA
> and bases the centerpiece of life to begin with.
> Thus everything seems to be beginning to fit with
> UV the power source and selective force for much of it.
>
> http://www.newscientist.com/news/news.jsp?id=ns99993778
> The World's No.1 Science & Technology News Service
> UV light may have sparked life on Earth
>
> Ultra-violet light, long thought to be an impediment to the early formation of
> long organic molecules, may in fact hold the key to the origin of life,
> according to a new study.
> (snipped)
>
> Life on Earth is thought to have evolved about 3.7 billion years ago, when
> there was no protective ozone layer encasing the planet and UV radiation was
> 100 times more intense than today.
>
> The nucleotides that make up RNA have three components - a sugar, a phosphate
> and nitrogen-containing base. "And these bases have very peculiar
properties of
> being extremely efficient at quenching UV light," says
Mulkidjanian, protecting
> the sugar and phosphate components which form the spine of the chain.
>
> The team fed data on the photochemistry of various organic molecules into a
> computer model designed to simulate the effects of UV light on
stability. "The
> effect was very pronounced in RNA," he says. In the presence of strong UV
> light, RNA was much more likely to form long chains than other molecules.
>
> Comment?

It is an interesting paper.  You realize, of course, Tom, that while
it supports your ideas regarding uv as a force for chemical selection
for RNA, it tends to contradict your ideas regarding a purine world and
regarding the significance of pyrimidine dimers.

My major criticism of this paper would be that the thermodynamics of
the polymerization reaction are treated unrealistically.  As a second
criticism, I would point out that this was computer modeling, not a
chemical experiment.  It excluded any possibility that the quenching
of the depolymerization reaction might be accompanied by an enhancement
of the cleavage of the bonds between bases and backbones.

Still, it is an interesting idea.  It would be fascinating to see the
results of a real experiment in which polymerization and base-backbone bond
formation are favored by dryness.  Then, perhaps as a result of periodic
rainstorms, the RNA polymers could be transfered to an aquaeous medium,
where they might otherwise be unlikely to appear.
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