Passage 36
Protein synthesis begins when the gene encoding a
protein is activated. The gene’s sequence of nucleotides is
transcribed into a molecule of messenger RNA (mRNA),
which reproduces the information contained in that
(5) sequence. Transported outside the nucleus to the cyto-
plasm, the mRNA is translated into the protein it
encodes by an organelle known as a ribosome, which
strings together amino acids in the order specified by the
sequence of elements in the mRNA molecule. Since the
(10) amount of mRNA in a cell determines the amount of the
corresponding protein, factors affecting the abundance
of mRNA’s play a major part in the normal functioning
of a cell by appropriately regulating protein synthesis.
For example, an excess of certain proteins can cause cells
(15) to proliferate abnormally and become cancerous; a lack
of the protein insulin results in diabetes.
Biologists once assumed that the variable rates at
which cells synthesize different mRNA’s determine the
quantities of mRNA’s and their corresponding proteins
(20) in a cell. However, recent investigations have shown that
the concentrations of most mRNA’s correlate best, not
with their synthesis rate, but rather with the equally vari-
able rates at which cells degrade the different mRNA’s
in their cytoplasm. If a cell degrades both a rapidly and
(25) a slowly synthesized mRNA slowly, both mRNA’s will
accumulate to high levels.
An important example of this phenomenon is the
development of red blood cells from their unspecialized