Talmudic Question #38

Not a novel question, but nevertheless…what’s the current thinking as to why early RNA life went the way of making double-stranded DNA instead of double-stranded RNA? (Yes, dsRNA is extra stable, but there are ways around that.)

Comments

[Rhea Miller]

Hmmmmm...never thought about it that way Paul...I like it. I suppose it's good organizational skills to differentiate and compartmentalize eh?

[Tim Sampson]

There is a theory that Patrick Forterre has put forth regarding exactly this question.

He proposes that during the RNA World, certain viruses evolved DNA in order to evade host defense. Subsequent parasitation of DNA viruses into RNA hosts could potentially have allowed the utilization of DNA by cellular entities. Especially when we look at the consistancy of recombinational events swapping genetic material between virus and host, and vice versa.

Science had a news review on this theory: Science 12 May 2006: V312, no5775, pp870-872
http://www.sciencemag.org/cgi/content/summary/312/5775/870

:-)

[stan zahler]

I suppose it's always useful to remind ourselves that the RNA World is a useful hypothesis, not (quite) a known fact.

stan zahler

[Paul Orwin]

I have no idea if this is the current thinking, but here's my thought. Assume that the "RNA world" existed, with self replicating ribozymes as the initiators, and presumably there was also some parallel (possibly earlier or later) development of compartments (membranes) which I'll ignore :). A subsequent event would seem to me to be the establishment of a template/effector separation (i.e. the effector function in one molecule, the template for making it in another). This could well have been a dsRNA at first, but it seems to me that evolution would favor the formation of a more distinct template (DNA) on the grounds that this would make the replicator more robust. How? I think possibly by further segregating effector function (ribozyme activity) from template structure.

Just as an example, a ribozyme that has RNAse activity might be more likely to cleave it's own dsRNA template, but not it's dsDNA template. Thus, at a population level, the protocells with the dsDNA template might be more robust, because this sort of mistake would be less likely.

An even cooler idea! maybe dsDNA evolved as a defense mechanism against RNA digesting ribozymes! I like that...

Fun stuff, as always!