Essential Cell Biology 5th edition

354 CHAPTER 10 Analyzing the Structure and Function of Genes
Figure 10–26 GFPs that fluoresce at
different wavelengths help reveal the
connections that individual neurons
make within the brain. This image shows
differently colored neurons in one region
of a mouse brain. The neurons express
different combinations of differently colored
GFPs, making it possible to distinguish
and trace many individual neurons within
a population. The stunning appearance of
these labeled neurons earned the animals
that bear them the colorful nickname
“brainbow mice.” (From J. Livet et al.,
Nature 450:56–62, 2007. With permission
from Macmillan Publishers Ltd.)
30 µm
that often behaves in the same way as the normal protein produced by
the gene. GFP fusion has become a standard strategy for tracking not
only the location but also the movement of specific proteins in living cells
(see How We Know, pp. 520−521).
ECB5 e10.33/10.26
The Study of Mutants Can Help Reveal the Function
of a Gene
Although it may seem counterintuitive, one of the best ways to determine
a gene’s function is to see what happens to an organism when the gene
is inactivated by a mutation. Before the advent of gene cloning, geneticists
would often study the mutant organisms that arise at random in a
population. The mutants of most interest were often selected because
of their unusual phenotype—fruit flies with white eyes or curly wings, for
example. The gene responsible for the mutant phenotype could then be
studied by breeding experiments, as Gregor Mendel did with peas in the
nineteenth century (discussed in Chapter 19).
Although mutant organisms can arise spontaneously, they do so infrequently.
The process can be accelerated by treating organisms with
radiation or chemical mutagens, which randomly disrupt gene activity.
Such random mutagenesis generates large numbers of mutant organisms,
each of which can then be studied individually. This “classical genetic
approach,” which we discuss in detail in Chapter 19, is most applicable to
organisms that reproduce rapidly and can be analyzed genetically in the
laboratory—such as bacteria, yeasts, nematode worms, and fruit flies—
although it has also been used to study zebrafish and mice, which require
more time to reproduce and develop.
RNA Interference (RNAi) Inhibits the Activity of Specific
Genes
DNA technology has made possible more targeted genetic approaches
to studying gene function. Instead of beginning with a randomly generated
mutant and then identifying the responsible gene, a gene of known
sequence can be inactivated deliberately, and the effects on the cell or
organism’s phenotype can be observed. Because this strategy is essentially
the reverse of that used in classical genetics—which goes from
mutants to genes—it is often referred to as reverse genetics.