Final answer:
In the NHEJ pathway, the Ku heterodimer (Ku70/Ku80) and DNA-PKcs are the main proteins that recognize and bind to double-stranded breaks in DNA, initiating repair.
Step-by-step explanation:
The proteins involved in the first step of Non-Homologous End Joining (NHEJ), during which double-stranded breaks are recognized, include the Ku heterodimer consisting of Ku70 and Ku80, and the DNA-dependent protein kinase catalytic subunit (DNA-PKcs). When DNA double-stranded breaks occur, the Ku heterodimer rapidly binds to the DNA ends, providing initial end protection and recruitment of DNA-PKcs. This complex facilitates the NHEJ process by recruiting additional factors and ensuring the DNA ends are held together for subsequent processing and ligation.