Biochem Lecture 8b
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Questions and Answers

What is the function of O6-methylguanine-DNA methyltransferase in repairing methylated guanine?

  • Deactivates the repair process
  • Creates DNA lesions
  • Converts guanine to a methylated form
  • Repairs methylated guanine to guanine (correct)

    • Which enzyme demethylates 1-methyladenine and 3-methylcytosine to adenine and cytosine in humans?

  • Exicisionuclease
  • O6-methylguanine-DNA methyltransferase
  • AlkB (correct)
  • Photolyase

    • What is the role of photolyase in E.coli DNA repair?

  • Creates single-strand DNA damage
  • Catalyzes the demethylation of DNA bases
  • Acts as a photoantenna to absorb blue-light photons (correct)
  • Repairs double-strand breaks

    • Which protein is an α-ketoglutarate-Fe2+ dependent dioxygenase that catalyzes the oxidative demethylation of certain DNA residues in humans?

    <p>AlkB</p> Signup and view all the answers

    What is the primary function of MTHFpolyGlu in E.coli photolyase?

    <p>Absorbs blue-light photons for DNA repair</p> Signup and view all the answers

    Which type of lesions is repaired by nucleotide excision, including cyclobutane pyrimidine dimers (CPDs) formed from UV light?

    <p>Thymine-thymine dimers</p> Signup and view all the answers

    What is the general outcome of a stable N2-dG adduct formed by benzo[a]pyrene exposure?

    <p>Mutations leading to cancer</p> Signup and view all the answers

    Which DNA repair pathway involves the direct removal of a photoproduct by enzymes like photolyases?

    <p>Direct repair</p> Signup and view all the answers

    Photolyases are enzymes involved in repairing DNA lesions caused by which specific type of radiation?

    <p>Ultraviolet radiation</p> Signup and view all the answers

    Which repair mechanism is capable of joining two DNA ends without nucleotide loss or addition?

    <p>Homologous recombination</p> Signup and view all the answers

    What role does microhomology play in the end joining process?

    <p>Guiding the insertion of new nucleotides at the repair junction</p> Signup and view all the answers

    Which repair mechanism is error-prone and can result in diverse DNA sequences at the repair junction?

    <p>Non-homologous end joining</p> Signup and view all the answers

    In the repair process involving homologous recombination repair, what restores the structure of the replication fork?

    <p>Nuclease and ligase enzymes</p> Signup and view all the answers

    Which repair mechanism is the preferred method for repairing DNA double-strand breaks that arise shortly after DNA replication?

    <p>Homologous recombination</p> Signup and view all the answers

    Which DNA repair mechanism is part of the 'SOS response' in bacteria when DNA damage is extensive?

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

    Which proteins are involved in the SOS response in bacteria?

    <p>UmuC and UmuD</p> Signup and view all the answers

    What is the function of DNA Pol V in bacterial DNA repair?

    <p>Processing past the damage</p> Signup and view all the answers

    Which polymerase in mammals assists when a T-T dimer halts replication fork by inserting two A residues opposite a T-T pyrimidine dimer?

    <p>DNA Pol h (eta)</p> Signup and view all the answers

    What is recognized by the cell as needing rescue in DNA repair when a replicative polymerase is stalled at a site of DNA damage?

    <p>Replicative polymerase stalled at DNA damage</p> Signup and view all the answers

    What is the role of ABC excinuclease in nucleotide excision repair in bacteria?

    <p>Hydrolyzes the fifth bond on the 3’-side of the lesion and the eighth bond on the 5'-side</p> Signup and view all the answers

    In nucleotide excision repair in eukaryotes, which enzyme removes the 27–29 nucleotides segment after cleavage of the DNA backbone?

    <p>DNA polymerase e</p> Signup and view all the answers

    What happens when a replication fork encounters an unrepaired lesion or strand break?

