Depurination Mechanism in DNA
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Questions and Answers

What occurs as a result of the loss of a purine base by spontaneous hydrolysis of the N-glycosidic bond?

  • A pyrimidine dimer is formed
  • A DNA base is skipped
  • An apurinic site is created (correct)
  • A wrong base is added to the newly synthesized strand

    • What is the rate of depurination under physiological conditions?

  • 500 bases/cell/day
  • 500000 bases/cell/day
  • 50000 bases/cell/day
  • 5000 bases/cell/day (correct)

    • What type of damage occurs when reactive oxygen species attack DNA?

  • Radiation damage
  • Depurination
  • Oxidative damage (correct)
  • Replication errors

    • What is the major product of DNA oxidation?

    <p>8-oxoguanine</p> Signup and view all the answers

    What type of radiation can cause DNA damage?

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

    What occurs when pyrimidines are exposed to UV light?

    <p>They form a pyrimidine dimer</p> Signup and view all the answers

    What is the result of thymine dimers in DNA?

    <p>Proper replication is prevented</p> Signup and view all the answers

    What happens to a cell if it does not undergo repair after DNA damage?

    <p>It becomes malignant</p> Signup and view all the answers

    What type of dimer is formed as a result of UV radiation?

    <p>Cyclobutane pyrimidine dimer</p> Signup and view all the answers

    What mechanism normally detects and repairs replication errors?

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

    What is one way that ionizing radiation damages DNA molecules?

    <p>By producing double strand breaks</p> Signup and view all the answers

    What type of mutagen can induce mutations by changing the chemical structure of DNA bases?

    <p>Base modifying agents</p> Signup and view all the answers

    What is the effect of intercalating agents on DNA structure?

    <p>They cause frameshift mutations</p> Signup and view all the answers

    What is the purpose of DNA repair mechanisms?

    <p>To identify and correct DNA lesions</p> Signup and view all the answers

    What type of repair strategy involves the direct reversal of DNA lesions?

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

    What is an example of a base analog?

    <p>2-amino purine</p> Signup and view all the answers

    What is the reason behind the rarity of replication errors in DNA?

    <p>High fidelity of DNA replication process</p> Signup and view all the answers

    What is the direction of the exonuclease activity of DNA polymerases?

    <p>3'-5'</p> Signup and view all the answers

    What is the purpose of the mismatch repair system?

    <p>To recognize and correct errors escaped from DNA polymerase proofreading</p> Signup and view all the answers

    What is the characteristic of the Homologous recombination repair mechanism?

    <p>Error-free</p> Signup and view all the answers

    What is the consequence of unrepaired double-strand breaks?

    <p>Chromosomal aberration</p> Signup and view all the answers

    What type of repair system is involved in correcting lesions that cause bulk distortions in DNA helix?

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

    What happens to mutations if they are not corrected by DNA repair systems?

    <p>They are replicated, become permanent, and inherited</p> Signup and view all the answers

    Which enzyme is involved in direct repair and transfers a methyl group from G to a cysteine residue within the enzyme itself?

    <p>O6-methylguanine methyltransferase</p> Signup and view all the answers

    What is the result of the action of DNA-N-glycosylases in base excision repair?

    <p>Removal of damaged bases resulting in an apurinic or apyrimidinic site</p> Signup and view all the answers

    What is the role of AP endonucleases in base excision repair?

    <p>To nick the damaged backbone at the 5' end of the AP site</p> Signup and view all the answers

    What is the consequence of a defect in nucleotide excision repair?

    <p>Both A and B</p> Signup and view all the answers

    What type of repair system corrects errors introduced during DNA replication?

    <p>Strand-directed mismatch repair</p> Signup and view all the answers

    What is the duration of the DNA region removed during nucleotide excision repair?

    <p>12-24 nucleotides</p> Signup and view all the answers

    What is the enzyme responsible for repairing UV-induced damage in plants, bacteria, and some animals?

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

    What type of mutation occurs when a single nucleotide base is altered in the DNA sequence?

