Biology Reference
In-Depth Information
Fig. 4.6
to make the music, but in itself, it is uniform, rather dull and doesn't do anything
on its own. The music produced by decoding the linear information on the tape, on
the other hand, is much more complex in structure and much more interesting - in
organisms, the proteins are the music.
The above simple description of gene expression omits an important aspect
of the genome - that gene expression is regulated in precise ways, both in
terms of how active a given gene is at a particular time, and where it is active
in the body of muticellular organisms. By active, we mean that the gene is being
transcribed into RNA. So genes can be switched on and off, and the switches are
proteins that bind to particular regions of the gene being regulated. We call these
proteins
transcription factors
for obvious reasons.
It might seem to be a paradox that organisms show high diversity at the anatom-
ical level but high similarity at the molecular level - the so-called “Unity of
Biochemistry” referred to earlier in this section. The explanation of this apparent
paradox is that much of the diversity at the anatomical level is due to how some
genes are regulated, and not in the genes themselves. By gene regulation remem-
ber, we mean where in the body, and when in the life of the organism, particular
genes are transcribed. It is now clear that much of evolution is at the level of gene
regulation rather than at the level of the function of the gene product.
Let me give you an example. About 15,000 years ago, the ancestors of stickle-
back fish lived exclusively in the oceans. But at the end of the last Ice Age many
stickleback populations found themselves in newly formed freshwater lakes. Today,
sticklebacks that live in the ocean have pelvic spines that offer some protection
