Mutation and Mutagenesis

Mutation and Mutagenesis - Genetic Code Glitches

• Mutation: Heritable change in DNA.
• Types:
  • Point Mutations (single base affected):
    • Silent: Codes for same amino acid (AA); no change in protein.
    • Missense: Codes for different AA; may alter protein function.
    • Nonsense: Codes for STOP codon (UAG, UAA, UGA); premature termination.
  • Frameshift Mutations: Insertion or deletion of bases not in multiples of 3; alters reading frame downstream, often creating non-functional protein.
• Mutagens (Causes):
  • Spontaneous: e.g., DNA replication errors, deamination.
  • Induced:
    • Physical: UV radiation (forms pyrimidine dimers, e.g., thymine dimers), Ionizing radiation (X-rays, gamma rays - cause DNA strand breaks).
    • Chemical: Base analogs (e.g., 5-bromouracil), Intercalating agents (e.g., ethidium bromide, acridine orange), Alkylating agents. and frameshift mutation)

⭐ Nonsense mutations (UAG, UAA, UGA) result in premature termination of protein synthesis, often leading to truncated, non-functional proteins. 📌 STop A Garbage, STop All Activity, STop Going Anywhere (UAG, UAA, UGA are stop codons).

Mutation and Mutagenesis - Agents of Alteration

Mutagens: Agents causing DNA damage, increasing mutation rates.

Table: Classification of Mutagens

Mutation and Mutagenesis - Cellular Fix-It Crew

• Cells employ diverse DNA repair systems to maintain genomic stability against spontaneous or induced damage.
• Major Repair Pathways:
  • Direct Reversal:
    • Photoreactivation: Light-dependent repair of pyrimidine dimers by photolyase.
  • Excision Repair:
    • Base Excision Repair (BER): Targets non-bulky lesions (e.g., uracil, alkylated bases).
    • Nucleotide Excision Repair (NER): Removes bulky adducts & pyrimidine dimers (e.g., UV damage).
    • Mismatch Repair (MMR): Corrects replication errors (base mismatches, small indels).
  • Double-Strand Break (DSB) Repair:
    • Homologous Recombination (HR): High-fidelity, uses sister chromatid.
    • Non-Homologous End Joining (NHEJ): Error-prone, direct ligation.

⭐ The SOS response in bacteria is an inducible, error-prone DNA repair system activated by extensive DNA damage, increasing mutation rates but allowing survival.

Mutation and Mutagenesis - Effects & Detection

• Effects of Mutations:

  • Phenotypic:
    • Silent: No amino acid (AA) change.
    • Missense: Different AA; conservative or non-conservative.
    • Nonsense: Premature STOP codon; truncated protein.
    • Frameshift: Insertion/deletion (not multiple of 3); altered reading frame.
  • Functional: Loss-of-function (null, hypomorph), Gain-of-function (hypermorph, neomorph), Lethal, Conditional (e.g., temperature-sensitive), Antibiotic resistance.

• Detection of Mutations:

  • Phenotypic Screening: Replica plating (auxotrophs, antibiotic resistance), specific indicator media.
  • Genotypic Screening: PCR-based methods (e.g., RFLP, ASO probes), DNA sequencing.

• Detection of Mutagens (Ames Test):

  

  ⭐ The Ames test utilizes histidine auxotrophs of Salmonella typhimurium to screen for chemical mutagens by detecting reversion to prototrophy, often with liver extract (S9) to simulate metabolic activation.

High‑Yield Points - ⚡ Biggest Takeaways

• Point mutations: silent (no change), missense (amino acid change), nonsense (premature STOP codon).
• Frameshift mutations: insertions/deletions not multiple of 3, alters downstream reading frame.
• Mutagens: Physical (UV radiation → pyrimidine dimers); Chemical (base analogs, intercalating agents).
• Ames test: detects chemical mutagens via reversion in auxotrophic bacteria (e.g., Salmonella His-).
• DNA repair: NER (Nucleotide Excision Repair) fixes pyrimidine dimers; MMR (Mismatch Repair) corrects replication errors.
• Transposons ("jumping genes"): cause insertional mutagenesis and can alter gene expression.