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

508 Chapter 8: Analyzing Cells, Molecules, and Systems
shoot
discs removed from
tobacco leaf
callus
shoot-inducing
medium
leaf discs incubated with
genetically engineered
Agrobacterium for 24 h
selection medium allows
only plant cells that
have acquired DNA from
the bacteria to
proliferate
Figure 8–70 Transgenic plants can
be made using recombinant DNA
techniques optimized for plants. A
disc is cut out of a leaf and incubated in
a culture of Agrobacterium that carries
a recombinant plasmid with both a
selectable marker and a desired genetically
engineered gene. The wounded plant cells
at the edge of the disc release substances
that attract the bacteria, which inject their
DNA into the plant cells. Only those plant
cells that take up the appropriate DNA and
express the selectable marker gene survive
and proliferate and form a callus. The
manipulation of growth factors supplied to
the callus induces it to form shoots, which
subsequently root and grow into adult
plants carrying the engineered gene.
transfer
shoot to rootinducing
medium
grow up
rooted
seedling
adult tobacco plant
carrying transgene
that was originally
present in the
bacterial plasmid
cells in culture, so transgenic plants can be created from plant cells transfected
with DNA in culture (Figure 8–70).
The ability to produce transgenic plants has greatly accelerated progress in
many areas of plant cell biology. It has played an important part, for example, in
isolating receptors for growth regulators and in analyzing the mechanisms of morphogenesis
and of gene expression in plants. These techniques have also opened
up many new possibilities in agriculture that could benefit both the farmer and
the consumer. They have made it possible, for example, to modify the ratio of lipid,
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starch, and protein in seeds, to impart pest and virus resistance to plants, and
to create modified plants that tolerate extreme habitats such as salt marshes or
water-stressed soil. One variety of rice has been genetically engineered to produce
β-carotene, the precursor of vitamin A. Were it to replace conventional rice, this
“golden rice”—so-called because of its faint yellow color—could help to alleviate
severe vitamin A deficiency, which causes blindness in hundreds of thousands of
children in the developing world each year.
Summary
Genetics and genetic engineering provide powerful tools for understanding the function
of individual genes in cells and organisms. In the classical genetic approach,
random mutagenesis is coupled with screening to identify mutants that are deficient
in a particular biological process. These mutants are then used to locate and
study the genes responsible for that process.
Gene function can also be ascertained by reverse genetic techniques. DNA engineering
methods can be used to alter genes and to re-insert them into a cell’s chromosomes
so that they become a permanent part of the genome. If the cell used for
this gene transfer is a fertilized egg (for an animal) or a totipotent plant cell in culture,
transgenic organisms can be produced that express the mutant gene and pass
it on to their progeny. Especially important for cell and molecular biology is the
ability to alter cells and organisms in highly specific ways—allowing one to discern
the effect on the cell or the organism of a designed change in a single protein or RNA
molecule. For example, genomes can be altered so that the expression of any gene
can be switched on or off by the experimenter.