order for those potential proteins to be pulled out of the DNA, there must
not only be ribosomes, but also RNA polymerase, which makes the mRNA
that gets transported to the ribosomes. And so we have to begin by assum-
ing a kind of "minimal support system" just sufficiently strong that it allows
transcription and translation to be carried out. This minimal support
system will thus consist in (1) some proteins, such as RNA polymerase,
which allow mRNA to be made from DNA, and (2) some ribosomes.How DNA Self-Replicates
It is not by any means coincidental that the phrases "sufficiently strong
support system" and "sufficiently powerful formal system" sound alike.
One is the precondition for a self-rep to arise, the other for a self-ref to
arise. In fact there is in essence only one phenomenon going on in two very
different guises, and we shall explicitly map this out shortly. But before we
do so, let us finish the description of how a strand of DNA can be a self-rep.
The DNA must contain the codes for a set of proteins which will copy
it. Now there is a very efficient and elegant way to copy a double-stranded
piece of DNA, whose two strands are complementary. This involves two
steps:(1) unravel the two strands from each other;
(2) "mate" a new strand to each of the two new single strands.This process will create two new double strands of DNA, each identical to
the original one. Now if our solution is to be based on this idea, it must
involve a set of proteins, coded for in the DNA itself, which will carry out
these two steps.
I t is believed that in cells, these two steps are performed together in a
coordinated way, and that they require three principal enzymes: DNA
endonuclease, DNA polymerase, and DNA ligase. The first is an "unzip-
ping enzyme": it peels the two original strands apart for a short distance,
and then stops. Then the other two enzymes come into the picture. The
DNA polymerase is basically a copy-and-move enzyme: it chugs down the
short single strands of DNA, copying them complementarily in a fashion
reminiscent of the Copy mode in Typogenetics. In order to copy, it draws
on raw materials-specifically nucleotides-which are floating about in the
cytoplasm. Because the action proceeds in fits and starts, with some unzip-
ping and some copying each time, some short gaps are created, and the
DNA ligase is what plugs them up. The process is repeated over and over
again. This precision three-enzyme machine proceeds in careful fashion all
the way down the length of the DNA molecule, until the whole thing has
been peeled apart and simultaneously replicated, so that there are now two
copies of it.(^530) Self-Ref and Self-Rep