Notes on Mutations and DNA Repair Mechanisms

Overview of Mutations

  • Types of Mutations:
    • Substitution Mutations:
    • Occurs when one base is replaced by another due to chemical changes.
    • Can lead to a different base pair after DNA replication.
    • Large Scale Mutations:
    • Deletions: Loss of a segment of the DNA.
    • Insertions: Addition of an extra segment into the DNA.
    • Translocations: A segment of DNA is moved from one location to another.
    • Inversions: A segment of DNA is reversed.
  • Causes of Larger DNA Mutations:
    • X-rays:
    • Can break the DNA backbone leading to mutations during repair processes.
    • UV Light:
    • Causes distortion in the DNA by creating dimers between adjacent thymine bases.
    • This change forces DNA polymerase to halt, necessitating repair.

Mechanism of Mutation Repair

  • Thymine Dimer Repair:
    • UV light causes neighboring thymine bases to bond improperly, creating a kink in DNA.
    • Enzymes cut out the damaged strand, allowing DNA polymerase to fill the gap.
    • DNA Ligase:
    • Joins two ends of the repaired strand to restore the DNA molecule.

DNA Polymerases and Repair Mechanisms

  • Low Fidelity Polymerase in E. Coli:
    • Used during repair, this enzyme has a higher chance of introducing errors.
  • Apoptosis:
    • Occurs in multicellular organisms when DNA damage is too severe for repair, leading to programmed cell death.
    • This process helps eliminate cells with extensive mutations, such as those from excessive UV exposure.

Types of Errors and Their Corrections

  • Deamination of Cytosine:
    • Loss of an amine group converts cytosine to uracil, which pairs incorrectly with adenine.
    • Specific enzymes identify and repair mismatches, removing uracil and replacing it with the correct base.
  • Proofreading Activity of DNA Polymerases:
    • Major enzymes like DNA polymerases I and III have proofreading capabilities to fix errors during replication.
    • Improves accuracy, reducing error rate to once per billion base pairs.

Mismatch Repair Mechanism (in E. Coli)

  • Methylation Tags:
    • Parental DNA strands are chemically tagged with methyl groups, distinguishing them from the newly synthesized strands.
    • Enzymes recognize mismatches based on methylation status:
    • Correct one: Methylated strand (old strand)
    • Erroneous one: Unmethylated strand (new strand)
    • Enzymes cut and replace the incorrect base after identifying the mismatch using methylation as an identifier.

Unequal Crossing Over

  • Meiosis and Mutations:
    • Unequal crossing over may lead to deletions or duplications of genes, such as photoreceptor genes in visual systems.
    • Advantageous mutations can lead to varied visual capabilities in species over time through evolution.

Summary of Learning Objectives

  • Understanding DNA Mutations:
    • Be able to describe different types of mutations and how they occur.
    • Predict the effects of such mutations based on DNA changes.
    • Interpret results of experiments related to mutations.
    • Define key terms discussed in the lessons on mutations and its repair mechanisms.