Overview of DNA Repair

Cells encounter various DNA damage types including base modifications, mismatches, and breaks. Single-strand breaks affect one DNA strand, while double-strand breaks damage both strands simultaneously. Environmental factors like radiation and chemicals cause these lesions. Cells have evolved specialized repair pathways to address each damage type and maintain genomic integrity.

Base pairing and its significance in DNA replication ensures accurate genetic information transfer. Repair systems use complementary base pairing to identify and correct errors. When incorrect bases are incorporated, repair enzymes recognize mismatches through disrupted pairing patterns. This allows cells to restore proper Watson-Crick base pairs and prevent mutations.

DNA damage can stall the cell cycle by halting replication fork progression. Sensor proteins detect stalled forks and activate checkpoint mechanisms. This pause allows time for repair machinery to fix lesions before replication resumes. If damage is too severe, cells may undergo apoptosis or enter permanent cell cycle arrest.

Cells use two primary pathways to repair double-strand breaks. Nonhomologous end joining directly ligates broken ends together quickly but less accurately. Homologous recombination uses a sister chromatid as a template for precise repair. The choice between pathways depends on cell cycle phase and break complexity.

Homologous recombination and strand invasion enable high-fidelity repair of double-strand breaks using identical DNA sequences. This pathway involves strand exchange between homologous chromosomes or sister chromatids. It restores genetic information accurately and is essential during meiosis and S/G2 phases of the cell cycle.

Different repair pathways recognize and fix specific damage types. Base excision repair removes damaged individual bases. Nucleotide excision repair excises bulky lesions affecting multiple bases. Mismatch repair corrects replication errors. This specialization ensures efficient and accurate repair of diverse DNA damage.

Restarting stalled replication forks is critical for completing DNA synthesis after damage repair. Specialized helicases and recombination proteins restart fork progression. This process prevents incomplete replication and ensures all genetic material is duplicated before cell division. Failure to restart forks can lead to genomic instability.