Messenger RNA and RibosomesAs was mentioned above, in many celis, DNA, the ruler of the cell, dwells in
its private "throne room": the nucleus of the cell. But most of the "living" in
a cell goes on outside of the nucleus, namely in the cytoplasm-the "ground"
to the nucleus' "figure". In particular, enzymes, which make practically every
life process go, are manufactured by ribosomes in the cytoplasm, and they do
most of their work in the cytoplasm. And just as in Typogenetics, the
blueprints for all enzymes are stored inside the strands-that is, inside the
DNA, which remains protected in its little nuclear home. So how does the
information about enzyme structure get from the nucleus to the ribo-
somes?
Here is where messenger RNA-mRNA--comes in. Earlier, mRNA
strands were humorously said to constitute a kind of DNA Rapid Transit
Service; by this is meant not that mRNA physically carries DNA anywhere,
but rather that it serves to carry the information, or message, stored in the
DNA in its nuclear chambers, out to the ribosomes in the cytoplasm. How is
this done? The idea is easy: a special kind of enzyme inside the nucleus
faithfully copies long stretches of the DNA's base sequence onto a new
strand-a strand of messenger RNA. This mRN A then departs from the
nucleus and wanders out into the cytoplasm, where it runs into many
ribosomes which begin doing their enzyme-creating work on it.
The process by which DNA gets copied onto mRNA inside the nucleus
is called transcription; in it, the double-stranded DNA must be temporarily
separated into two single strands, one of which serves as a template for the
mRNA. Incidentally, "RNA" stands for "ribonucleic acid", and it is very
much like DNA except that all of its nucleotides possess that special oxygen
atom in the phosphate group which DNA's nucleotides lack. Therefore the
"deoxy" prefix is dropped. Also, instead of thymine, RNA uses the' base
uracil, so the information in strands of RNA can be represented by arbi-
trary sequences of the four letters 'A', 'C', 'G', 'U'. Now when mRNA is
transcribed off of DNA, the transcription process operates via the usual
base-pairing (except with U instead of T), so that a DNA-template and its
mRNA-mate might look something like this:DNA: ........ CGT AAATCAAGTCA ........
mRNA: ........ GCAUUUAGUUCAGU ........(template)
("copy")RNA does not generally form long double strands with itself, although it
can. Therefore it is prevalently found not in the helical form which so
characterizes DNA, but rather in long, somewhat randomly curving
strands.
Once a strand of mRNA has escaped the nhcleus, it encounters those
strange subcellular creatures called "ribosomes"-but before we go on to
explain how a ribosome uses mRNA, I want to make some comments about
enzymes and proteins. Enzymes belong to the general category of
biomolecules called proteins, and the job of ribosomes is to make all pro-Self-Ref and Self-Rep 517