Lecture on DNA Mutations

Introduction

• DNA Replication: High accuracy in living organisms, but errors occur approximately once in every billion nucleotides.
• Mutations: Errors in DNA replication, can be increased by mutagens.

Types of Mutations

Base Substitution Mutations

• Definition: A single nucleotide is replaced by another.
• Types:
  • Silent Mutations:
    • No change in amino acid sequence.
    • Affect genotype, not phenotype.
  • Missense Mutations:
    • Change in a single amino acid.
    • Impact can be harmful, neutral, or beneficial.
  • Nonsense Mutations:
    • Changes codon to a stop codon.
    • Generally harmful, causing premature termination of protein synthesis.

Frameshift Mutations

• Definition: Insertion or deletion of nucleotides, alters reading frame.
• Effects: Changes every amino acid following the mutation site, often results in non-functional proteins.
• Types:
  • Frameshift Insertion: Addition of nucleotides.
  • Frameshift Deletion: Removal of nucleotides.

Mutagens and Their Effects

Ionizing Radiation

• Sources: X-rays, gamma rays.
• Effects: Causes electrons to be lost, leading to highly reactive ions and free radicals, potentially breaking the sugar-phosphate backbone of DNA.

Non-ionizing Radiation

• Source: UV light.
• Effects: Causes thymine bases to bind covalently (thymine dimers), hindering DNA replication and transcription.

Nucleoside Analogues

• Characteristics: Structurally similar to normal bases but with different pairing properties.
• Example: 5-bromo uracil, which pairs with guanine instead of adenine, leading to base substitution mutations.

Chemical Mutagens

• Nitrous Acid: Alters adenine to pair with cytosine, causing base substitution mutations.
• Acridine, Benzopyrene, Ethidium Bromide:
  • Function: Cause insertions or deletions leading to frameshift mutations.
  • Properties: Intercalate into DNA, causing bulges during replication.

DNA Repair Mechanisms (mentioned briefly)

• Photolyases
• Excision repair enzymes
• Mismatch repair enzymes

These notes summarize the key points on DNA mutations, types, causes, and the effects of various mutagens presented in the lecture.