different copyists are all at work simultaneously, each one of them copying
the same original manuscript from a clef which flutists cannot read into a
clef which they can read. As each copyist finishes a page of the original
manuscript, he passes it on to the next copyist, and starts transcribing a new
page himself. Meanwhile, from each score emerging from the pens of the
copyists, a set of flutists are reading and tooting the melody, each flutist
delayed with respect to the others who are reading from the same sheet.
This rather wild image gives, perhaps, an idea of some of the complexity of
the processes which are going oR in each and every cell of your body during
every second of every day ...Which Came First-The Ribosome or the Protein?
We have been talking about these wonderful beasts called ribosomes; but
what are they themselves composed of? How are they made? Ribosomes are
composed of two types of things: (1) various kinds of proteins, and (2)
another kind of RNA, called ribosomal RNA (rRNA). Thus, in order for a
ribosome to be made, certain kinds of proteins must be present, and rRNA
must be present. Of course, for proteins to be present, ribosomes must be
there to make them. So how do you get around the vicious circle? Which
comes first-the ribosome or the protein? Which makes which? Of course
there is no answer because one always traces things back to previous
members of the same class-just as with the chicken-and-the-egg
question-until everything vanishes over the horizon of time. In any case,
ribosomes are made of two pieces, a large and a small one, each of which
contains some rRNA and some proteins. Ribosomes are about the size of
large proteins; they are much much smaller than the strands of mRNA
which they take as input, and along which they move.Protein FunctionWe have spoken somewhat of the structure of proteins-specifically en-
zymes; but we have not really mentioned the kinds of tasks which they
perform in the cell, nor how they do them. All enzymes are catalysts, which
means that in a certain sense, they do no more than selectively accelerate
various chemical processes in the cell, rather than make things happen
which without them never could happen. An enzyme realizes certain path-
ways out of the myriad myriad potentialities. Therefore, in choosing which
enzymes shall be present, you choose what shall happen and what shall not
happen-despite the fact that, theoretically speaking, there is a nonzero
probability for any cellular process to happen spontaneously, without the
aid of catalysts.
Now how do enzymes act upon the molecules of the cell? As has been
mentioned, enzymes are folded-up polypeptide chains. In every enzyme,
there is a cleft or pocket or some other clearly-defined surface feature
where the enzyme binds to some other kind of molecule. This location is528 Self-Ref and Self-Rep