    <p>Replication generally halts and the fork may collapse.</p> Signup and view all the answers

    Which repair mechanism involves using another chromosome as a template?

    <p>Recombinational DNA repair</p> Signup and view all the answers

    How does an excinuclease participate in nucleotide excision repair for both bacteria and humans?

    <p>Cleaves a segment of DNA containing damage</p> Signup and view all the answers

    Which repair mechanism involves replicating inaccurately over many types of lesions?

    <p>Translesion Repair</p> Signup and view all the answers

    What is the main mechanism of direct repair in bacteria and humans using photolyases?

    <p>Use of light energy to repair pyrimidine dimers</p> Signup and view all the answers

    Which enzyme is involved in nucleotide excision repair in bacteria?

    <p>Excisionuclease</p> Signup and view all the answers

    What is the function of photolyases in DNA repair?

    <p>Repair UV-induced DNA damage by photoreactivation</p> Signup and view all the answers

    Which protein binds to DNA double-strand breaks and improves their subsequent binding by the NHEJ polymerase, nuclease, and ligase complexes?

    <p>Ku70–Ku80 protein heterodimer</p> Signup and view all the answers

    Which repair mechanism typically takes place after DNA has been duplicated but before the cell has divided?

    <p>Homologous recombination</p> Signup and view all the answers

    Which repair mechanism alters the original DNA sequence when fixing a broken chromosome?

    <p>Nonhomologous end joining</p> Signup and view all the answers

    Which enzyme typically cuts and pastes genetic material from one chromosomal location to another in some transposons?

    <p>Transposase</p> Signup and view all the answers

    When does nonhomologous end joining usually take place during the cell cycle?

    <p>When cells have not yet duplicated their DNA</p> Signup and view all the answers

    Which repair mechanism is a source of DNA exchange and contributes to genetic diversity?

    <p>Homologous recombination</p> Signup and view all the answers

    What is the primary function of homologous recombination in DNA repair?

    <p>Restoring the original DNA sequence</p> Signup and view all the answers

    Which repair mechanism restores the original DNA sequence after a break has occurred?

    <p>Homologous recombination</p> Signup and view all the answers

    'Double strand breaks can be repaired by two mechanisms'. Which one of the following mechanisms is more difficult to accomplish?

    <p>Homologous recombination</p> Signup and view all the answers

    What type of DNA lesions are repaired by nucleotide excision, including those formed from UV light?

    <p>Thymine dimers</p> Signup and view all the answers

    Which specific type of radiation primarily causes the formation of cyclobutane pyrimidine dimers (CPDs) in DNA?

    <p>UV light</p> Signup and view all the answers

    What is the general function of Excinucleases in the repair pathway involving large distortions in DNA?

    <p>Removal of DNA segments</p> Signup and view all the answers

    What is the repair pathway that involves removing a DNA segment by Excinucleases?

    <p>Nucleotide excision repair</p> Signup and view all the answers

    How do nucleotide additions play a role in nonhomologous end joining during DNA repair?

    <p>They guide end joining using microhomology</p> Signup and view all the answers

    What is the role of AlkB protein in humans related to DNA repair?

    <p>Catalyzes oxidative demethylation of certain DNA residues</p> Signup and view all the answers

    Which of the following is a function of FADH– in the E.coli photolyase repair mechanism?

    <p>Donates an electron to the pyrimidine dimer</p> Signup and view all the answers

    Which repair mechanism is more error-prone and results in altered DNA sequences when fixing a broken chromosome?

    <p>Nonhomologous end joining</p> Signup and view all the answers

    When does homologous recombination typically take place in relation to DNA duplication and cell division?

    <p>After DNA duplication but before cell division</p> Signup and view all the answers

    What is the primary function of the Ku70-Ku80 protein heterodimer in DNA repair?

    <p>Promotes NHEJ by improving binding to broken DNA ends</p> Signup and view all the answers

    Which mechanism is involved in repairing DNA double-strand breaks when cells have not yet duplicated their DNA?

