Another example of a fail-safe system is a glycosylase that removes a T opposite a G. Such a T:G mismatch can arise, as we have seen, by spontaneous deamination of 5-methylcytosine,which occurs frequently in the DNA of vertebrates. 

TLS Online TPP Program

#Id: 6140

#Unit 3. Fundamental Processes

UV-induced cyclobutane pyrimidine dimers (CPDs), are not well recognized by XPC. In this case, the DNA damage-binding (DDB) complex assists in recruiting XPC to this type of damage.

TLS Online TPP Program

#Id: 6141

#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 6142

#Unit 3. Fundamental Processes

transcription-coupled repair (TC-NER) TFIIH, itself a large complex, which includes the products of two XP genes, XPB and XPD. 

TLS Online TPP Program

#Id: 6143

#Unit 3. Fundamental Processes

Cleavages are made on either side of the lesion by endonucleases encoded by the XPG and XPF genes. XPG is related to an endonuclease called FEN1 that cleaves DNA during the base excision repair pathway

TLS Online TPP Program

#Id: 6144

#Unit 3. Fundamental Processes

Finally, the single-stranded stretch including the damaged bases can then be replaced by new synthesis, and the final remaining nick is ligated by a complex of ligase III and XRCC1.

TLS Online TPP Program

#Id: 6145

#Unit 3. Fundamental Processes

Cockayne syndrome can be caused by mutations in either of two genes (CSA and CSB), both of whose products appear to be part of or bound to TFIIH, and can also be caused by specific mutations in XPB or XPD.