all of life, and there are many mysteries connected with it. But in essence it
is easy to describe. Let us first give a picturesque image, and then render it
more precise. Imagine the mRNA to be like a long piece of magnetic
recording tape, and the ribosome to be like a tape recorder. As the tape
passes through the playing head of the recorder, it is "read" and converted
into music, or other sounds. Thus magnetic markings are "translated" into
notes. Similarly, when a "tape" of mRNA passes through the "playing
head" of a ribosome, the "notes" which are produced are amin:J acids, and
the "pieces of music" which they make up are proteins. This is what transla-
tion is all about; it is shown in Figure 96.The Genetic CodeBut how can a ribosome produce a chain of amino acids when it is
reading a chain of nucleotides? This mystery was solved in the early
1960's by the efforts of a large number of people, and at the core of
the answer lies the Genetic Code-a mapping from triplets of nucleotides
into amino acids (see Fig. 94). This is in spirit extremely similar to
the Typogenetic Code, except that here, three consecutive bases (or
nucleotides) form a codon,
whereas there, CVA GAV only two were
needed. Thus C u A g A u there must be
4x4x4 (equals 64) different
entries in the A typical segment of mRNA table, instead
of sixteen. A read first as two triplets ribosome clicks
down a strand (above), and second as three of RNA three
nucleotides at duplets (below): an example a time-which
is to say, one of hemiolia in biochemistry. codon at a time
-and each time it does so, it
appends a single new amino acid to the protein it is presently manufactur-
ing. Thus, a protein comes out of the ribosome amino acid by amino acid.Tertiary StructureHowever, as a protein emerges from a ribosome, it is not only getting
longer and longer, but it is also continually folding itself up into an ex-
traordinary three-dimensional shape, very much in the way that those
funny little Fourth-of-July fireworks called "snakes" simultaneously grow
longer and curl up, when they are lit. This fancy shape is called the
protein's tertiary structure (Fig. 95), while the amino acid sequence per se is
called the primary structure of the protein. The tertiary structure is implicit
in the primary structure, just as in Typogenetics. However, the recipe for
deriving the tertiary structure, if you know only the primary structure, is by
far more complex than that given in Typogenetics. In fact, it is one of the
outstanding problems of contemporary molecular biology to figure out
some rules by which the tertiary structure of a protein can be predicted if
only its primary structure is known.
Self-Ref and Self-Rep 519