Final answer:
To repair DNA, cells identify the damaged strand by its disrupted structure and then use the complementary, undamaged strand as a guide for correction. Single strand damage and double strand breaks are addressed using different repair mechanisms such as nucleotide excision repair and homologous recombination, ensuring genetic integrity is maintained.
Step-by-step explanation:
Cells have evolved sophisticated DNA repair mechanisms to correct damage to their genetic material. The process of determining which strand to repair involves recognizing the damaged DNA and using the complementary, undamaged strand as a template for correction. When it comes to single strand damage, the intact strand guides the repair. However, for more severe cases such as double stranded breakage, cells may employ mechanisms like homologous recombination, which also relies on the sister chromatid for accurate repair.
During nucleotide excision repair (NER), a bulky helix-distorting change is recognized, and the damaged DNA is excised, followed by resynthesis using the undamaged strand as a template. Specialized versions of this process, such as Transcription-Coupled Repair (TCR), prioritize repair of actively transcribed genes.
In cases where the damage occurred prior to replication, repair mechanisms involve proteins like RecA that find homologous sequences to facilitate the repair without causing deletions or insertions in the DNA sequence.