Certain mutations make microorganisms resistant to antibiotics or increase
their pathogenicity. There are many naturally occurring mutagens, such as radi-
ation from x-rays, gamma rays, and ultraviolet light. These rays break the cova-
lent bonds between certain bases of DNA-producing fragments.
Ultraviolet light binds together adjacent thymines in a DNA strand, forming
thymine dimers that cannot function in protein synthesis. Unless repaired, these
dimers cause damage or death to cells due to improper transcription or replica-
tion of DNA. Some bacteria can repair damage caused by ultraviolet radiation
by employing light-repairing enzymes that separate the dimer into the original
two thymines. This process is called photoreactivation.
MUTATION RATE
Mutations occur naturally and can be induced by mutation-causing agents in the
environment. However, not all cells experience mutation even if they are exposed
to mutation-causing agents.
Scientists measure the impact that mutation has on an organism by determin-
ing the mutation rate. The mutation rateis the number of mutations per cell divi-
sion. For example, suppose you observe the growth of 100 cells that began from
a parent cell. If 90 of those cells replicate the parent cell and 10 cells are muta-
tions, than the mutation rate is 10 percent.
Measuring the mutation rate is a way to compare the number of mutations
that occur naturally to the number of mutations that occur when a cell is exposed
to a potential mutation-causing agent.
First, scientists measure the mutation rate that occurs naturally when a cell is
not exposed to a potential mutation-causing agent. Next, the mutation rate is cal-
culated when a cell is not exposed to a potential mutation-causing agent. The
results of these two observations are compared. If both mutation rates are rela-
tively the same, then the substance being tested is not a mutation-causing agent.
However, the substance is a mutation-causing agent if its mutation rate is appre-
ciably higher than the natural mutation rate.
Quiz
- The point at which the double-stranded DNA molecule unwinds is called
(a) polymerase
(b) replication fork
(c) hydrogen bonding
(d) ribosomes
CHAPTER 7 Microbial Genetics^127