Comparison of DNA's Self-Rep Method with QuiningNote that in the enzymatic action on the DNA strands, the fact that
information is stored in the DNA is just plain irrelevant; the enzymes are
merely carrying out their symbol-shunting functions, just like rules of
inference in the MIU-system. It is of no interest to the three enzymes that
at some point they are actually copying the very genes which coded for
them. The DNA, to them, is just a template without meaning or interest.
It is quite interesting to compare this with the Quine sentence's
method of describing how to construct a copy of itself. There, too, one has
a sort of "double strand"-two copies of the same information, where one
copy acts as instructions, the other as template. In DNA, the process is
vaguely parallel, since the three enzymes (DNA endonuclease, DNA
polymerase, DNA ligase) are coded for injust one of the two strands, which
therefore acts as program, while the other strand is merely a template. The
parallel is not perfect, for when the copying is carried out, both strands are
used as template, not just one. Nevertheless, the analogy is highly sugges-
tive. There is a biochemical analogue to the use-mention dichotomy: when
DNA is treated as a mere sequence of chemicals to be copied, it is like
mention of typographical symbols; when DNA is dictating what operations
shall be carried out, it is like use of typographical symbols.
Levels of Meaning of DNAThere are several levels of meaning which can be read from a strand of
DNA, depending on how big the chunks are which you look at, and how
powerful a decoder you use. On the lowest level, each DNA strand codes
for an equivalent RNA strand-the process of decoding being transcription.
If one chunks the DNA into triplets, then by using a "genetic decoder", one
can read the DNA as a sequence of amino acids. This is translation (on top of
transcription). On the next natural level of the hierarchy, DNA is readable
as a code for a set of proteins. The physical pulling-out of proteins from
genes is called gene expression. Currently, this is the highest level at which we
understand what DNA means.
However, there are certain to be higher levels of DNA meaning which
are harder to discern. For instance, there is every reason to believe that the
DNA of, say, a human being codes for such features as nose shape, music
talent, quickness of reflexes, and so on. Could one, in principle, learn to
read off such pieces of information directly from a strand of DNA, without
going through the actual physical process of epigenesis-the physical
pulling-out of phenotype from genotype? Presumably, yes, since-in
theory-one could have an incredibly powerful computer program
simulating the entire process, including every cell, every protein, every tiny
feature involved in the replication of DNA, of cells, to the bitter end. The
output of such apseudo-epigenesis program would be a high-level description
of the phenotype.
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