    <p>Nonhomologous end joining</p> Signup and view all the answers

    What is the role of the homologous recombination pathway in terms of genetic diversity?

    <p>Source of DNA exchange and genetic diversity</p> Signup and view all the answers

    Study Notes

    DNA Repair Mechanisms and Functions

    • O6-methylguanine-DNA methyltransferase repairs methylated guanine, restoring DNA integrity by directly removing methyl groups.
    • ALKBH family enzymes demethylate 1-methyladenine and 3-methylcytosine to their respective bases, adenine and cytosine, in humans.
    • Photolyase in E. coli repairs UV-induced DNA damage by utilizing light energy to reverse cyclobutane pyrimidine dimers.
    • Tet enzyme family members, such as TET1, are α-ketoglutarate-Fe2+ dependent dioxygenases that catalyze the oxidative demethylation of 5-methylcytosine in DNA.
    • MTHFpolyGlu in E. coli photolyase acts as a cofactor that aids in the repair of DNA by transferring electrons during photoreactivation.
    • Nucleotide excision repair (NER) mechanisms handle various types of DNA lesions, including cyclobutane pyrimidine dimers (CPDs) caused by UV light.
    • Stable N2-dG adducts formed by benzo[a]pyrene exposure can block DNA replication and transcription, leading to mutations.
    • Direct DNA repair of photoproducts is achieved through photolyases, which specifically repair UV-induced lesions.
    • Photolyases target lesions created by UV radiation, utilizing visible light to drive repair reactions.
    • Nonhomologous end joining (NHEJ) efficiently joins DNA ends without loss or addition of nucleotides.
    • Microhomology aids NHEJ by providing short, matching sequences that facilitate the alignment and joining of broken DNA ends.
    • Error-prone repair mechanisms, such as NHEJ, can introduce mutations at repair junctions due to imprecise joining.
    • In homologous recombination repair, the replication fork's structure is restored using the sister chromatid as a template.
    • Homologous recombination is preferred for repairing double-strand breaks that occur soon after DNA replication.
    • The SOS response in bacteria, a mechanism to manage extensive DNA damage, involves various regulatory proteins and stress responses.
    • DNA Pol V plays a critical role in bacterial DNA repair by allowing DNA synthesis across damaged areas, albeit in an error-prone manner.
    • In mammals, specialized polymerases assist when replication forks stall at thymine-thymine dimers.
    • Stalled replicative polymerases signal the cell to activate DNA repair mechanisms to prevent replication failure.
    • ABC excinuclease initiates nucleotide excision repair by cutting DNA at damage sites in bacteria.
    • In eukaryotic nucleotide excision repair, the endonuclease removes 27-29 nucleotide segments after DNA strand cleavage.
    • If a replication fork encounters an unrepaired lesion, it leads to fork collapse, risking the loss of genetic information.
    • Homologous recombination uses an undamaged chromosome as a template for accurate repair.
    • Excinucleases recognize and excise damaged DNA segments, addressing substantial distortions in DNA.
    • Nucleotide excision repair (NER) removes damaged DNA segments facilitated by excinucleases.
    • Nonhomologous end joining may involve the addition of nucleotides to aid in the repair process.
    • AlkB protein in humans repairs 1-methyladenine and 3-methylcytosine by oxidation, reverting them to adenine and cytosine.
    • FADH– serves as a reactive intermediate in E. coli photolyase, playing a role in light-mediated DNA repair.
    • Error-prone repair mechanisms can alter original DNA sequences, particularly in nonhomologous end joining scenarios.
    • Homologous recombination generally occurs after DNA duplication and before cell division, aiding in accurate repair.
    • Ku70-Ku80 heterodimer is crucial for the recognition and initiation of repair at double-strand breaks in DNA.
    • The homologous recombination pathway contributes to genetic diversity by facilitating recombination and exchange between alleles.

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