8.4 The Regulation of Gene Expression
272 MHR • Unit 3 Molecular Genetics
Every living cell has the ability to respond to its
environment by changing the kinds and amounts
of polypeptides it produces. Whether these
polypeptides are the pigments that give fur a
particular colour, the hormones that govern the
development of sex organs, or the enzymes that
metabolize nutrients, they all originate from the
same processes of transcription and translation that
you have studied so far in this chapter. By exercising
control over these processes, a cell can regulate its
gene expression.
For example, the white colour of arctic foxes
(Alopex lagopus) in winter, as seen in Figure 8.24,
helps to conceal the foxes from both prey and
predators. As the temperature warms and the snow
melts, the ground will take on the browns of the
arctic tundra, and the foxes will also turn brown. The
expression of the genes that govern the coat colour
of these animals varies with the temperature, thus
helping them to stay camouflaged in a changing
landscape. Some animals show even more dramatic
changes in gene expression during their lifetimes.
For example, if a male slipper limpet (Crepidola
fornicata) is surrounded by other males, as shown
in Figure 8.25, it will turn into a female.Figure 8.24Seasonal changes in the coat colour of the
arctic fox are the result of changes in gene expression.Figure 8.25Slipper limpets live in stacked colonies. If no
female limpets are present, some males will turn into females.Gene Expression in Prokaryotes
Figure 8.26 on the next page summarizes the
main steps in the protein synthesis pathway in
prokaryotes. Each step or control point (listed
below) offers the cell an opportunity to prevent
or otherwise influence the synthesis of an enzyme
that is not needed.
transcriptional control By exercising control
here, the cell can speed up or slow down the
transcription of mRNA from the gene that codes
for the polypeptide.
post-transcriptional control The cell may
transcribe the mRNA but break it down before
translation. Alternatively, it may lengthen or
shorten the molecule’s poly-A tail to control the
time in which the mRNA remains stable and active.
post-translational control After synthesizing the
polypeptide, the cell may modify it chemically or
vary the rate at which the polypeptide becomes
a functional protein. Alternatively, the cell may
break the polypeptide down before it becomes a
functional protein.EXPECTATIONSExplain how regulatory proteins act as control mechanisms for genetic
expression.
Demonstrate how more than one form of control can operate on a single
gene at any one time.
Discuss some of the reasons for the differences in control mechanisms
in eukaryotes and prokaryotes.
Interpret micrographs that demonstrate the cellular structures involved
in protein synthesis.