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5 September, 15:12

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

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  1. 5 September, 17:32
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    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.
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