Practice question

Ku heterodimer composed of 70 kDa and 80 kDa subunits forms preformed ring with central aperture approximately 30 angstroms accommodating double-stranded DNA ends without sequence specificity. Abundant nuclear protein diffuses, threading onto broken terminus within seconds, protecting end from nucleolytic degradation and acting as scaffold for non-homologous end joining machinery. Ku recruits DNA-dependent protein kinase catalytic subunit, Artemis nuclease, polymerases mu and lambda, and XRCC4-Ligase IV complex. By synapsing two Ku-bound ends, it holds broken chromosome together. Rapid binding also antagonizes resection, steering repair away from homologous recombination.