164 ■ CHAPTER 09 What Genes Are
GENETICS
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DNA damageCut CutDNA polymeraseTemplate
strandNew strand(^1) Repair proteins
detect and tag
the damaged
DNA strand.
(^4) A repair DNA polymerase fills
the gap in the DNA with the
correct sequence of bases.
(^2) Repair enzymes
cut the DNA on
both sides of the
damage.
(^3) The damaged
segment of DNA
is removed and
degraded.
Figure 9.10
Repair proteins fix DNA damage
Large complexes of DNA repair proteins work
together to fix damaged DNA.
Q1: Summarize how DNA repair works and
why the repair mechanisms are essential
for the normal function of cells and whole
organisms.
Q2: Is DNA repair 100 percent effective?
Q3: What would happen to an organism if
its DNA repair became less effective?
creates a solution containing DNA polymerase,
primers, and loose nucleotides, and the solution
goes into a PCR machine, a device developed
for this specific process. The machine cycles
between heating, which separates the strands,
and cooling, which enables the added enzymes
to build two new strands of DNA through
complementary base-pairing. These heating
and cooling steps are repeated 25–40 times to
replicate millions of exact copies of the targeted
region of DNA.
PCR remains a central tool in most biology
labs. The Harvard team, for example, used PCR
to replicate pig DNA for their sequencing reac-
tions, which require many copies of the same
sequence in order to succeed. DNA sequenc-
ing machines, which produce a data file listing
the complete sequence of nucleotides in a given
strand of DNA, were used to identify all the
places in the pig genome where PERV had been
integrated.
Making Mutations
To inactivate all 62 copies of PERV in the pig
genome at once, the Church team decided to
try to prevent DNA polymerase from success-
fully replicating the PERV DNA. To that end,
the team planned to use CRISPR to mutate a
small region of the PERV DNA: a gene named
pol (see Figure 9.6). A change to the sequence
of nucleotides in an organism’s DNA is called a
mutation. The extent of a mutation can range
from the change of one nucleotide in a single base