To: All Msg #69, Apr2093 10:25AM Subject: abiogenesis questions from J. Thomas In article
From: Chris Colby
To: All Msg #69, Apr-20-93 10:25AM
Subject: abiogenesis questions from J. Thomas
Organization: animal -- coelomate -- deuterostome
From: email@example.com (Chris Colby)
In article <1qbo86$cbs@usenet.INS.CWRU.Edu> bk186@cleveland.Freenet.Edu
(Julie Thomas) writes:
>1. Remember the commercial "how many licks does it take to get to
>the center of a Tootsie Pop?". Well, how many nucleotides
>does it take to make a self-replicator?
The L19 RNA from self-splicing Tetrahymena ribosomal RNA is 395
nucleotides long and has polymerase function (as well as a
nuclease function). So, one specific RNA sequence 395 nucleotides
long can polymerize _something_ (I don't know what the fidelity of
it is.) There are probably many other lengths and permutations of
specific nucleotides that yeild some polymerase function. I'm not
sure how much work has been done in this realm. I know some
researchers have used an artificial selection system to start with
random nucleotide sequences and end up with molecules of specific
catalytic functions. There have been papers in Nature (and I think
Science) on this in the past couple of years.
Are you trying to argue that since we don't know the exact structure
of the initial self-replicator, we can't hypothesize that _a_ initial
self-replicating structure existed. I'm not sure if I follow your
logic, here. We don't know all the details of abiogenesis (far from
it). Does that mean we can't produce hypotheses regarding it?
(I haven't been following this thread very closely -- if you have
answered this, sorry.)
>2. Do you have any evidence that a replicator homogenous for stereoisomers
>has a "reproductive advantage"? Or how was it that you put it? Oh yeah,
>"or are you just making this up"?
>3. What "reproductive advantage" does an L-peptide have over a R-peptide?
I have a hypothesis about how proteins ended up homochiral. (I don't
read the literature, so this probably has been thought of. It seems
sort of obvious to me that this might be one way to get from heterochiral
"soup" to homochiral proteins.)
There is a reasonable amount of evidence that nucleic acid replication
preceeded the incorporation of proteins into an organisms metabolism.
What if the first replicator to utilize amino acids, made chains
(protiens) from a single amino acid of a specific chirality. (Lets
say L - alanine to pick one at random). Now, over time, the organism
mutated and was able to add other amino-acids to it's poly-L-alanine
protein. Whatever machinery it had was able to add L-alanine to
L- alanine. Any modification may have been enough to allow incorp-
oration of other L- amino acids, but not D- amino acids(*). Thus,
protiens ended up homochiral.
(*) I think that the amino-acyl tRNA synthetases (hope that is
what they are called) in current organisms are highly stereo-
specific. But, you can get mutations that put the "wrong" L-
amino acid onto a tRNA -- and hence get it incorporated into
a protein. Somebody correct me if I'm screwing the pooch here.
I really don't know that much about molecular biology.
This is a highly speculative hypothesis, but I would think it
would be possible to gather evidence for or against it. (And as
I said before, this might not be an original idea.) So, here is
one way to get from a mix of L- and D- amino acids to homo-
chiral proteins. Notice that there doesn't have to be any
specific advantage of L- over D-; proteins just ended up that
way (if my hypothesis is close enough) do to 'historical
Chris Colby --- email: firstname.lastname@example.org ---
"'My boy,' he said, 'you are descended from a long line of determined,
resourceful, microscopic tadpoles--champions every one.'"
--Kurt Vonnegut from "Galapagos"
E-Mail Fredric L. Rice / The Skeptic Tank