When the chains pair-up, key goes with lock; hook with eye.

This time imagine that there is a break in both of the paired chains.  This type of damage, called a double-strand break.  Double-strand breaks can be caused by particularly damaging events such as alpha radiation.

Both strands have been broken.

This double-strand break can be very bad news for the cell. There are two methods for repairing this – template repair and end joining. If the cell has already made a copy of the DNA, because it’s about to divide, then the copy can be used to fix the double break. First the cell must tidy-up the broken ends ...

A double strand break.

Then the copy is used to fix one strand.

The copy is used to help the repair.

The second strand can then be fixed ...

The template helps the cell get the repair right.

and finally the two copies separated.

When the chains pair-up, key goes with lock; hook with eye.

This repair is very reliable, but needs the second copy to be available.  So the cell can only use this repair method when it's getting ready to divide.  Only then will it have a complete second copy of the instruction manual.