DNA Replication and Repair Mechanisms
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Questions and Answers

What process generates two Holliday intermediates during DNA repair?

  • Branch migration (correct)
  • Sealing by DNA ligase
  • DNA replication
  • Cleavage by resolvases

    • What enzyme is primarily responsible for sealing nicks in the DNA after resolution of Holliday intermediates?

  • Holliday intermediate resolvases
  • DNA helicase
  • DNA ligase (correct)
  • Recombinase

    • What type of repair can begin after the undamaged complement is generated from the strand with a DNA lesion?

  • Homologous recombination
  • Methylation repair
  • Base excision repair (BER) (correct)
  • Nucleotide excision repair (NER) (correct)

    • How do most cells typically resolve the Holliday intermediates during DNA repair?

    <p>Primarily through a non-crossover pathway (A)</p> Signup and view all the answers

    What role do recombinases play in the context of DNA repair?

    <p>They adapt the repair system to various fork structures. (D)</p> Signup and view all the answers

    What is the primary consequence of DNA lesions during replication?

    <p>They become problematic primarily after DNA replication. (A)</p> Signup and view all the answers

    Which process allows DNA synthesis to continue over a lesion?

    <p>Translesion synthesis (B)</p> Signup and view all the answers

    What happens when a replication fork encounters a lesion undergoing nucleotide excision repair?

    <p>The replication fork collapses, leading to a double-strand break. (B)</p> Signup and view all the answers

    In which scenario does the replication machinery stall during DNA synthesis?

    <p>When the fork encounters a lesion that cannot be repaired. (A)</p> Signup and view all the answers

    What type of mutation can occur if an O6-meG lesion in the template strand is replicated?

    <p>A C→T transition mutation (B)</p> Signup and view all the answers

    What typically happens when replication machinery is blocked by a lesion but resumes further downstream?

    <p>A single-strand gap occurs with no complementary strand present. (C)</p> Signup and view all the answers

    What characterizes translesion DNA polymerases?

    <p>They are specialized for particular repair scenarios. (A)</p> Signup and view all the answers

    In normal conditions, when do most lesions cause replication forks to stall?

    <p>Before any repair processes are initiated. (C)</p> Signup and view all the answers

    What is the primary advantage of recombinational DNA repair over translesion DNA synthesis?

    <p>It does not cause mutations. (A)</p> Signup and view all the answers

    What occurs when a replication fork encounters a break in a template strand?

    <p>Recombinational repair is triggered to reconstruct the fork. (D)</p> Signup and view all the answers

    What is the first step in the recombinational double-strand break repair when a replication fork collapses?

    <p>The broken end is processed to create a 3' single-stranded extension. (D)</p> Signup and view all the answers

    What does branch migration accomplish in the recombinational repair process?

    <p>It moves DNA branches along and reconfigures base pairing. (A)</p> Signup and view all the answers

    What happens during fork regression when a replication fork stalls?

    <p>The lesion-containing strand is reunited with its complementary strand. (C)</p> Signup and view all the answers

    What is a Holliday intermediate?

    <p>A temporary DNA molecule during recombinational repair. (B)</p> Signup and view all the answers

    What role does the dedicted replication restart complex play after repairing a replication fork?

    <p>It aids in reloading the helicase onto the DNA. (C)</p> Signup and view all the answers

    Which of the following DNA repair processes requires the lesion to occur in only one strand of duplex DNA?

    <p>Base excision repair (D)</p> Signup and view all the answers

    What mechanism can be used for filling in single-stranded DNA regions left by DNA polymerase during replication?

    <p>Gap repair through recombinational mechanisms. (B)</p> Signup and view all the answers

    Which statement best describes branch migration?

    <p>It can occur via random movement in either direction. (C)</p> Signup and view all the answers

    What happens if a lesion is not repaired after forming a Holliday intermediate?

    <p>The lesion can still be paired with a complementary strand for future repair. (D)</p> Signup and view all the answers

    In what situation is recombinational DNA repair preferred over translesion DNA synthesis?

    <p>When the goal is to avoid introducing mutations. (B)</p> Signup and view all the answers

    Which enzyme is involved in the process of resolving the Holliday intermediate?

    <p>Holliday intermediate resolvases. (D)</p> Signup and view all the answers

    What initiates the repair of double-strand breaks (DSBs) in recombinational DNA repair?

    <p>Processing of broken DNA ends (A)</p> Signup and view all the answers

    Which enzyme class is essential for the DNA strand invasion step during homologous recombination?

    <p>Recombinases (C)</p> Signup and view all the answers

    What structural intermediate is formed during the initial stages of DSB repair in recombinational DNA repair?

    <p>D-loop (A)</p> Signup and view all the answers

    What role do DNA polymerases play in the repair of DSBs?

