THE DNA WORLD
The entire story up till now was pretty much run by RNA.
This molecule can do it all.
can be a template that directs its own replication, or it can be a catalyst that accelerates
We considered its catalytic activity as a replicase in previous
lectures, but RNA is much more versatile than that.
Its ability to fold into three-
dimensional shapes according to the interactions of its internal complementary base pairs
means it can facilitate all kinds of chemical reactions.
Thus, one would think that cells
could survive just fine with RNA as the master chemical.
In fact, although RNA is a
good jack-of-all-trades, it is a master of none.
First, as a
, RNA is relatively unstable, and although some mutation is
good in order to generate diversity, the original requirement for persistence of chemical
form demands that the RNA not change too much.
RNA makes about 1 mistake for
every 1000 nucleotides polymerized, so it really needs to be shorter than 1000
nucleotides, or persistence will be insufficient.
A more stable replicator would allow a
longer, more complex nucleotide, and thus more potential functions.
Second, as a
, RNA molecules fold into different shapes according to the
hydrogen bonding in their various nucleotide sequences.
But the variety of such shapes
is actually somewhat limited.
A more versatile catalyst could facilitate a much broader
range of reactions.
Lets start with this second deficiency first, and return later to the first
So, as best we can sort out, based on a wide variety of observations and
experiments, the original RNA world gradually handed over the duty of catalysis to a
much more versatile class of molecule, proteins.
Proteins consist of a linear string of
Such strings of amino acids have much greater versatility than do strings of
ribonucleotides (RNA) for two reasons.
First, there are 20 amino acids from which to
choose, leading to many more combinations for a polymer of any particular length.
in structural differences between amino acids on a protein is much
greater than the range in structure available in the 4 ribonucleotides.
Notice in the
following figure the huge diversity in the side groups of amino acids, the white parts of
each of the 20 amino acids.
Thus, the number of different side groups, as well as their
large range in chemical structure, has the consequence that proteins are vastly more
versatile than RNA at catalyzing cellular reactions.
In fact the vast majority of cellular
reactions in modern cells are catalyzed by protein enzymes rather than RNA ribozymes.
RNA retains its catalytic function in critical processes as we will see, but protein is by far
the most ubiquitous catalyst.