Figure 2 FLASH ANIMATION

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Figure 2 | The essential features of nucleotide excision repair.  a | Nucleotide excision repair (NER) operates on base damage caused by exogenous agents (such as mutagenic and carcinogenic chemicals and photoproducts generated by sunlight exposure) that cause alterations in the chemistry and structure of the DNA duplex . b | Such damage is recognized by a protein called XPC, which is stably bound to another protein called HHRAD23B (R23). c | The binding of the XPC–HHRAD23 heterodimeric subcomplex is followed by the binding of several other proteins (XPA, RPA, TFIIH and XPG). Of these, XPA and RPA are believed to facilitate specific recognition of base damage. TFIIH is a subcomplex of the RNA polymerase II transcription initiation machinery which also operates during NER. It consists of six subunits and contains two DNA helicase activities (XPB and XPD) that unwind the DNA duplex in the immediate vicinity of the base damage. This local denaturation generates a bubble in the DNA, the ends of which comprise junctions between duplex and single-stranded DNA. d | The subsequent binding of the ERCC1–XPF heterodimeric subcomplex generates a completely assembled NER multiprotein complex. e | XPG is a duplex/single-stranded DNA endonuclease that cuts the damaged strand at such junctions 3' to the site of base damage. Conversely, the ERCC1–XPF heterodimeric protein is a duplex/single-stranded DNA endonuclease that cuts the damaged strand at such junctions 5' to the site of base damage. This bimodal incision generates an oligonucleotide fragment 27–30 nucleotides in length which includes the damaged base. f | This fragment is excised from the genome, concomitant with restoring the potential 27–30 nucleotide gap by repair synthesis. Repair synthesis requires DNA polymerases or , as well as the accessory replication proteins PCNA, RPA and RFC. The covalent integrity of the damaged strand is then restored by DNA ligase. g | Collectively, these biochemical events return the damaged DNA to its native chemistry and configuration. ERCC1, excision repair cross-complementing 1; PCNA, proliferating cell nuclear antigen; POL, polymerase; RFC, replication factor C; RPA, replication protein A; TFIIH, transcription factor IIH; XP, xeroderma pigmentosum.
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