Jumping Genes
The spotted and streaked patterns seen in
Indian corn result from genes that have moved
from one chromosomal location to another.
Such genes are called transposons (trans
POH zahns). When a transposon jumps to
a new location, it often inactivates a gene or
causes mutations. In Indian corn, some
pigment genes are not expressed in some
cells because they have been disrupted by
jumping genes.
The Discovery of Transposons
In the 1950s, the geneticist Barbara
McClintock discovered transposons while
studying corn. Most scientists rejected her
ideas for more than 20 years. The idea that
genes could change locations on the chromo-
some contradicted the prevailing view that
genes and chromosomes are stable parts
of the cell. Over time, additional research
supported her hypothesis, and her model
gradually gained acceptance. In 1983,
McClintock received a Nobel Prize for
her discoveries involving transposons.Importance of
Transposons
All organisms,
including humans,
appear to have
transposons.
Transposons proba-
bly play a role in
spreading genes for
antibiotic resistance
among bacteria.
Transposons that
affect flower color
in morning glory
flowers have been
found. Transposons
may also have
medical applications,
such as helping scien-
tists discover how white blood cells make antibod-
ies and what causes cancer.
Although the movement of transposons is very
rare, transposons are important because they can
cause mutations and bring together different
combinations of genes. The transfer of these mobile
genes could be a powerful mechanism in evolution
and could help solve certain mysteries about evolu-
tion, such as how larger organisms developed from
single cells and how new species arise.Exploring FurtherFurther
The piece of DNA that overlaps the promoter site and serves as
the on-off switch is called an .Because of its position, the
operator is able to control RNA polymerase’s access to the three lac-
tose-metabolizing genes.
In bacteria, a group of genes that code for enzymes involved in the
same function, their promoter site, and the operator that controls
them all function together as an (AHP uhr ahn). The operon
that controls the metabolism of lactose is called the and
is shown in Figure 6.
What determines whether the lacoperon is in the “on” or “off”
mode? When there is no lactose in the bacterial cell, a repressor turns
the operon off. A is a protein that binds to an operator and
physically blocks RNA polymerase from binding to a promoter site.
The blocking of RNA polymerase consequently stops the transcrip-
tion of the genes in the operon, as shown in Figure 6.
When lactose is present, the lactose binds to the repressor and
changes the shape of the repressor. The change in shape causes
the repressor to fall off of the operator, as shown in Figure 6. Now
the bacterial cell can begin transcribing the genes that code for the
lactose-metabolizing enzymes. By producing the enzymes only
when the nutrient is available, the bacterium saves energy.repressorlacoperonoperonoperator216 CHAPTER 10How Proteins Are Made
Barbara McClintock