Checks to see if the DNA has been damaged and if the cell is large enough and healthy enough to divide.

Depending on the type of damage in the DNA, different enzymes can act to correct a base damage or to correct mutations in the sequence.

Explanation:

Base excision repair (BER) : in this case, there is a subtle base damage in which a DNA glycosylase, extrudes a base in a damaged base pair. Then, the apurinic or apyrimidinic site attracts DNA repair enzymes removing the remaining nucleotide and adding a normal one. in Eukaryotes, it is the DNA polymerase B who plays this role.

Nucleotide Excision Repair (NER) : there is a bulky base damage that is repaired by an enzyme called uvrABC endonuclease in bacteria. It is not necessary the activity of any DNA polymerase in this case.

Double-strand DNA breaks: In mammals, this damage could be repair by homologous recombination. The process could be carried out by enzymes Ku and DNA-PK which binds together the DNA ends.

Mismatch repair: cells have a mismatch repair system that, in DNA replication, recognizes the parental strand (because it is methylated) and correct the error in the complementary strand.

Errors during DNA replication: some DNA polymerases have an exonuclease activity in 3'-5' and 5'-3'. The 3'-5' exonuclease activity is important in a process called proofreading, in which a newly added wrong base is removed and replaced with the correct one. The 5'-3' activity is important in the DNA repair process: it can remove and replace a strand in any place of the DNA.