Was There Once an RNA World of Life?

John J Kuszewski johnk at jhunix.hcf.jhu.edu
Fri Aug 21 13:52:34 EST 1992


Loren I. Petrich writes:
>
>	This does not look like the result of spotaneous generation,
>unless one allows for extreme improbabilities, so it is presumed that
>there has been a lot of evolution behind it, evolution that may have
>left some vestiges, in the fashion of the numerous vestigial features
>generated by later evolution (I wonder if anyone has compiled a
>reasonably comprehensive list).

Interesting request.  How many molecular fossils can you name?  Here's
a start:

rRNA performs catalytic functions in ribosomes (several papers in Science
in the last few weeks)

tRNA CCA adding enzyme is the first telomerase (Nancy Maizels and Alan Weiner
in PNAS a few years ago)

self-splicing introns and the role of snRNAs in other intron splicing

NTPs as energy-storage media

other nucleoside-based cofactors (NAD, CoA, etc.)

mRNA

>
>	A system where there are two kinds of nucleic acids and where
>RNA was the original molecule, with DNA a specialization for long-term
>information storage. This is from several reasons, one being that it
>is RNA in transfer and ribosomal nucleic acids, when DNA could do just
>as well 
>

No, DNA couldn't do just as well.  The extra hydroxyl group is often used
in structural Hbonds or catalysis.

One point that several others have missed in their arguments about bubbles
on the seashore:  Membranes don't do any good without reasonably selective
holes in them.  Thus, membranes and membrane channels have to have evolved 
at the same time.  In fact, all high-energy synthesis reactions (in 
mitochondria, eubacteria, chloroplasts, etc.) work by setting up a pH
gradient across a membrane--another obvious molecular fossil.
Membranes' intimate involvement in so many synthetic paths, as well as
the obvious kinetic problems causes by having your genome and enzymes
dissolved throughout an entire pond, indicates that the evolved very 
early.  Any comments on this?  Anyone have any idea how to make a 
membrane channel out of RNA?


-- 
John Kuszewski
johnk at jhunix.hcf.jhu.edu
I'm not an idiot, but I play one on USENET



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