Because these categories of DNA damage are different, there is a need for multiple repair systems.
The first type of repair system we will discuss is the excision repair system. To excise simply means to remove, so this repair systems works by removing the area of damage. Special enzymes recognize damaged DNA. This repair system comes in two forms: Base-excision repair and short-patch nucleotide excision.
Base-pair Excision Repair
In base-pair excision, single base-pairs are identified and removed. The resultant gap is then filled with a DNA polymerase and the nick is sealed by a DNA ligase.
Short-patch Excision Repair
Short-patch excision varies from base-pair excision in that its enzymes will recognize and remove "short patches" of DNA that are damaged. These short patches of damage arise from bulky lesions such as thymine dimers. This form of damage is radiation-induced and leads to the formation of a bond between adjacent thymine bases on the same strand of DNA. This bond leads to a distortion in the DNA that makes a short stretch around the thymine dimer unable to base pair correctly. The short-patch excision repair system recognizes such distortions and cuts the damaged strand on both sides of the damaged region leaving a 12 base pair gap in the strand. A helicase then unwinds the stretch of the helix with the damage that can then be filled and sealed with DNA polymerase and ligase. The short-patch excision repair can also be used to correct damage resulting from unnatural bases.