    <p>Extend the invading strands (C)</p> Signup and view all the answers

    What does the pathway known as synthesis-dependent strand annealing (SDSA) involve?

    <p>Displacement of lengthened invading strands (C)</p> Signup and view all the answers

    What is a potential consequence of improper homologous recombination?

    <p>Complete deletion of genetic information (C)</p> Signup and view all the answers

    What is the function of Holliday junction resolvases?

    <p>Cleave Holliday intermediates (C)</p> Signup and view all the answers

    What type of DNA structure is referred to as a Holliday intermediate?

    <p>A four-branched crossover junction (D)</p> Signup and view all the answers

    What happens to the breaks left by the cleavage of Holliday intermediates?

    <p>They are sealed by DNA ligase (C)</p> Signup and view all the answers

    During which phase is homologous recombination most likely to occur?

    <p>During DNA replication (B)</p> Signup and view all the answers

    What does the term 'DSB repair' refer to specifically?

    <p>Repair of double-strand breaks (C)</p> Signup and view all the answers

    What effect do oxidative DNA damage and ionizing radiation have on DNA?

    <p>They can cause double-strand breaks (C)</p> Signup and view all the answers

    What is retained when both Holliday intermediates are cleaved at the same site during repair?

    <p>Chromosomal segments (B)</p> Signup and view all the answers

    Study Notes

    DNA Replication and Repair Mechanisms

    • DNA damage is common and detrimental, especially during replication. Damage impacts replication in various ways.

    Outcomes of Lesions During Replication

    • Translesion Synthesis (TLS): DNA synthesis continues over the lesion using specialized DNA polymerases. This is often the case with a lesion that doesn't significantly distort the DNA. A common result of TLS is a mutation.
    • Lesion Repair and Fork Collapses: If repair is initiated but incomplete when the replication fork arrives, the fork collapses, creating a double-strand break (DSB). Recombinational repair fixes this.
    • Replication Fork Stalling: Replisome halts at the lesion. Replication can restart downstream, leaving the lesion behind. The lesion is in a single-stranded gap. This is more common in delaying strand..
    • Replication Bypass and Restart: The replication machinery halts but picks up again. The lesion is left behind, in a single-stranded gap. This bypass is more common on the lagging strand due to how Okazaki fragments are generated.

    DNA Double-Strand Break Repair (DSBR)

    • Double-strand breaks (DSBs) are lethal if not repaired. They can originate from oxidative damage or radiation.
    • DSBR utilizes homologous recombination; requires an undamaged homologous DNA molecule, for example, a sister chromatid.
    • DSBR Pathway:
      • Broken DNA ends are processed.
      • The 3’ strands invade the homologous chromosome.
      • The invading strand is replicated.
      • The invading strand is used for replication to restore the damaged piece of DNA using the homologous chromosome as a template.
    • Repair pathways utilize recombination.
      • The DNA double crossover intermediate has multiple resolution options.
        • Synthesis-Dependent Strand Annealing (SDSA): The invading strands are displaced, and any gaps are filled.
        • Double-Strand Break Repair (DSBR): The strands are linked until replication is completed, forming a Holliday intermediate. Holliday Junction Resolvases cleaves the Holliday intermediate in ways that retain or exchange the chromosomal DNA segments.

    Replication Fork Repair

    • Collapsed Replication Forks: A broken template strand leads to a DSB. Recombinational repair recreates the replication fork. The steps are similar to DSBR.
      • Broken DNA end is processed to create a single-stranded extension.
      • The strand then invades the intact chromosome.
      • Branch migration moves the branch point in one direction, usually.
      • The branch can be resolved, and replication can restart without mutations..
    • Stalled Replication Forks: The fork stops at the lesion. This triggers fork regression (fork moves backward) to bring the lesion into contact with its complementary strand. It results in a four-branched Holliday intermediate. It also allows further repair. This lesion can be repaired by DNA excision repair or other relevant pathways..

    Gap Repair

    • A lesion can cause single-stranded gap in which TLS fills in the missing DNA segment. If TLS is mutagenic, recombinational gap repair happens in another way.
    • Gap Repair Pathway:
      • A recombinase binds to the single-stranded gap.
      • Recombinase promotes strand invasion into the undamaged double-stranded DNA of the replication fork.
      • Branch migration generates Holliday intermediates.
      • The displaced strand acts as a template allowing further DNA replication.
    • The Holliday intermediates are resolved, resulting in non-crossover pathways, and gaps are filled. NER, BER can begin at the lesion.

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    Description

    Explore the essential processes of DNA replication and the various repair mechanisms that deal with DNA lesions. This quiz examines translesion synthesis, lesion repair, and the consequences of replication fork stalling. Test your knowledge on how cells manage DNA damage during replication!

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