UNCC BIOL3111 Exam 3 Chapter 6 Study Questions
During DNA replication in a bacterium, a C is accidentally incorporated instead of an A into one newly synthesized DNA strand. Imagine the error was not corrected and that it has no effect on the ability of the progeny to grow and reproduce. A- After this original bacterium has divided once, what proportion of its progeny would you expect to contain the mutation? B- What proportion after three more rounds of DNA replication and cell division?
A- 1/2, 50% B- 1/4, 25%
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.
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.
A pregnant mouse is exposed to high levels of a chemical. many of the mice in her little 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.
A- In the deformed mice, somatic cells but not germ cells were mutated. D- The toxic chemical affects development but is not mutagenic.
Which of the following choices below represents the correct way to repair the mismatch? Mismatch: New Strand C, Template T. (a) New Strand A, Template T (b) New Strand C, Template G (c) New Strand C, Template A (c) New Strand G, Template T
A- New Strand A, Template T
Nonhomologous end joining can result in all but which of the following? (a) the recovery of lost nucleotides on a damaged DNA strand (b) the interruption of gene expression (c) loss of nucleotides at the site of repair (d) translocations of DNA fragments to an entirely different chromosome
A- the recovery of lost nucleotides on a damaged DNA strand.
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
B- 1/4
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.
B- Thymine dimers are covalent links between thymidines on opposite DNA strands.
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
B- base-pair change
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
C- A nick in the new strand
Several members of the same family were diagnosed with the same kind of cancer when they were unusually young. Which one of the following is the most likely explanation for this phenomenon? It is possible that the individuals with the cancer have _______________________. (a) inherited a cancer-causing gene that suffered a mutation in an ancestor's somatic cells. (b) inherited a mutation in a gene required for DNA synthesis. (c) inherited a mutation in a gene required for mismatch repair. (d) inherited a mutation in a gene required for the synthesis of purine nucleotides.
C- Inherited a mutation in a gene required for mismatch repair.
Which best completes this statement; Nonhomologous end joining is a process by which a double-stranded DNA end is joined ______________. (a) to a similar stretch of sequence on the complementary chromosome. (b) after repairing any mismatches. (c) to the nearest available double-stranded DNA end. (d) after filling in any lost nucleotides, helping to maintain the integrity of the DNA sequence.
C- to the nearest available double-stranded DNA end.
Even through 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
D- 100-fold
The events listed below are all necessary for homologous recombination to occur properly. A. Holliday junction cut and ligated B. strand invasion C. DNA synthesis D. DNA ligation E. double-strand break F. nucleases create uneven strands Which of the following is the correct order of events during homologous recombination? (a) E, B, F, D, C, A (b) B, E, F, D, C, A (c) C, E, F, B, D, A (d) E, F, B, C, D, A
D- E, F, B, C, D, A
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.
D- Mutations in coding sequences are more likely to be deleterious to the organism than mutations in noncoding sequences.
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 by the cells of these individuals? (a) chemical damage (b) X-ray irradiation damage (c) mismatched bases (d) ultraviolet irradiation damage
D- ultraviolet irradiation damage
True or False? If False, explain why. The DNA template used to repair the broken strand is the homologous chromosome inherited from the other parent.
False. Although it is called homologous recombination, this is not a process that depends on the proximity of parental homologs. When used as a mechanism for DNA repair, homologous recombination uses the sister chromatids in an undamaged, newly replicated (homologous) DNA helix as a template.
True or False? If False, explain why. Homologous recombination cannot occur in prokaryotic cells, because they are haploid, and therefore have no extra copy of the chromosome to use as a template for repair.
False. Homologous recombination also occurs in prokaryotic cells, and typically occurs very shortly after DNA replication, when the newly replicated duplexes are in close proximity.
True or False? If False, explain why. Depurination of DNA is a rare event that is caused by ultraviolet irradiation.
