Any of a number of processes to repair damaged DNA. In general, the damaged portions of the molecule are removed by enzymes (this could range from a single nitrogenous base to a large number of them, and the enzymes doing the removing depend on the specific type of damage), leaving holes where bases should be. Then other enzymes remove an entire segment of DNA, in the middle of which was the hole or holes. A DNA polymerase I then comes and fills the gap with new nitrogenous bases, based on what bases are on the opposite (presumably unharmed) strand of DNA (if both strands are harmed, a process is invoked which inserts bases at random into the strand). Finally a ligase enzyme seals the phosphate backbone back together.