Could you back that up with some studies or
> references, or specific arguments?

TT
The best explanation I'm aware of regarding why RNA is not a sensible
candidate for the first replicator is in the first part of:

Genetic takeover - and the mineral origins of life, A. G. Cairns-Smith, 
Cambridge University Press, 1982;

However much the same sentiment has been expressed by others - e.g.:

``But where did RNA come from? To date, no one has been able to form RNA
  under in the laboratory under conditions that mimic those believed to
  have existed on primitive Earth. Some scientists also question whether
  nucleic acids with a backbone of ribose, or any other sugar molecule,
  would be stable enough to survive the harsh conditions of early Earth.

TH
I don't think stable is the best word here. It would be more likely that
whatever started life was ENHANCED by the harsh conditions.
That puts a new spin on things. And you look at UV for instance in other ways.
Did you see my post on the study suggesting how RNA would profit by UV.
And note its high absorbency - that can't be a coincidence.

The making of RNA is as you say, a missing component.
But I don't see how anything else could be complicated enough to become the
first ribozyme.
So in essence you have to have something along those lines before you have the
necessary components to move toward life.

TT
  So it is generally agreed that RNA must have evolved from an earlier
  form. While TNA is a good candidate, other polymers that exhibit
  self-replication and base pairing could have evolved into RNA.
  Pyranosyl RNA (p-RNA) and peptide nucleic acid (PNA) are two of
  these alternatives. [...]''

TH
I don't know that I agree. Or that it is generally agreed to. These ideas are
interesting - but they suggest more time needed, and some reason for them to
keep changing (Thus suggesting they aren't working as is and must continually
be fixed.)
All that requires time. And the window for the origin is amazingly short.

In my scenario I suggest there is simply some missing step that once
discovered, will produce RNA , pretty much as it is now - that is something in
UV or the sun heat. wet-dry cycle.

As to stability - I think the high heat of the cycle, and its wet, dry aspects,
would favor those chemicals that could fold, h-bond, and ultimately w-c
h-bonds.  In other words it was this chemical adaptation to harsh conditions
that made RNA - not prevented RNA.

Note this
'Two areas remain problematic. First,

directed polymerization is compatible with diverse chemical structures; it is
not known why nucleic acids like DNA (Watson and Crick, 1953) took on a central
role.'

- this to me suggests that RNA had some property that
made it the leading candidate and that it did not go
through stages to get there.

" Second,

in any duplex, the new complementary strand blocks further polymerization
(Blum, 1957; Szathmary and Gladkih, 1989). "

Which supports my idea that there was replication of
single unfolded strands - not duplexes: that means a single folded RNA
ribozyme, read and copied a single non folded RNA - in its unfolded annealed
stage of the heat cycle. And the most thermally stable RNA would also be the
most prevalent RNA - so whatever was read was thermally stable.

The quotes above from,
Icarus 168 (2004) 18-22

www.elsevier.com/locate/icarus

Fast tidal cycling and the origin of life

TT
 - http://nai.arc.nasa.gov/news_stories/news_detail.cfm?ID=189

RNA seems /particularly/ unlikely as the first replicator - since
its replication is blocked by isomeric impurities in its bases -
as described by J. Maynard Smith on p. 73 of "The Major Transitions
in Evolution".


TH
I'd like to know more about this - a web page anywhere
on this that you know of?
-- 
__________
 |im |yler  http://timtyler.org/  tim{at}tt1lock.org  Remove lock to reply.

 >>
---
þ RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info{at}bbsworld.com

---
 * RIMEGate(tm)V10.2áÿ* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 8/4/04 12:34:44 PM