False; depurination occurs constantly in our cells through spontaneous hydrolysis of the bond linking the DNA base to the deoxyribose sugar.
True or False? If False, explain why. Nonhomologous end joining is a mechanism that ensures that DNA double-strand breaks are repaired with a high degree of fidelity to the original DNA sequence.
False; homologous recombination can repair double-strand breaks without any change in DNA sequence, but nonhomolous end joining always involves a loss of genetic information because the ends are degraded by nucleases before they can be ligated back together.
True or False? If False, explain why. After damaged DNA has been repaired, nicks in the phosphate backbone are maintained as a way to identify the strand that was repaired.
False; it is believed that the nicks are generated during DNA replication as a means of easy identification of the newly synthesized strand but are sealed by DNA ligase shortly after replication is completed.
True or False? If False, explain why. Ionizing radiation and oxidative damage can cause DNA double-strand breaks.
True
True or False? If False, explain why. The 3′ overhang "invades" the homologous DNA duplex, which can be used as a primer for the repair DNA polymerase.
True
True or False? If False, explain why. The first step in repair requires a nuclease to remove a stretch of base pairs from the 5′ end of each strand at the site of the break.
True
A mismatched base pair causes ____________ in the DNA backbone. If this were the only indication of an error in replication, the overall _____________________ would be much higher. This is because __________ alone does not tell the mismatch repair system _________________________________________________________________.
distortion, rate of mutation. distortion, which base is incorrect and which was originally part of the chromosome.
Homologous recombination is an important mechanism in which organisms use a "backup" copy of the DNA as a template to fix ____________________________________ without loss of genetic information. Which of the following is NOT necessary for homologous recombination to occur? (a) 3′ DNA strand overhangs (b) 5′ DNA strand overhangs (c) a long stretch of sequence similarity (d) nucleases
double-strand breaks, B- 5' DNA stand overhangs
In addition to the repair of DNA ____________________________, homologous recombination is a mechanism for generating genetic diversity by swapping segments of parental chromosomes. During which process does swapping occur? (a) DNA replication (b) DNA repair (c) meiosis (d) transposition
double-strand breaks, C- meiosis
Affected individuals in some families with a history of early-onset colon cancer have been found to carry mutations in ____________________. Mutations arising in _____________________ are not inherited. A defect in DNA synthesis or nucleotide biosynthesis would probably _______________.
mismatch repair genes, somatic cells, be lethal.
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 _____________. 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
mutation A- TTAT
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 ____________. 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
mutation C- TCT
WHY DISTORTION ALONE IS NOT SUFFICIENT- Without additional marks that identify the difference between the _________________________________, the repair would only be corrected 50% of the time by random chance. The error rate (and therefore the __________________) would still be less than in a system that lacked the mismatch repair enzymes (1 mistake per ______________), but greater than the error rate in a system that accurately identifies the newly synthesizes strand (1 mistake per _______________.)
newly synthesized strand and the template strand. mutation rate, 1 per 10^7 base pairs 1 per 10^9 base pairs
The deamination of cytosine generates a _____________. This is a naturally occurring nucleic acid base, and so does not represent a DNA lesion caused by damage due to chemicals or irradiation. Why is this base recognized as "foreign" and why is it important for cells to have a mechanism to recognize and remove uracil when it is found in the DNA duplex?
uracil base. Uracil is an RNA base and it is recognized as a mutational lesion because, as it is formed from the deamination of cytosine, it will be paired with a guanine in the context of the DNA duplex. Uracil pairs by forming two hydrogen bonds, similar to thymine, and is thus a poor partner for guanine, which forms three hydrogen bonds with cytosine. The mismatch causes a distortion of the DNA backbone, allowing the repair machinery to recognize the uracil as a lesion. Because uracil pairs preferably with adenine (its partner in double-stranded RNA), the deamination of cytosine to uracil is highly mutagenic. If unrepaired, it can result in the transition of a C-G base pair to a T-A base pair.
