Practice question

After double-strand break, Ku heterodimer comprising Ku70 and Ku80 forms ring-shaped structure that threads onto each DNA end with high affinity irrespective of sequence, protecting ends from excessive resection. Ku serves as scaffold recruiting DNA-dependent protein kinase catalytic subunit, end-processing factors, and Ligase IV complex. By blocking homologous recombination machinery, Ku channels repair toward NHEJ, especially in mammalian G1 cells or immune lymphocytes generating immunoglobulin diversity. Absence of Ku shifts repair toward microhomology-mediated alternative end joining or homologous recombination pathways, increasing resection length and genome instability.