Page 66 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

7.2 Molecular Mechanisms That Alter DNA Sequence 225

Figure 7.6 Replica plating verifies that bacterial resistance 7.2 Molecular Mechanisms That
is the result of preexisting mutations. (a) Pressing a master
plate onto a velvet surface transfers some cells from each bacterial Alter DNA Sequence
colony onto the velvet. Pressing a replica plate onto the velvet
then transfers some cells from each colony onto the replica plate.
Investigators track which colonies on the master plate can grow on learning objectives
the replica plate (here, only penicillin-resistant ones). (b) Colonies on
a master plate without penicillin are transferred sequentially to three 1. Outline natural processes that can produce mutations
replica plates with penicillin. Resistant colonies grow in the same
positions on all three replicas, showing that some colonies on the by damaging DNA.
master plate had multiple resistant cells even before exposure to 2. Explain how errors in DNA replication can cause
the antibiotic. mutations.
(a) The replica plating technique 3. Define mutagen and describe how mutagens are used
1. Invert master plate; pressing against 2. Invert second plate in genetic research.
velvet surface leaves an (replica plate); pressing
imprint of colonies. Save plate. against velvet surface 4. Describe how the Ames test can detect potential
picks up colony imprint. carcinogens.

S
S R
S
The creation of a heritable mutation is the outcome of sev-
eral competing processes: mutation, repair, and replication
Master plate Penicillin in medium
No penicillin in medium (Fig. 7.7). First, of course, a random event must occur to
alter the DNA. Two different kinds of events initiate DNA
3. Incubate changes: Either DNA can be damaged by chemical reac-
plate.
tions or irradiation, or alternatively, mistakes can happen
Velvet when DNA is copied during replication.
When DNA changes first occur, they are not yet
actual mutations but only potential mutations. The reason
is that most of them are quickly repaired by a variety of
enzymatic systems within cells. These DNA repair
Replica plate machines are engaged in a continual race with DNA rep-
S = penicillin-sensitive 4. Only penicillin-resistant lication (Fig. 7.7). If repair of damaged DNA or misincor-
bacteria colonies grow. Compare
R = penicillin-resistant with position of colonies porated nucleotides occurs before the next round of DNA
bacteria on original plate. replication, then the sequence is corrected and no muta-
tion will result. However, if the repair enzymes do not
(b) Mutations occur prior to penicillin exposure correct the problem before the next round of DNA repli-
7
10 colonies of Make three replica plates. cation, the mutation becomes established permanently in
penicillin-sensitive bacteria Incubate to allow penicillin- both strands of the double helix and a heritable mutation
resistant colonies to grow. is the outcome.
In this section, we describe some of the most important
mechanisms that can change DNA sequences. The subsequent

Master plate Figure 7.7 Point mutations result when DNA replication
No penicillin in medium Penicillin in medium wins the race with DNA repair. An alteration that occurs in
DNA is heritable only if DNA repair fails to reverse the change
before the next round of DNA replication.
DNA repair
Velvet
Penicillin in medium
Natural process DNA
or mutagen damage DNA
Wild-type replication Mutation
DNA base
sequence
Natural DNA replication
Penicillin in medium error
or in presence of mutagen
Penicillin-resistant
colonies grow in the same
position on all three plates. DNA repair