Overview
This lecture explains the sources and types of DNA mutations, their impact on protein synthesis, and the cellular mechanisms used to repair DNA and maintain genomic integrity.
DNA Stability and Replication Errors
- DNA is highly stable, but not immune to errors or mutations.
- DNA polymerase makes an error about once every 100,000 nucleotides during replication.
- Given the large genome size, even a low error rate can lead to thousands of errors per cell division.
- Uncorrected errors in coding regions can be especially harmful.
Sources of DNA Mutations
- Mutagens are agents that cause DNA mutations, including radiation (UV, gamma rays, X-rays) and chemicals (cleaning products, cigarette smoke, heavy metals).
- Some mutagens are endogenous, created within the body.
- Avoidable exposures (e.g., smoking, unnecessary chemicals) should be minimized to reduce mutation risk.
Types of DNA Mutations
- Copy Number Variations: Large sections of DNA are deleted or duplicated, affecting gene dosage.
- Point Mutations: Single nucleotide changes in DNA.
- Silent Mutation: Alters DNA sequence but not the amino acid, so protein remains unchanged.
- Nonsense Mutation: Converts a codon to a stop codon, resulting in a truncated, often nonfunctional protein.
- Missense Mutation: Changes the amino acid in the protein.
- Conservative: Amino acid change has similar properties, modest effect.
- Non-conservative: Amino acid change is very different, major impact on protein structure/function.
- Frameshift Mutations: Insertion or deletion of nucleotides shifts the reading frame, altering every amino acid after the mutation and often producing nonfunctional proteins.
DNA Repair Mechanisms
- Over 100 enzymes continuously monitor and repair DNA errors.
- Mismatch Repair: Fixes base pairing errors after replication; defects are linked to hereditary cancers.
- Nucleotide Excision Repair: Removes damaged DNA segments (lesions), replaces with correct DNA using polymerase and ligase.
- Not all errors are fixed; mutations that escape repair and are replicated become permanent.
Maintaining DNA Integrity
- Repair mechanisms are robust but not infallible; some mutations are inevitable.
- Avoiding avoidable mutagen exposure (e.g., use sunscreen, avoid smoke, wear protective equipment) is important for DNA protection.
- Ensuring DNA integrity is crucial for cellular and organismal longevity.
Key Terms & Definitions
- Mutagen — An agent that induces DNA mutations.
- Point Mutation — A change in a single nucleotide in DNA.
- Silent Mutation — Mutation that does not alter the amino acid sequence.
- Nonsense Mutation — Mutation that introduces a premature stop codon.
- Missense Mutation — Mutation that results in a different amino acid in the protein.
- Frameshift Mutation — Mutation caused by insertion/deletion, altering the reading frame.
- Mismatch Repair — DNA repair system correcting erroneous base pairs.
- Nucleotide Excision Repair — DNA repair system removing bulky DNA lesions.
Action Items / Next Steps
- Review types of DNA mutations and their effects.
- Study DNA repair mechanisms, focusing on mismatch and nucleotide excision repair.
- Practice avoiding unnecessary mutagen exposure in daily life.