Essential Cell Biology 5th edition

Generating Genetic Variation
301
small risk of changing each time a cell divides. Changes that affect a single
nucleotide pair are called point mutations. These typically arise from
rare errors in DNA replication or repair (discussed in Chapter 6).
The point mutation rate has been determined directly in experiments with
bacteria such as E. coli. Under laboratory conditions, E. coli divides about
once every 20–25 minutes; in less than a day, a single E. coli can produce
more descendants than there are humans on Earth—enough to provide
a good chance for almost any conceivable point mutation to occur. A
culture containing 10 9 E. coli cells thus harbors millions of mutant cells
whose genomes differ subtly from a single ancestor cell. A few of these
mutations may confer a selective advantage on individual cells: resistance
to a poison, for example, or the ability to survive when deprived of
a standard nutrient. By exposing the culture to a selective condition—
adding an antibiotic or removing an essential nutrient, for example—one
can find these needles in the haystack; that is, the cells that have undergone
a specific mutation enabling them to survive in conditions where
the original cells cannot (Figure 9−5). Such experiments have revealed
that the overall point mutation frequency in E. coli is about 3 changes for
each 10 10 nucleotide pairs replicated. With a genome size of 4.6 million
nucleotide pairs, this mutation rate means that approximately 99.99%
of the time, the two daughter cells produced in a round of cell division
will inherit exactly the same genome sequence of the parent E. coli cell;
mutant cells are therefore produced only rarely.
The overall mutation rate in humans, as determined by comparing the
DNA sequences of children and their parents (and estimating how many
times the parental germ cells divided before producing gametes), is
about one-third that of E. coli—which suggests that the mechanisms that
mutant E. coli cell
that requires histidine
to proliferate
INNOCULATE
CULTURE
AS CELLS DIVIDE,
RANDOM MUTATIONS
ARISE SPONTANEOUSLY
SAMPLE OF CELLS
SPREAD ON
PETRI DISH
medium lacking
histidine
rare colony of cells that
contains a new mutation
enabling proliferation in
the absence of histidine
MUTATION IN His GENE
TGA
ACT
inactive
His
gene
rich medium,
which includes
histidine, allows
all bacteria
to multiply
bacteria in which
different mutations
have occurred
NEW MUTATION
IN His GENE
TGG
ACC
active
His
gene
UGA mRNA
premature stop codon
mutation eliminates
enzyme required to
make histidine
UGG
mRNA
enzyme
new mutation
restores production of
enzyme required to
make histidine
Figure 9−5 Mutation rates can be measured in the laboratory. In this experiment, an E. coli strain that carries a deleterious point
mutation in the His gene—which is needed to manufacture the amino acid histidine—is used. The mutation has converted a G-C
nucleotide pair to an A-T, resulting in a premature stop signal in the mRNA produced from the mutant gene (left box). As long as
histidine is supplied in the growth medium, this strain can grow and divide normally. If a large number of mutant cells (say 10 10 ) is
spread on an agar plate that lacks histidine, the great majority will die. The rare survivors will contain a new mutation in which the A-T is
changed back to a G-C. This “reversion” corrects the original ECB5 e9.05/9.05
defect and allows the bacterium to make the enzyme it needs to survive
in the absence of histidine. Such mutations happen by chance and only rarely, but the ability to work with very large numbers of E. coli
cells makes it possible to detect this change and to accurately measure its frequency.