RNA vs. DNA

[aroger at ac.dal.ca]

In article <1993Apr22.142300.22900 at yang.earlham.edu>, derekr at yang.earlham.edu writes:
> Thanks to all who answered my question about RNA vs. DNA evolution.  As I
> suspected, RNA apparently evolved before DNA.  I will now go straighten my
> friend out.
> 
> -Derek
Before you go and straighten your friends out let us point out that there 
is little point in trying to explain an improbable situation (the primordial 
DNA world) with an equally improbable one (the RNA world). There have 
been numerous articles writen on the impossibility of spontaneously 
generating RNA (see anything by Joyce or Orgel); it is often thought that 
there would have to be a pre-RNA world perhaps based on a simpler 
backbone and purines only.
	Also there have been some really strange ideas portrayed as fact in 
this discussion which should be pointed out. First of all RNA has not really 
been shown to catalyse its own replication: the most exciting experiments 
(by Szostak) have shown ribozymes that ligate a set of oligos based on 
their complimentarity to a template. Orgel and Joyce have also done some 
work on non-enzymatic replication of poly C templates.
	Second it is not relevent to talk about RNA recognising amino acids 
as the actual reaction is catalysed by a protein which recognizes both the 
tRNA and the amino acid.
	There are convincing reasons to regard RNA as an earlier step than 
DNA in the evolution of genetic systems. For instance ribonucleotides play 
a fundamental role in metabolism as components of cofactors.  NAD, FAD 
and Coenzyme A all have ribonucleotide moieties.  ATP is the most 
general energy carrier in cells, GTP is centrally involved in protein 
synthesis, UTP is involved in the synthesis of polysaccharides and 
peptidoglycan.  In addition, RNA plays a central role in genetics.  It is 
involved in coding (mRNA), as a cofactor (tRNA) and as the catalyst 
(rRNAs) of protein synthesis.  All deoxyribonucleotides are derived from 
ribonucleotides and thymine from uracil.  
	All of these examples constitute circumstantial evidence for a pre-
DNA RNA world.  The latter is by no means proven and probably was 
only a step in the origin of a genetic system and not the first step.

Patrick Keeling
Andrew Roger