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Read any introductory text on the subject of DNA/RNA and a simple
picture emerges:
RNA has one strand, while DNA has two strands which are inverses of each
other. Either way, each strand consists of four kinds of molecule
chained together, representing four possible "letters". Three "letters"
make a codon, which selects an animo acid. A gene describes how to make
a protien; just read the codons and they tell you what animo acids to
chain together, and there's your protien.
If you're lucky, you'll even get a little table listing all 4^3 possible
codons and which of the 20 possible amino acids each one represents.
(And you'll notice that the 3rd letter often makes no difference!) More
to the point, there are a couple of "stop" codons which mark the end of
the protien sequence.
That all sounds pretty simple, but back up a sec: This table can exist
because EVERY ORGANISM KNOWN TO SCIENCE uses exactly the same 20 amino
acids, *and* the same DNA/RNA molecules with the same four letters,
*and* encodes animo acids in exactly the same way. (If this weren't so,
you'd have to have a different table for each kind of organism.)
This is of course why you can take the gene for spider silk and insert
it into a goat and have it synthesize the protien; both spiders and
goats use DNA molecules in exactly the same way.
So all living organisms, from bacteria and fungi to mammals and birds
(and, hell, even viruses) have this "DNA computer" that reads "programs"
written in DNA and uses it to produce protiens. The only difference is
that each organism is running a different program - it has different
DNA. The machine that "runs" it is the same. Which isn't that
surprising, given that all life presumably evoled from a single common
ancester.
Now, dig a little deeper and you'll find that all is not quite so simple
and straight-forward. If a *human being* had designed the system, they
would have designed something simple like this. But life was not
designed, it happened by accident. And the more you study it, the more
you realise that it often goes about things in the most over-complicated
way possible! ;-)
Firstly, the books tell you that a protien is just a chain of amino
acids, but that's not strictly true. Some protiens are actually several
"seperate", unattached chains, which are knitted together as they're
synthesized in such a way that you can't easily pull them apart.
Strictly they're seperate molecules (they aren't chemically bonded), but
in reality they function as a single chemical entity.
Not only that, but some protiens are "modified" after being built. You
read off the DNA and chain the designated animo acids together, but then
you take the resulting molecule and add extra bonds or glue bits onto it
or otherwise modify it. So the final result is no longer a simple chain
of 20 animo acids; it has become a bit more complex than that.
(For example... hemoglobin contains iron. That's not an animo acid!)
It gets slightly weirder than that, though:
http://en.wikipedia.org/wiki/Selenocysteine
This is a standard amino acid, but there's no codon for it. Instead, the
RNA strand has a kind of a kink in it, and that turns the UGA (uracil,
guanine, adenine) codon from being a stop codon into being a
selenocysteine codon. (Over a certain range of bases, anyway.)
So the "standard genetic code" is modified by a loop in the RNA. Weird!
OK, so it's not quite as simple as it looks. But stick this in your pipe
and smoke it:
http://en.wikipedia.org/wiki/Pyrrolysine
Pyrrolysine is an animo acid similar to lysine. But here's the
interesting thing: The RNA code for pyrrolysine is "UAG" (uracil,
adenine, guanine). The thing is, in *most* living organisms, that's a
stop codon. But in certain archaea species, instead of being a stop
signal, it selects pyrrolysine.
Read that again: The *same* genetic code has a *different* meaning in
different organisms.
In these obscure organisms, there are 22 available amino acids, not 21.
And that means that the genetic code itself can change over time. That
least the question: Did the original genetic code have more than 21
amino acids? Did these organisms evolve a new amino acid, or did all
life originally have it and subsequently lost it? And when did all this
happen??
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