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The most important factor for carcinogenesis is the DNA absorption, that produces specific pyrimidine photoproducts.The energy absorbed by DNA produces molecular changes that involve single bases, interactions between adjacent and nonadjacent bases, and between DNA and proteins. The relative proportions of DNA photoproducts can be very sensitive to wavelength.The major photoproducts produced by UVB are dimerizations between adjacent pyrimidines.Other minor photoproducts include purinepurine and purinepyrimidine photoadducts, photohydrations, and photooxidations. They may, however, behave as premutagenic lesions in specific sites.Some conditions, especially dehydration and binding of unique proteins, can change DNA conformation and result in other kinds of UV photoproducts   being formed.In bacterial spores, for example, DNA adopts a special conformation similar to dehydrated DNA and is surrounded by small acidsoluble spore proteins, such that a unique photoproduct is formed, thyminyl, dihydrothymine. These may be more important in photoaging, for example, than DNA photoproducts formed by UVB.Model for the sequential assembly of the various components of nucleotide excision repair.Transcription coupled repair involves initial response to damage by stalling of the RNA polymerase II apparatus, and coupling by the CSA and CSB proteins.Subsequent steps proceed in common, consisting of loading the XPG nuclease, the XPARPA DNA binding proteins, and the ERCCXPF nuclease.In part, it is the need for interacting proteins in repair that gives rise to complex overlapping symptoms in some patients with mutations in these genes.These processes involve sequential steps of photoproduct recognition, assembly of the DNA binding proteins, remodeling by helicases and excision nucleases, displacement of the excised fragment, and polymerization of the replacement patch. The efficiency of NER is determined by the particular photoproduct, the bases flanking the damage, DNA conformation, bound proteins, transcriptional activity of both the gene and DNA strand containing the damage, and additional factors including p, GADD, and others. The rate of excision of photoproducts, therefore, represents a dynamic balance between strand breakage and rejoining, subject to many modulating factors on an individual nucleotide and gene basis.Only a small fraction of dimers are acted upon at any one time and the process removes them sequentially over a long period.The repair process, in principle, involves removal of a nt oligonucleotide containing the photoproduct by precisely positioned cleavages nt on the side of the photoproduct, and nt on the side. Once this oligonucleotide is removed, the resulting gap is filled in by DNA polymerase delta, proliferating cell nuclear antigen. The individual factors of NER associate sequentially and independently on UV photoproducts, to a first approximation, without preassembly of a repairosome complex. The nuclease complex plus the nt single strand <a href="http://www.targetmol.com/compound/Estriol">sell Estriol</a> fragment is released by the action of transcription factor TFIIH which contains both helicases.Two major branches of NER are distinguished by the relationship to transcriptional activity of the genes being repaired described as transcriptioncoupled repair. The initial damage recognition mechanism for TCR may be the stalled RNA pol II, itself.Two genes, CSA CSB, are involved specifically in TCR.UV damage in CS cells results in a failure of DNA and RNA synthesis to recover to normal levels after UV irradiation.

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