called its active site, and any molecule which gets bound there is called a
substrate. Enzymes may have more than one active site, and more than one
substrate. Just as in Typogenetics, enzymes are indeed very choosy about
what they will operate upon. The active site usually is quite specific, and
allows just one kind of molecule to bind to it, although there are sometimes
"decoys"-other molecules which can fit in the active site and clog it up,
fooling the enzyme and in fact rendering it inactive.
Once an enzyme and its substrate are bound together, there is some
disequilibrium of electric charge, and consequently charge-in the form of
electrons and protons-flows around the bound molecules and readjusts
itself. By the time equilibrium has been reached, some rather profound
chemical changes may have occurred to the substrate. Some examples are
these: there may have been a "welding", in which some standard small
molecule got tacked onto a nucleotide, amino acid, or other common
cellular molecule; a DNA strand may have been "nicked" at a particular
location; some piece of a molecule may have gotten lopped off; and so
forth. In fact, bio-enzymes do operations on molecules which are quite
similar to the typographical operations which Typo-enzymes perform.
However, most enzymes perform essentially only a single task, rather than
a sequence of tasks. There is one other striking difference between Typo-
enzymes and bio-enzymes, which is this: whereas Typo-enzymes operate
only on strands, bio-enzymes can act on DNA, RNA, other proteins, ribo-
somes, cell membranes-in short, on anything and everything in the cell. In
other words, enzymes are the universal mechanisms for getting things done
in the cell. There are enzymes which stick things together and take them
apart and modify them and activate them and deactivate them and copy
them and repair them and destroy them ...
Some of the most complex processes in the cell involve "cascades" in
which a single molecule of some type triggers the production of a certain
kind of enzyme; the manufacturing process begins and the enzymes which
come off the "assembly line" open up a new chemical pathway which allows
a second kind of enzyme to be produced. This kind of thing can go on for
three or four levels, each newly produced type of enzyme triggering the
production of another type. In the end a "shower" of copies of the final
type of enzyme is produced, and all of the copies go off and do their
specialized thing, which may be to chop up some "foreign" DNA, or to help
make some amino acid for which the cell is very "thirsty", or whatever.Need for a Sufficiently Strong Support SystemLet us describe nature's solution to the puzzle posed for Typogenetics:
"What kind of strand of DNA can direct its own replication?" Certainly not
every strand of DNA is inherently a self-rep. The key point is this: any
strand which wishes to direct its own copying must contain directions for
assembling precisely those enzymes which can carry out the task. Now it is
futile to hope that a strand of DNA in isolation could be a self-rep; for inSelf-Ref and Self-Rep^529