DNA Repair
6-39 Sickle-cell anemia is an example of an inherited disease. Individuals with this disorder have
misshapen (sickle-shaped) red blood cells caused by a change in the sequence of the β-globin
gene. What is the nature of the change?
(a) chromosome loss
(b) base-pair change
(c) gene duplication
(d) base-pair insertion
6-40 Even though DNA polymerase has a proofreading function, it still introduces errors in the newly
synthesized strand at a rate of 1 per 10 7 nucleotides. To what degree does the mismatch repair
system decrease the error rate arising from DNA replication?
(a) 2-fold
(b) 5-fold
(c) 10-fold
(d) 100-fold
6-41 Which of the choices below represents the correct way to repair the mismatch shown in Figure
Q6-41?
A is the answer
Figure Q6-41
,6-43 Beside the distortion in the DNA backbone caused by a mismatched base pair, what additional
mark is there on eukaryotic DNA to indicate which strand needs to be repaired?
(a) a nick in the template strand
(b) a chemical modification of the new strand
(c) a nick in the new strand
(d) a sequence gap in the new strand
6-44 A pregnant mouse is exposed to high levels of a chemical. Many of the mice in her litter are
deformed, but when they are interbred with each other, all their offspring are normal. Which
two of the following statements could explain these results?
(a) In the deformed mice, somatic cells but not germ cells were mutated.
(b) The original mouse’s germ cells were mutated.
(c) In the deformed mice, germ cells but not somatic cells were mutated.
(d) The toxic chemical affects development but is not mutagenic.
6-45 The repair of mismatched base pairs or damaged nucleotides in a DNA strand requires a
multistep process. Which choice below describes the known sequence of events in this process?
(a) DNA damage is recognized, the newly synthesized strand is identified by an existing nick
in the backbone, a segment of the new strand is removed by repair proteins, the gap is
filled by DNA polymerase, and the strand is sealed by DNA ligase.
, (b) DNA repair polymerase simultaneously removes bases ahead of it and polymerizes the
correct sequence behind it as it moves along the template. DNA ligase seals the nicks in
the repaired strand.
(c) DNA damage is recognized, the newly synthesized strand is identified by an existing nick
in the backbone, a segment of the new strand is removed by an exonuclease, and the
gap is repaired by DNA ligase.
(d) A nick in the DNA is recognized, DNA repair proteins switch out the wrong base and
insert the correct base, and DNA ligase seals the nick.
6-46 Human beings with the inherited disease xeroderma pigmentosum have serious problems with
lesions on their skin and often develop skin cancer with repeated exposure to sunlight. What
type of DNA damage is not being recognized in the cells of these individuals?
(a) chemical damage
(b) X-ray irradiation damage
(c) mismatched bases
(d) ultraviolet irradiation damage
6-47 You are examining the DNA sequences that code for the enzyme phosphofructokinase in skinks
and Komodo dragons. You notice that the coding sequence that actually directs the sequence of
amino acids in the enzyme is very similar in the two organisms but that the surrounding
sequences vary quite a bit. What is the most likely explanation for this?
(a) Coding sequences are repaired more efficiently.
(b) Coding sequences are replicated more accurately.
(c) Coding sequences are packaged more tightly in the chromosomes to protect them from
DNA damage.
(d) Mutations in coding sequences are more likely to be deleterious to the organism than
mutations in noncoding sequences.
6-48 In somatic cells, if a base is mismatched in one new daughter strand during DNA replication, and
is not repaired, what fraction of the DNA duplexes will have a permanent change in the DNA
sequence after the second round of DNA replication?
(a) 1/2
(b) 1/4
, (c) 1/8
(d) 1/16
6-49 Sometimes, chemical damage to DNA can occur just before DNA replication begins, not giving
the repair system enough time to correct the error before the DNA is duplicated. This gives rise
to mutation. If the cytosine in the sequence TCAT is deaminated and not repaired, which of the
following is the point mutation you would observe after this segment has undergone two rounds
of DNA replication?
(a) TTAT
(b) TUAT
(c) TGAT
(d) TAAT
6-51 Sometimes, chemical damage to DNA can occur just before DNA replication begins, not giving
the repair system enough time to correct the error before the DNA is duplicated. This gives rise
to mutation. If the adenosine in the sequence TCAT is depurinated and not repaired, which of
the following is the point mutation you would observe after this segment has undergone two
rounds of DNA replication?
(a) TCGT
(b) TAT
(c) TCT
(d) TGTT
6-52 Which of the following statements is not an accurate statement about thymine dimers?
(a) Thymine dimers can cause the DNA replication machinery to stall.
(b) Thymine dimers are covalent links between thymidines on opposite DNA strands.
(c) Prolonged exposure to sunlight causes thymine dimers to form.
(d) Repair proteins recognize thymine dimers as a distortion in the DNA backbone.
6-53 Indicate whether the following statements are true or false. If a statement is false, explain why it
is false.
