There are certain proteins that must be available at all times for
cells to keep functioning. For instance, histone proteins are vital so
that cells with nuclei can organise their DNA into chromosomes,
as we shall see in Chapter 3. Other proteins are more specialised,
and may be needed only at a certain stage of development, or by
certain tissues. An example is the hormone insulin, which is
manufactured in the pancreas, and used to control the amount of
glucose in the blood (blood sugar). Insulin is not produced, or used
in, the brain, lung or any other tissues.
However, every cell in the organism has the potential to express
genes for both essential and specialised proteins. This must be so,
because the DNA of the original cell of the organism (the fertilised
egg in humans, for example) is copied into each cell as the
organism grows and therefore contains a full complement of genes.
The genes for essential proteins are known as housekeeping or
constitutive genes and these are expressed all the time. Inducible
genes, which code for specialised proteins are turned on only when
and where expression is required, otherwise the cell would waste
precious biochemical energy turning out proteins that were not
actually required. Worse, the unwanted proteins might interfere in
the cell's biochemistry. Such a scenario would not be conducive to
the organism's survival. If the insulin gene were left switched on all
the time, for instance, the hormone would have a devastating effect
on blood chemistry that would be fatal within a short time. All the
blood sugar would end up being stored in the liver instead of
fuelling vital organs such as the brain.
Gene expression is regulated by *on-ofF switches. There is
more to a gene than the piece that is copied in transcription.
Remember that this runs in the direction 3' to 5'. It turns out that
there is DNA 'upstream' from the 3' end - just before the start of
the sequence that RNA copies - which acts as the switch. The first
example of one of these switches was discovered by Fran~oisJacob
andJacques Monod, working at the Pasteur Institute in Paris in the
late 1950s and early 1960s. They started by exploring the way in
which bacteria change their pattern of gene expression in response