• It is also called double stranded DNA repair
because in this phenomenon repair of the two
broken ends of the DNA takes place and protect
the genome from further degradation .
•Double stranded breaks are generated by
exposure to ionizing radiation or chemical
mutagens or during recombination events .
• A multi-component protein complex directs a DNA
Ligase to the break.
• The complex contains a protein called Ku, having two
non – identical subunit binds to two broken ends of
• The binding of Ku is accomplished by the DNA -PK
• It activates a third protein XRCC4, which interacts with
the mammalian DNA Ligase –IV directing this repair
to join the double strands broken ends.
• Like recombination, non-homologous ends are joined
by this process.
• Therefore, this type of repair process is also
called non- homologous end joining repair