A. Ionizing radiation and oxidative damage can cause DNA double-strand breaks. T
6-39 Sickle-cell anemia is an example of an inherited disease. Individuals with this disorder have
misshapen (sickle-shaped) red blood cells caused by a change in the sequence of the β-globin
gene. What is the nature of the change?
(a) chromosome loss
(b) base-pair change
(c) gene duplication
(d) base-pair insertion
6-40 Even though DNA polymerase has a proofreading function, it still introduces errors in the newly
synthesized strand at a rate of 1 per 10 7 nucleotides. To what degree does the mismatch repair
system decrease the error rate arising from DNA replication?
(a) 2-fold
(b) 5-fold
(c) 10-fold
(d) 100-fold
6-41 Which of the choices below represents the correct way to repair the mismatch shown in Figure
Q6-41?
A is the answer
Figure Q6-41
,6-43 Beside the distortion in the DNA backbone caused by a mismatched base pair, what additional
mark is there on eukaryotic DNA to indicate which strand needs to be repaired?
(a) a nick in the template strand
(b) a chemical modification of the new strand
(c) a nick in the new strand
(d) a sequence gap in the new strand
6-44 A pregnant mouse is exposed to high levels of a chemical. Many of the mice in her litter are
deformed, but when they are interbred with each other, all their offspring are normal. Which
two of the following statements could explain these results?
(a) In the deformed mice, somatic cells but not germ cells were mutated.
(b) The original mouse’s germ cells were mutated.
(c) In the deformed mice, germ cells but not somatic cells were mutated.
(d) The toxic chemical affects development but is not mutagenic.
6-45 The repair of mismatched base pairs or damaged nucleotides in a DNA strand requires a
multistep process. Which choice below describes the known sequence of events in this process?
(a) DNA damage is recognized, the newly synthesized strand is identified by an existing nick
in the backbone, a segment of the new strand is removed by repair proteins, the gap is
filled by DNA polymerase, and the strand is sealed by DNA ligase.
, (b) DNA repair polymerase simultaneously removes bases ahead of it and polymerizes the
correct sequence behind it as it moves along the template. DNA ligase seals the nicks in
the repaired strand.
(c) DNA damage is recognized, the newly synthesized strand is identified by an existing nick
in the backbone, a segment of the new strand is removed by an exonuclease, and the
gap is repaired by DNA ligase.
(d) A nick in the DNA is recognized, DNA repair proteins switch out the wrong base and
insert the correct base, and DNA ligase seals the nick.
6-46 Human beings with the inherited disease xeroderma pigmentosum have serious problems with
lesions on their skin and often develop skin cancer with repeated exposure to sunlight. What
type of DNA damage is not being recognized in the cells of these individuals?
(a) chemical damage
(b) X-ray irradiation damage
(c) mismatched bases
(d) ultraviolet irradiation damage
6-47 You are examining the DNA sequences that code for the enzyme phosphofructokinase in skinks
and Komodo dragons. You notice that the coding sequence that actually directs the sequence of
amino acids in the enzyme is very similar in the two organisms but that the surrounding
sequences vary quite a bit. What is the most likely explanation for this?
(a) Coding sequences are repaired more efficiently.
(b) Coding sequences are replicated more accurately.
(c) Coding sequences are packaged more tightly in the chromosomes to protect them from
DNA damage.
(d) Mutations in coding sequences are more likely to be deleterious to the organism than
mutations in noncoding sequences.
6-48 In somatic cells, if a base is mismatched in one new daughter strand during DNA replication, and
is not repaired, what fraction of the DNA duplexes will have a permanent change in the DNA
sequence after the second round of DNA replication?
(a) 1/2
(b) 1/4
, (c) 1/8
(d) 1/16
6-49 Sometimes, chemical damage to DNA can occur just before DNA replication begins, not giving
the repair system enough time to correct the error before the DNA is duplicated. This gives rise
to mutation. If the cytosine in the sequence TCAT is deaminated and not repaired, which of the
following is the point mutation you would observe after this segment has undergone two rounds
of DNA replication?
(a) TTAT
(b) TUAT
(c) TGAT
(d) TAAT
6-51 Sometimes, chemical damage to DNA can occur just before DNA replication begins, not giving
the repair system enough time to correct the error before the DNA is duplicated. This gives rise
to mutation. If the adenosine in the sequence TCAT is depurinated and not repaired, which of
the following is the point mutation you would observe after this segment has undergone two
rounds of DNA replication?
(a) TCGT
(b) TAT
(c) TCT
(d) TGTT
6-52 Which of the following statements is not an accurate statement about thymine dimers?
(a) Thymine dimers can cause the DNA replication machinery to stall.
(b) Thymine dimers are covalent links between thymidines on opposite DNA strands.
(c) Prolonged exposure to sunlight causes thymine dimers to form.
(d) Repair proteins recognize thymine dimers as a distortion in the DNA backbone.
6-53 Indicate whether the following statements are true or false. If a statement is false, explain why it
is false.
A. Ionizing radiation and oxidative damage can cause DNA double-strand breaks. T
