prevented from being transcribed, which means that a whole set of related
enzymes remains unsynthesized.
What about positive feedback and feedforward? Here again, there are
two options: (1) unclog the clogged enzym:es, or (2) stop the repression of
the relevant operon. (Notice how nature seems to love double-negations!
Probably there is some very deep reason for this.) The mechanism by which
repression is repressed involves a class of molecules called inducers. The
role of an inducer is simple: it combines with a repressor protein before the
latter has had a chance to bind to an operator on a DNA molecule; the
resulting "repressor-inducer complex" is incapable of binding to an
operator, and this leaves the door open for the associated operon to be
transcribed into mRNA and subsequently translated into protein. Often
the end product or some precursor of the end product can act as an
inducer.Feedback and Strange Loops ComparedIncidentally, this is a good time to distinguish between simple kinds of
feedback, as in the processes of inhibition and repression, and the
looping-back between different informational levels, shown in the Central
Dogmap. Both are "feedback" in some sense; but the latter is much deeper
than the former. When an amino acid, such as tryptophan or isoleucine,
acts as feedback (in the form of an inducer) by binding to its repressor so
that more of it gets made, it is not telling how to construct itself; it is just
telling enzymes to make more of it. This could be compared to a radio's
volume, which, when fed through a listener's ears, may cause itself to be
turned down or up. This is another thing entirely from the case in which
the broadcast itself tells you explicitly to turn your radio on or off, or to
tune to another wavelength-or even how to build another radio! The
latter is much more like the looping-back between informational levels, for
here, information inside the radio signal gets "decoded" and translated into
mental structures. The radio signal is composed of symbolic constituents
whose symbolic meaning matters-a case of use, rather than mention. On
the other hand, when the sound is just too loud, the symbols are not
conveying meaning; they are merely being perceived as loud sounds, and
might as well be devoid of meaning-a case of mention, rather than use.
This case more resembles the feedback loops by which proteins regulate
their own rates of synthesis.
It has been theorized that the difference between two neighJjoring cells
which share the exact same genotype and yet have different functions is
that different segments of their genome have been repressed, and there-
fore they have different working sets of proteins. A hypopothesis like this
could account for the phenomenal differences between cells in different
organs of the body of a human being.Self-Ref and Self-Rep^545