Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

488 Chapter 8: Analyzing Cells, Molecules, and Systems
large populations—thousands or tens of thousands of individual organisms—isolating
mutant individuals is much more efficient if one generates mutations with
chemicals or radiation that damage DNA. By treating organisms with such mutagens,
very large numbers of mutant individuals can be created quickly and then
screened for a particular defect of interest, as we discuss shortly.
An alternative approach to chemical or radiation mutagenesis is called insertional
mutagenesis. This method relies on the fact that exogenous DNA inserted
randomly into the genome can produce mutations if the inserted fragment interrupts
a gene or its regulatory sequences. The inserted DNA, whose sequence is
known, then serves as a molecular tag that aids in the subsequent identification
and cloning of the disrupted gene (Figure 8–44). In Drosophila, the use of
the transposable P element to inactivate genes has revolutionized the study of
gene function in the fly. Transposable elements (see Table 5–4, p. 288) have also
been used to generate mutations in bacteria, yeast, mice, and the flowering plant
Arabidopsis.
Genetic Screens Identify Mutants with Specific Abnormalities
Once a collection of mutants in a model organism such as yeast or fly has been
produced, one generally must examine thousands of individuals to find the
altered phenotype of interest. Such a search is called a genetic screen, and the
larger the genome, the less likely it is that any particular gene will be mutated.
Therefore, the larger the genome of an organism, the bigger the screening task
becomes. The phenotype being screened for can be simple or complex. Simple
phenotypes are easiest to detect: one can screen many organisms rapidly, for
example, for mutations that make it impossible for the organism to survive in the
absence of a particular amino acid or nutrient.
More complex phenotypes, such as defects in learning or behavior, may require
more elaborate screens (Figure 8–45). But even genetic screens that are used to
dissect complex physiological systems can be simple in design, which permits the
simultaneous examination of large numbers of mutants. As an example, one particularly
elegant screen was designed to search for genes involved in visual processing
in zebrafish. The basis of this screen, which monitors the fishes’ response
to motion, is a change in behavior. Wild-type fish tend to swim in the direction of
a perceived motion, whereas mutants with defects in their visual processing systems
swim in random directions—a behavior that is easily detected. One mutant
discovered in this screen is called lakritz, which is missing 80% of the retinal ganglion
cells that help to relay visual signals from the eye to the brain. As the cellular
organization of the zebrafish retina is similar to that of all vertebrates, the study
of such mutants should also provide insights into visual processing in humans.
Because defects in genes that are required for fundamental cell processes—
RNA synthesis and processing or cell-cycle control, for example—are usually
lethal, the functions of these genes are often studied in individuals with
Figure 8–44 Insertional mutant of the
snapdragon, Antirrhinum. A mutation
in a single gene coding for a regulatory
protein causes leafy shoots (left) to develop
in place of flowers, which occur in the
normal plant MBoC6 (right). The m8.53/8.45 mutation causes
cells to adopt a character that would be
appropriate to a different part of the normal
plant, so instead of a flower, the cells
produce a leafy shoot. (Courtesy of Enrico
Coen and Rosemary Carpenter.)
1 mm
Figure 8–45 A behavioral phenotype
detected in a genetic screen. (A) Wildtype
C. elegans engage in social feeding.
The worms migrate around until they
encounter their neighbors and commence
feeding on bacteria. (B) Mutant animals
feed by themselves. (Courtesy of Cornelia
Bargmann, Cell 94: cover, 1998. With
permission from Elsevier.)