Answers ■ A17Figure 9.6
Q1: What common mechanism is employed by the guide RNA to
find its target DNA sequence?
A1: Complementary base-pairing between the guide RNA and one
strand of the target DNA.
Q2: How many strands of DNA must Cas9 cut to be effective?
A2: Both strands must be cut.
Q3: Does Cas9 also cause the deletion of DNA from the genome?
A3: No. Normal DNA repair proteins cause the deletion of DNA
from the genome.
Figure 9.7
Q1: What are the structures that result from the first level of
coiling around proteins called?
A1: Nucleosomes.
Q2: What makes up a “bead” and what makes up the “string” in the
beads-on-a-string structure of DNA?
A2: The beads are the nucleosomes; the string is the double-
stranded DNA linking them together.
Q3: What is the name for the structure that is more compact than
the beads-on-a-string structure but less compact than an actual
chromosome?
A3: Chromatin fiber.
Figure 9.8
Q1: Which of the DNA strands here are the template strands?
Why are they called “template” strands?
A1: The strands from the original double helix are the template
strands. They are called this because new DNA is built using the
information from them as a template.
Q2: What must be broken before replication can begin?
A2: Hydrogen bonds between base pairs.
Q3: In your own words, explain why replication is described as
“semiconservative.”
A3: It is semiconservative because each new double helix of DNA
is composed of one “conserved” strand from the original DNA
molecule and one new strand.
Figure 9.9
Q1: PCR replicates DNA many times to increase the amount
available for analysis. Why is this process called “amplification”?
A1: Because PCR substantially increases, or “amplifies,” the
quantity of DNA.
Q2: During the PCR cycle, what causes the DNA strands to
separate?
A2: Heat.
Q3: Identify a difference between how PCR and DNA replication
are accomplished.A3: PCR is the targeted amplification of a specific region of DNA,
while DNA replication is the duplication of an entire chromosome.Figure 9.10
Q1: Summarize how DNA repair works and why the repair
mechanisms are essential for the normal function of cells and
whole organisms.A1: An error is detected and tagged, then the damaged section of
DNA is removed and replaced. Without DNA repair, mutations
would persist and potentially result in cell death.Q2: Is DNA repair 100 percent effective?A2: No.Q3: What would happen to an organism if its DNA repair became
less effective?A3: Its cells would have more trouble operating properly as the
DNA’s genetic instructions became less accurate.Figure 9.11
Q1: What are the three types of point mutations?A1: Substitution, insertion, and deletion.Q2: Sickle-cell disease is an autosomal recessive genetic disorder.
How many mutated hemoglobin alleles do people with sickle-cell
disease have?A2: Tw o.Q3: Because of improved treatments, individuals with sickle-cell
disease are now living into their forties, fifties, or longer. How
might this extension of life span affect the prevalence of sickle-cell
disease in the population?A3: Sickle-cell disease will increase because people will be able to
survive long enough to reproduce and pass on the sickle-cell allele.Figure 9.12
Q1: Name a step in this process that is similar to the original
CRISPR method that removed the PERVs from the pig genome.
(Hint: Review Figure 9.6.)A1: CRISPR is employed to remove a gene from the genome.Q2: Name a step in this process that is not included in the original
CRISPR method that removed the PERVs from the pig genome.A2: Human stem cells are added to the pig embryos.Q3: What parts of this process would scientists need to change in
order to develop several different human organs in a single pig?A3: Scientists would need to use CRISPR to target only the genes
that are required for the development of each organ in the pig. Those
organs would then be replaced by human organs via use of the same
human stem cells added to create a single human kidney in the pig.