Here's some quotes from "The RNA World" from Strickberger's
textbook, Evolution and some comments by me:
1." because it is difficult to conceive how informational nucleotide sequences could have evolved
simultaneously with the polypeptide sequences necessary to replicate them, researchers place
emphasis on discoveries that some RNAs pssess catalytic activity that might have enabled self
replication."
Two comments - IMO nucleotide sequences were chosen for thermal stability in a heat cycle that
denatured strands completely in high heat and then annealed variants in low. Each go round gave a
selective advantage to certain sequences
- GC over AU over mistmatched bases. And the most stable sequence did not completely denature but
only partially. Thus it was selected over those that completely denatured - etc. Also the most
thermally stable fold from the sequence was selected. Look at tRNA today - it is built to
withstand denaturing. The other comment is this - don't look for a SELF replicator. Look for a
(more likely) environmentally forced replicator (the heat that denatures the nucleotide strands).
This could have gone on for thousands of years thus giving time for the SELF replicator to emerge.
2"...Joyce and Orgel suggest that the minimum length for RNA catalytic activity is possibly "a
triple stem-loop containing 40-60 nucleotides. Such an RNA replicase could hardly have arisen by
chance..."
But it could come from thermal selection over thousands of years. This shifts the meaning of life as
not that which feeds and breeds, (there must have been some reason for chemicals to react to the
environment through those evolved strategies) but that which survives in a heat cycle by strategies
that include feeding and breeding.
3."As Moore puts it, "Why would a device for making polypeptides evolve in an organism that had no
use for protein?"
Moore is correct IMO. What we need to find is symbiosis between amino acids and tRNA that in someway
would help the survival of both.
4."Ribas de Pouplana and coworkers show that an amino acid activating enzyme used in protein
synthesis was likely preceded during evolution by a transfer RNA."
I agree and somewhere there's a symbiosis between tRNA and amino acids that supports making
peptide bonds out of the aa's, and perhaps helps tRNA by keeping it from denaturing. I tend to
think its some type of h-bond that connects the two. And that it did this first at the acceptor
stem of the tRNA.
Comment? Tom Hendricks