chapter 11

Chapter 11: DNA Damage, Repair, and Mutation

Course Information

  • Course Name: BIO 200

  • Instructor: Santamaria

  • Institution: University of Tampa

  • Semester: Fall 2025

Key Questions

  1. What is the molecular nature of mutations?

  2. What is the basis of spontaneous versus induced mutations?

  3. What are biological repair mechanisms?

  4. What are the examples of human genetic diseases caused by mutations?

Chapter Overview

  • Wild-type DNA sequence: Represents the standard sequence of DNA that is considered normal and accurate.

  • DNA repair: The instrumental process that rectifies errors or damages to DNA.

  • Types of DNA damage:

    • Base loss, alteration, mismatch, or crosslink.

    • Strand break.

  • Inaccurate DNA repair: Results in mutations; leads to a mutant DNA sequence.

Effects of Mutations on Fitness

  • Misconception: Not all mutations are harmful.

  • Role of mutations:

    • Necessary for the creation of new alleles or genetic variants.

    • Capable of being beneficial, neutral, or deleterious:

      • Beneficial: Enhances fitness.

      • Neutral: No significant effect on fitness.

      • Deleterious: Negatively impacts fitness.

Statistics on Mutations

  • According to various studies, approximately 40-75% of mutations may be beneficial, neutral, or marginally impactful.

Types of Point Mutations

  • Definition: Point mutations involve the alteration of a single base pair (bp).

    • Can arise from:

    • Base-pair substitutions.

    • Indels (insertions or deletions).

  • Example Substitution: Change from DNA sequence AGT to ACT.

Types of Base-Pair Substitutions

  • Transitions: Alterations that replace a purine with a purine or a pyrimidine with a pyrimidine.

  • Transversions: Changes that replace a purine with a pyrimidine or vice versa.

  • Commonality: Transitions are more common than transversions.

Indels

  • Description: Involve either the insertion or deletion of a nucleotide.

  • Complexity: Often categorized broadly as “indel” due to challenges in distinguishing between insertion and deletion events.

Consequences of Point Mutations

  • Dependence: Consequences depend on the location within the gene and mutation type.

Base-Pair Substitution Effects

  1. Silent Mutation: No change in the amino acid coded for, often occurring at the third position in a codon.

  2. Missense Mutation: Change results in a different amino acid; may be:

    • Conservative: Similar properties.

    • Nonconservative: Different properties.

  3. Nonsense Mutation: Introduces a premature stop codon.

Indel Effects in Open Reading Frames

  • If the deletion or insertion is not a multiple of three, a frameshift occurs, altering the reading frame of the downstream sequence.

Mutations Outside Coding Regions

  • Mutations may also impact regulatory elements or introns, potentially affecting RNA binding and splicing.

Timing and Occurrence of Mutations

  • Random mutations can happen at a constant rate in populations as demonstrated by Luria and Delbrück.

  • Current understanding: Mutations can occur in any cell at any time and are generally random.

Mechanism of Spontaneous Mutations

  • Spontaneous mutations can result from:

    • Tautomerization: Structural isomers of bases interconvert, affecting base pairing.

    • Ionization: Exchange of protons between bases, leading to mispairing.

    • Potential errors during replication by DNA polymerase may remain undetected.

Tautomerization

  • Structural isomers interconvert due to shifts in hydrogen atoms.

  • Example: Normal vs. tautomeric forms leading to incorrect base pairing.

Ionization

  • Ionization can affect base pairing, leading to mutations.

Replication Slippage

  • Occurs during DNA replication, particularly in repeated sequences causing insertions or deletions due to misalignment.

Other Mechanisms Leading to Mutations

  • Environmental factors may chemically react resulting in mutations:

    • Depurination: Loss of purine due to bond hydrolysis between the base and sugar.

    • Deamination: Loss of an amine group, changing C to T and A to G.

    • Oxidative damage: Many varieties of oxidative damage can occur in mammalian DNA.

Induced Mutations

  • In contrast to spontaneous mutations, induced mutations arise from external agents:

    • Mutagens: Chemical or physical agents like UV light or ionizing radiation can induce mutations.

Chemical Inducers

  1. Alkylating agents: Add alkyl groups (e.g., ethyl groups) leading to improper base pairing.

  2. Base Analogs: Similar compounds to DNA bases that can lead to mispairing and mutations.

Physical Agents

  • UV light: can cause pyrimidine dimers, which stall DNA polymerase during replication.

DNA Repair Mechanisms

  • Not all changes become permanent mutations; the cell has mechanisms to repair DNA damage.

Photoreactivation

  • Enzymatic process performed by photolyases that repair damage from UV light, converting pyrimidine dimers back to normal pairs.

Base Excision Repair (BER)

  • Targets small DNA lesions, recognizing damaged bases, removing them, and correctly filling in the gaps using template strands.

Mismatch Repair

  • Fixes improper base pairing and induces fidelity in replication by detect-and-excise mechanisms, enhancing overall DNA accuracy by 1000 times.

Example Questions

  1. Ionizing radiation in Chlamydomonas leading to lack of flagella suggests a mutation. Possible type options include: A. missense mutation, B. nonsense mutation, C. frameshift mutation, D. regulatory-region mutation, E. all of these.

  2. Mutation transition from 5’-TGAGTCTGAAGTCC-3’ to 5’-TGAGTCGGAAGTCC-3’ classified as: A. transversion, B. transition, C. insertion, D. deletion, E. frameshift mutation.

  3. Point mutation resulting in UGU to UGA indicates: A. conservative substitution, B. missense mutation, C. synonymous mutation, D. nonsense mutation, E. frameshift mutation.

  4. Factors giving rise to mutations include: A. UV light, B. exposure to oxygen radicals, C. replication slippage, D. exposure to alkylating agents, E. all of these.