Unit 5: DNA Repair, Mutations and Transposons

1. Mechanism of DNA Repair

DNA is a highly stable molecule, but it is constantly subjected to damage from environmental factors and errors during replication. If left unrepaired, these damages can lead to mutations or cell death. Living cells have evolved several sophisticated mechanisms to repair damaged DNA.

Types of DNA Repair Mechanisms

• Direct Repair: Some damages can be repaired directly without removing any nucleotides. For example, Photoreactivation uses the enzyme DNA photolyase to break pyrimidine dimers caused by UV light.
• Excision Repair: This involves removing the damaged segment and replacing it with new DNA. It is divided into:
  • Base Excision Repair (BER): Corrects damage to a single nitrogenous base using enzymes called DNA glycosylases.
  • Nucleotide Excision Repair (NER): Repairs larger lesions, like thymine dimers, by removing a short oligonucleotide fragment containing the damage.
• Mismatch Repair (MMR): Corrects errors that escaped proofreading during DNA replication, such as mispaired bases.
• Double-Strand Break Repair: Severe breaks in both strands are repaired via Homologous Recombination or Non-Homologous End Joining (NHEJ).

2. Types and Mechanism of Gene Mutation, Mutagens

A mutation is a sudden, heritable change in the DNA sequence of an organism. Mutations can be spontaneous or induced by external factors.

Types of Gene Mutations

• Point Mutations: Changes involving a single nucleotide pair.
  • Transitions: Replacement of a purine with another purine (A ↔ G) or a pyrimidine with another pyrimidine (C ↔ T).
  • Transversions: Replacement of a purine with a pyrimidine or vice versa.
• Silent Mutations: A change in a codon that does not change the amino acid produced (due to the degeneracy of the genetic code).
• Missense Mutations: A change that results in a different amino acid being incorporated into the protein.
• Nonsense Mutations: A change that creates a premature stop codon, leading to a truncated, usually non-functional protein.
• Frameshift Mutations: Additions or deletions of nucleotides (not in multiples of three) that shift the reading frame of the mRNA.

Mutagens

Definition: Agents that significantly increase the frequency of mutations are called mutagens.

3. Basic Concept of Transposons and Transposable Element

Transposable elements (TEs), also known as "jumping genes," are DNA sequences that can move from one location to another within the genome. They were first discovered by Barbara McClintock in maize.

Key Characteristics

• They can increase genome size and introduce genetic diversity through their movement.
• They can cause mutations if they insert themselves into a functional gene.

Classification of Transposons

• Class I (Retrotransposons): Move via an RNA intermediate. They are "copied and pasted" into a new location (e.g., LINEs and SINEs in humans).
• Class II (DNA Transposons): Move directly as DNA. They are "cut and pasted" from one site to another using the enzyme transposase.

4. Exam Focus Enhancements