Single-strand break repair is easier than double-strand break repair because the unaltered strand of DNA can be used as a template. It is important for the cell to determine which strand of the DNA has the accurate sequence and which strand has the defect. This will allow the cell to use the correct strand of DNA as a template to choose the correct base without corrupting the genetic sequence.
If a normal base is placed in the wrong position during replication and is not corrected by proofreading, the cell will use mismatch repair system (MMR). In this type of DNA repair, a surveillance protein detects the incorrect base and recruits repair enzymes to the site. Enzymes such as endonuclease and exonuclease remove the incorrect base. DNA polymerase will add the correct base in the same position. Finally, DNA ligase will seal the nick in the DNA strand.
If a base has been chemically altered such that the base no longer has a normal structure, the cell will need to use either base-excision repair (BER) or nucleotide-excision repair (NER). BER uses an enzyme called glycosylase to remove the damaged base. Then two more enzymes, AP endonuclease and DNA polymerase, add correct base(s) to the position(s). BER is used when one base is affected. NER is used when the damage is more bulky such as when two bases are linked (dimerized) due to UV damage. This process follows the same steps as BER, but requires the removal of a section of DNA to include the areas upstream and downstream of the damage.
