OK, so to recap: DNA is converted to RNA, which is used to build 
proteins, which do all the nifty stuff that makes life happen.

That's a fairly complicated structure. So one might legitimately ask, 
"how did life come into existence?" Presumably the first life forms were 
far simpler than anything that exists today. But can we say anything 
more definite than that?

One hypothesis has come to be known as the "RNA world". Now if, like me, 
you read a book which has a page or two about the inner workings of the 
living cell, you probably come away with the impression that DNA and RNA 
are like data tapes containing a computer program, and the cellular 
machinery reads this program and builds the chemicals it describes.

Dig deeper into molecular biology, however, and you discover an 
interesting thing: DNA and RNA are chemicals too. They have chemical 
reactions of their own. In particular, plain ordinary RNA with the 
correct sequences encoded within it can actually catalyse chemical 
reactions, just as proteins can.

A protein that catalyses something is called an enzyme. An RNA fragment 
that does the same is a ribozyme.

According to the "RNA world" hypothesis, RNA was the first biomolecule 
to come into existence. The first life forms were composed entirely of 
RNA. The RNA was both the genome and the catalyst. Later, DNA evolved as 
an improved genome storage molecule, and proteins evolved as an improved 
catalyst molecule. (Not necessarily in that order.) And the various cell 
functions which still use RNA to this day are remnants of the RNA world.

That's the hypothesis, anyway. The very first "life form" was just a 
strand of RNA which happened to catalyse its own duplication (probably 
not terribly accurately). Scientists have succeeded in constructing RNA 
strands which actually do this in the laboratory. (Admittedly their 
starting point came from a biological organism.)

Presumably once you have an RNA strand that can assemble duplicates of 
itself provided the right nucleotides are available, the next step is 
evolving ribozymes that synthesize more of those nucleotides from 
whatever precursors are available, speeding up the whole process. I'm 
not sure at what point this system would have acquired a "cell 
membrane", but such as the obvious advantage of concentrating the 
nucleotides synthesized next to the RNA strands that want to use them.

So what roles does RNA play today? Well, most obviously, every single 
piece of DNA to be turned into protein has to be turned into RNA first. 
Oh, and the thing that turns RNA into protein? It's made of protein and 
RNA. (And the "active site" itself is RNA. The protein components just 
hold it together.)

Not only that. Short strands of RNA glue themselves to the 22 different 
amino acids and facilitate the attachment of the correct amino acid to 
the growing protein chain.

So, in summary, the machinery for turning RNA into protein is itself 
built out of RNA, for the most part. You can imagine a world consisting 
only of RNA life, and one of them evolving the ability to string 
together two or three different amine groups to make the first simple 
proteins.

Proteins, you will remember work primarily because of their shape. 
Different amine groups attract or repel each other by a combination of 
different types of forces, resulting in a molecule resembling a tangled 
piece of string. The exact shape of these tangles tend to align other 
chemical molecules in a particular way, causing them to hit each other 
at just the right angle for a chemical reaction to happen - something 
which might be very rare if the protein weren't there. In other words, 
proteins catalyse reactions.

Recall also that DNA forms a double-helix, which each strand being the 
"mirror image" of the other, which facilitates repair. Each base on one 
strand has a matching base on the other strand, in the so-called 
"Crick-Watson pairs":

   Adenine pairs with Thymine.
   Guanine pairs with Cytosine.

RNA is very similar to DNA, except that it's a single spiral strand, 
rather than a double helix. The "backbone" chemical is slightly 
different, and Thymine is replaced by Uracil. (Thymine and Uracil, 
despite sounding totally different, are nearly identical molecules.)

In DNA, the Crick-Watson pairs hold the two mirror image strands 
together [the correct way around]. In RNA, these same base-pairs can 
cause the RNA to stick to itself, tangling up in complicated but 
predictable ways. This yields a tangled structure... rather like a 
protein. In other words, a catalyst.

RNA catalysts are not nearly as versatile as protein catalysts, and RNA 
is not nearly as reliable for gene storage as DNA. But it /can/ do both 
of these functions, to a limited but probably "good enough" degree. 
Hence, the RNA world hypothesis.

Of course, the fact that this scenario could /work/ doesn't mean that it 
/was/ what happened...

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Am 14.01.2011 16:57, schrieb Invisible:

> I'm
> not sure at what point this system would have acquired a "cell
> membrane", but such as the obvious advantage of concentrating the
> nucleotides synthesized next to the RNA strands that want to use them.

There's a theory that "cell membranes" would have existed even before 
self-replicating stuff: With the chemical composition of the atmosphere 
back then, rain drops might have been covered with a layer of naturally 
occurring molecules with both a hydrophobe and a hydrophile end, forming 
cells in which life might have started.

> So what roles does RNA play today? Well, most obviously, every single
> piece of DNA to be turned into protein has to be turned into RNA first.
> Oh, and the thing that turns RNA into protein? It's made of protein and
> RNA. (And the "active site" itself is RNA. The protein components just
> hold it together.)

AFAIK viruses, too, commonly use RNA.

Maybe the first life can be thought of as a bunch of different ribozymes 
entering a symbiosis.

In that sense, viruses would be the oldest type of life.

Post a reply to this message

>> I'm
>> not sure at what point this system would have acquired a "cell
>> membrane", but such as the obvious advantage of concentrating the
>> nucleotides synthesized next to the RNA strands that want to use them.
>
> There's a theory that "cell membranes" would have existed even before
> self-replicating stuff:

That sounds reasonably plausible also.

In fact, the only implausible thing seems to be thinking that we will 
ever truly know what actually happened. ;-)

>> So what roles does RNA play today?
>
> AFAIK viruses, too, commonly use RNA.

Yes.

In fact, human skin produces enzymes that chop up RNA, specifically as a 
defence against RNA viruses.

> Maybe the first life can be thought of as a bunch of different ribozymes
> entering a symbiosis.
>
> In that sense, viruses would be the oldest type of life.

Well, if you think about "life" as originating as a set of 
self-catalysing chemical reactions that eventually evolved into a 
complex system with a genome, then a virus (which is basically a genome 
with no copying machinery) would logically seem to pre-date more complex 
life.

Now discuss plasmids. ;-)

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