Exam Preparation Notes on DNA Repair Mechanisms

Assignment Overview

  • Total Marks: 30
  • Breakdown:
    • Question 1: 4 marks
    • Question 2: 4 marks
    • Question 3: 4 marks
    • Question 4: 3 marks
    • Question 5: 3 marks
    • Question 6: 6 marks
    • Question 7: 6 marks

Nucleotide Excision Repair

Effect of Missing Proteins

  1. UvrA

    • Role: Initiates the process of nucleotide excision repair by identifying DNA lesions.
    • Effect of Absence: Without UvrA, the repair system cannot recognize the damaged DNA, preventing repair and leading to accumulation of unrepaired lesions, which may result in mutations.

  2. UvrC

    • Role: Responsible for cutting the damaged DNA strand on both sides of the lesion.
    • Effect of Absence: If UvrC is missing, the cut necessary for excision cannot occur. The DNA will remain damaged without repair, leading to structural abnormalities or replication errors.

  3. UvrD

    • Role: Functions as a helicase to unwind DNA, facilitating the removal of the damaged section.
    • Effect of Absence: Lacking UvrD means the damaged segment cannot be efficiently excised from the DNA strand, resulting in persistent DNA damage.

  4. DNA Polymerase

    • Role: Fills in the gap left after the damaged DNA is excised.
    • Effect of Absence: Without DNA polymerase, the newly excised region cannot be repaired, causing potential propagation of unrepaired DNA during replication.

DNA Repair Mechanisms

Types of DNA Changes and Corresponding Repair Mechanisms

  1. Mutagen-induced Base Change (Eukaryotic Cell)

    • Mechanism: Nucleotide Excision Repair
    • Rationale: This mechanism is suited for structural alterations from mutagens.

  2. DNA Sequence Mistake (DNA Polymerase)

    • Mechanism: Mismatch Repair
    • Rationale: Corrects errors that occur during DNA replication by identifying mismatched bases.

  3. Thymine Dimer (Bacterial Cell)

    • Mechanism: Nucleotide Excision Repair
    • Rationale: Thymine dimers result from UV damage and can be effectively repaired by this method.

Mismatch Repair Process

Importance of Strand Discrimination
  • Rationale: Distinguishing between the template strand (parent) and newly synthesized strand (daughter) is vital for accurately correcting errors without modifying the correct sequence.
  • Mechanism: This is typically achieved through methylation; in bacteria, parental strands are often methylated while newly synthesized strands are not, allowing mismatch recognition.

mRNA Sequences and Codon Analysis

Sequence Comparison
  • Wild-type mRNA (5' to 3'): AUG GGA AGC UGG GGC CUU
  • Mutant mRNA: AUG GGA UGC UGG GGC CUU
    • Contains a codon change resulting in a potential amino acid substitution.
  • Reversion A: AUG GGA GGC UGG GGC CUU
    • Changes the mutant codon restoring original function but differs from wild-type.
  • Reversion B: AUG GGA AGC UGG GGC CUU
    • True reversion; restores the wild-type codon.
  • Reversion C: AUG GGA UGC UGG GGC CUU
    • Remains a mutant; characteristics unchanged.
Classification of Mutations
  • True Reversion: Restores original codon (Reversion B).
  • Second-site Reversion: Introduces a mutation restoring function without reverting codon (Reversion A).
  • Silent Mutation: Change in codon without altering amino acid.
  • Missense Mutation: Change in amino acid due to codon change (Mutant).

Translation and Mutation Effects

  1. Original DNA Coding Strand: 5' - ATG AAG TTT GGC CCA TAA - 3'

    • Corresponding mRNA: 5' - AUG AAG UUU GGC CCA UAA - 3'
    • Reading Frame: Start from AUG, translates into: Leu - Lys - Phe - Gly - Pro - Stop.

  2. After Point Mutation Change (Second 'A' to 'T'):

    • New DNA Sequence: 5' - ATG TAG TTT GGC CCA TAA - 3'
    • New mRNA Sequence: 5' - AUG AUC UUU GGC CCA UAA - 3'
    • New Reading Frame: Start from AUG, translates into: Met - Ile - Phe - Gly - Pro - Stop.

  3. Assessment of Mutation Effects:

    • The mutation results in a missense change altering one amino acid (from Lys to Ile).