Molecular Biology: Base Excision Repair

Mutation Basics

• A mutation is a stable and heritable change in a DNA sequence.
• Mutations create phenotypes distinct from the wildtype allele, which can arise from base substitutions, duplications, deletions, or chromosomal aberrations like inversions and translocations.

Classification of Mutations

• Dominant mutations (M2) can express a mutant phenotype in heterozygotes, while recessive mutations (m1) typically yield a wildtype appearance in heterozygotes.
• Semidominant mutations showcase varying severity in homozygotes compared to heterozygotes.

Muller's Classification of Mutants

• Developed by H.J. Muller in 1932, this classification includes:
  • Nullomorphs: Complete loss of gene function.
  • Hypomorphs: Reduced gene function, insufficient for wildtype phenotype.
  • Hypermorphs: Enhanced or excess gene activity.
  • Antimorphs: Mutations that inhibit wildtype protein function.
  • Neomorphs: Mutations that create new functions.

Types of Mutants

• Nullomorphs: No functional gene product remains; can have various defects in gene expression.
• Hypomorphs: Produce residual activity but insufficient to manifest a normal phenotype, often recessive.
• Hypermorphs: Generate overactive protein products or excess normal proteins; useful for examining genetic processes.
• Antimorphs: Produce products that negatively affect wildtype function; typically dominant.

Mutations Overview

• They can be classified into loss-of-function and gain-of-function mutations.
• Essential terms include wild-type phenotype, forward and reverse mutations, and neutral mutations.

Suppressor Mutations

• Intragenic suppressor mutations occur within the same gene, compensating for previous mutations.
• Intergenic suppressor mutations occur in different genes and can also counteract mutations.

Mutation Rate

• The mutation rate indicates how frequently wild-type alleles convert to mutant alleles, expressed per cell division or gamete.
• Factors influencing mutation rates include the occurrence of primary DNA changes and probabilities related to mutation detection.
• Mutation rates for viral and bacterial organisms are approximately 1 to 100 mutations per 10 billion cells, while eukaryotic genes show rates of 1 to 10 mutations per million gametes.

Causes of Mutations

• Spontaneous replication errors:
  • Tautomeric shifts, wobble formation, strand slippage, unequal crossing over.
• Spontaneous chemical changes:
  • Depurination and deamination.

Detection of Mutations

• Luria and Delbrück fluctuation test helps establish spontaneous mutation rates.
• Ames test (1974) uses Salmonella typhimurium to detect mutagenicity, focusing on reversions of specific mutations.

DNA Damage and Repair

• DNA damage can result from radiation, chemical mutagens, and spontaneous decay.
• More than 99% of DNA lesions are repaired, and various pathways exist for DNA repair:
  • Direct reversal mechanisms (alkyltransferases).
  • Base excision repair (specific glycosylases address modified bases).
  • Nucleotide excision repair (removes bulky lesions).
  • Mismatch repair systems based on recognition and correction of errors in DNA replication.

Nonhomologous End-Joining and Homologous Recombination

• Double-strand breaks can be repaired by nonhomologous end-joining or homologous recombination.
• Homologous recombination uses the sister chromatid as a template to repair broken strands.

Faulty DNA Repair Examples

• Xeroderma pigmentosum: Autosomal recessive disorder linked to failure in repairing UV-induced thymine dimers, leading to high skin cancer risk.
• Li-Fraumeni syndrome: Associated with mutations in p53, increasing propensity for various cancers due to impaired cell cycle regulation and DNA repair mechanisms.

Explore the intricate processes of base excision repair in this quiz. Dive into the roles of uracil glycosylase and AP endonuclease, understanding how these enzymes interact with modified bases in both prokaryotic and eukaryotic systems. Test your knowledge on the mechanisms of DNA repair at a molecular level.