Page 65 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

224 Chapter 7 Anatomy and Function of a Gene: Dissection Through Mutation

Figure 7.5 The Luria-Delbrück fluctuation experiment. (a) Hypothesis 1: If resistance arises only after exposure to a bactericide,
all bacterial cultures of equal size should produce roughly the same number of resistant colonies. Hypothesis 2: If random mutations
conferring resistance arise before exposure to bactericide, the number of resistant colonies in different cultures should vary (fluctuate) widely.
(b) Actual results showing large fluctuations suggest that mutations in bacteria occur as spontaneous mistakes independent of exposure to a
selective agent.
(a) Two hypotheses for the origin of bactericide resistance
Hypothesis 1: Resistance is a physiological response to a bactericide

1 2 3 4

Hypothesis 2: Resistance arises from random mutation

1 2 3 4
(b) Fluctuation test results
1 2 3 4 5 6 7 8 9 10 11
Cultures

Number
of colonies 1 0 107 0 0 5 0 5 0 6 3

Time of exposure to selective agent

observation of a substantial fluctuation in the number of contain bacteriophages, they would nevertheless remain
resistant colonies in different petri plates, Luria and resistant to this bactericidal virus.
Delbrück concluded that bacterial resistance arises from We next describe some of the many kinds of random
mutations that exist before exposure to bacteriophages. events that can cause mutations. We also discuss how cells
After exposure, however, the bactericide in the petri plate cope with these events and minimize mutation creation.
becomes a selective agent that kills off nonresistant cells,
allowing only the preexisting resistant ones to survive.
Figure 7.6 illustrates how researchers used another tech- essential concepts
nique, known as replica plating, to demonstrate even
more directly that the mutations conferring bacterial re- • Mutations are heritable alterations in the base sequence
sistance occur before the cells encounter the bactericide of DNA.
that selects for their resistance. • Point mutations change one or a few base pairs; they
These key experiments showed that bacterial resis- include substitutions (transitions and transversions) and
tance to phages and other bactericides is the result of mu- small insertions and deletions.
tations, and these mutations do not arise in particular • Spontaneous mutation rates are low and vary among
genes as a directed response to environmental change. different genes and organisms.
Instead, mutations occur spontaneously as a result of ran- • The more cells divide, the more likely it is that mutations
dom processes that can happen at any time and hit the will accumulate in their genomes.
genome at any place. Once such random changes occur, • Results of the fluctuation test and replica plating
however, they usually remain stable. If the resistant mu- experiments showed that resistance mutations arise
tants of the Luria-Delbrück experiment, for example, randomly in bacterial cells prior to bactericide exposure.
were grown for many generations in medium that did not