Final answer:
Double-stranded DNA enters a cell and through homologous recombination, one strand is used to repair DNA with the help of the RecA protein. The process ensures accurate repair without deletions or insertions, reflecting the evolutionary conservation of these mechanisms across species.
Step-by-step explanation:
Homologous Recombination and DNA Repair
When double-stranded DNA enters a cell, one strand may be degraded while the complementary strand participates in homologous recombination, provided it is homologous to the host cell's chromosome. The protein RecA (or its homologs RadA in Archaea and Rad51 in Eukaryotes) plays a crucial role in this process. During the repair of double-stranded breaks, proteins like MRX/MRN and Sae2 recognize the break site, while helicases such as Sgs1 unwind the DNA. The RecA protein, after forming a nucleoprotein filament around the remaining single strand, searches for homologous sequences in the DNA and promotes strand invasion. This results in the accurate repair of the DNA without any deletions or insertions.
The DNA repair mechanisms are evolutionarily conserved across species, suggesting their importance in maintaining the integrity and accessibility of essential genetic information. In single-strand break repair, the RecA-DNA filament also ensures accurate repair, highlighting the fidelity of the homologous recombination repair pathway.