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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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