13 mm.25 mm.First complete replica of T4 invader is pro-
duced.
Lysozyme (a protein) attacks host cell wall,
breaking open the bacterium, and the "bicen-
tuplets" emerge.Thus, when a T4 phage invades an E. coli cell, after the brief span of about
twenty-four or twenty-five minutes, the cell has been completely subverted,
and breaks open. Out pop about two hundred exact copies of the original
virus-"bicentuplets"-ready to go attack more bacterial cells, the original
cell having been largely consumed in the process.
Although from a bacterium's point of view this kind of thing is a
deadly serious menace, from our large-scale vantage point it can be looked
upon as an amusing game between two players: the invader, or "T" player
(named after the T-even class of phages, including the T2, T4, and others),
and the "C" player (standing for "Cell"). The objective of the T player is to
invade and take over the cell of the C player from within, for the purpose
of reprod ucing itself. The objective of the C player is to protect itself and
destroy the invader. When described this way, the molecular TC-game can
be seen to be quite parallel to the macroscopic TC-game described in the
preceding Dialogue. (The reader can doubtless figure out which player-T
or C--corresponds to the Tortoise, and which to the Crab.)
Recognition, Disguises, LabelingThis "game" emphasizes the fact that recognition is one of the central themes
of cellular and subcellular biology. How do molecules (or higher-level
structures) recognize each other? It is essential for the functioning of
enzymes that they should be able to latch onto special "binding sites" on
their substrates; it is essential that a bacterium should be able to distinguish
its own DN A from that of phages; it is essential that two cells should be able
to recognize each other and interact in a controlled way. Such recognition
problems may remind you of the original, key problem about formal
systems: How can you tell if a string has, or does not have, some property
such as theoremhood? Is there a decision procedure? This kind of question
is not restricted to mathematical logic: it permeates computer science and,
as we are seeing, molecular biology.
The labeling technique described in the Dialogue is in fact one of E.
coli's tricks for outwitting its phage invaders. The idea is that strands of
DNA can be chemically labeled by tacking on a small molecule-methyl-to
various nucleotides. Now this labeling operation does not change the usual
biological properties of the DNA; in other words, methylated (labeled)
DNA can be transcribed just as well as unmethylated (unlabeled) DNA, and
so it can direct the synthesis of proteins. But if the host cell has some special(^540) Self-Ref and Self-Rep