    <p>Point mutation</p> Signup and view all the answers

    What is the result of a nonsense mutation?

    <p>A stop codon is formed, resulting in a truncated protein</p> Signup and view all the answers

    What type of mutation occurs when the number of nucleotides inserted or deleted is not a multiple of three?

    <p>Frameshift mutation</p> Signup and view all the answers

    What is a transition in the context of point mutations?

    <p>A purine is replaced with another purine</p> Signup and view all the answers

    What type of mutation has no effect on the protein sequence?

    <p>Silent mutation</p> Signup and view all the answers

    What is the term for a missense mutation where the new amino acid is chemically similar to the original one?

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

    Study Notes

    DNA Damage and Mutation

    • DNA damage occurs through various mechanisms, including:
      • Depurination: loss of purine bases due to spontaneous hydrolysis of N-glycosidic bonds, resulting in apurinic sites (AP sites)
      • Replication errors: incorrect incorporation of nucleotides during DNA replication, leading to base pair deletions, insertions, or substitutions
      • Oxidative damage: chemical modification of nitrogenous bases due to reactive oxygen species (ROS), resulting in mispairing
      • Exogenous DNA damage: radiation and chemical mutagens

    Exogenous DNA Damage

    • Radiation damage:
      • UV light: forms pyrimidine dimers (thymine dimers) through intra-strand crosslinking, altering DNA structure and preventing proper replication
      • Ionizing radiation: causes direct damage to DNA molecules, producing single-strand breaks (SSBs) and double-strand breaks (DSBs), and indirect damage through production of free radicals
    • Chemical mutagens:
      • Base modifying agents: alter the chemical structure of DNA bases, leading to mispairing (e.g., alkylating agents like SAM)
      • Base analogs: replace standard DNA bases, leading to base pair substitutions (e.g., 2-amino purine replacing adenine)
      • Intercalating agents: insert themselves between adjacent DNA bases, affecting DNA structure and leading to frameshift mutations (e.g., acridine orange, benzopyrene)

    DNA Repair Pathways

    • Direct repair:
      • Photolyases: repair UV-induced damage in plants, bacteria, and some animals (excluding humans) by splitting dimers
      • O6-methylguanine methyltransferase: transfers a methyl group from G to a cysteine residue within the enzyme, repairing damage
    • Base excision repair (BER):
      • Enzymes like uracil DNA glycosylase recognize and remove damaged bases, resulting in AP sites
      • AP endonucleases nick the damaged backbone, and DNA polymerase and ligase repair the region
    • Nucleotide excision repair (NER):
      • Corrects lesions causing bulk distortions in DNA helix, such as UV-induced pyrimidine dimers
      • Involves recognition, excision, and resynthesis of the damaged region
    • Strand-directed mismatch repair (MMR):
      • Corrects errors introduced during DNA replication, such as base substitutions, deletions, and insertions
      • Recognizes and corrects errors that escaped DNA polymerase proofreading machinery
    • Double-strand breaks repair:
      • Two types: non-homologous end joining (error-prone) and homologous recombination (error-free)

    Mutations

    • Frameshift mutation: alteration in the reading frame due to insertion or deletion of nucleotides
    • Point mutation: alteration in a single nucleotide base, resulting in a change in a single base pair (substitution)
      • Transition: replacement of one purine with another purine or one pyrimidine with another pyrimidine
      • Transversion: replacement of one purine with one pyrimidine or vice versa
    • Types of point mutations:
      • Silent mutation: no effect on protein sequence
      • Nonsense mutation: truncation of protein due to premature stop codon
      • Missense mutation: alteration in amino acid sequence, potentially affecting protein function

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    DNA Damage, Mutations, and Repair Mechanisms PDF

    Description

    This quiz covers the process of depurination, a spontaneous hydrolysis of the N-glycosidic bond that links purine bases to deoxyribose in DNA, and its consequences on DNA replication and mutation. Learn about the repair mechanisms of apurinic sites.

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