XPC detect distortions = UvrA in E. coli

Formation of the bubble involves the helicase activities of the proteins XPA and XPD (the equivalent to UvrB in E. coli) and the single-strand-binding protein RPA. The bubble creates cleavage sites 5’ to the lesion for a nuclease known as ERCC1-XPF and 3’ to the lesion for nuclease XPG (representing the function of UvrC). In higher cells, the resulting DNA strand is 24–32 nucleotides long. As in bacteria, the DNA strand is released to create a gap that is filled in by the action of DNA polymerase and ligase.

XPB and XPD are both helicases; the XPB helicase is required for promoter melting during transcription, while the XPD helicase performs the unwinding function in NER

TLS Online TPP Program

#Id: 2992

#Unit 3. Fundamental Processes

DNA polymerase I has a unique 5 '-3' exonuclease activity that can be combined with DNA synthesis to perform nick translation.

TLS Online TPP Program

#Id: 2993

#Unit 3. Fundamental Processes

The first DNA-synthesizing enzyme to be characterized was DNA polymerase I, which is a single polypeptide of 103 kD, The chain can be cleaved into two parts by proteolytic treatment.

The larger cleavage product (68 kD) is called the Klenow fragment. It is used in synthetic reactions in vitro, It contains the polymerase and the proofreading 3'-5'  exonuclease activities.

The small fragment (35 kD) possesses a 5 '-3' exonucleolytic activity, which excises small groups of nucleotides, up to - 10 bases at a time. This activity is coordinated with the synthetic/ proofreading activity. It provides DNA polymerase I with a unique ability to start replication in vitro at a nick in DNA.

TLS Online TPP Program

#Id: 2994

#Unit 3. Fundamental Processes

Nick translation has been a major technique for introducing radioactively labeled nucleotides into DNA in vitro.

TLS Online TPP Program

#Id: 2996

#Unit 3. Fundamental Processes

The coupled 5'-3' synthetic/ 3'-5' exonucleolytic action is used most extensively for filling in short single-stranded regions in double-stranded DNA. These regions arise during lagging strand DNA replication.

TLS Online TPP Program

#Id: 2997

#Unit 3. Fundamental Processes

These Okazaki fragments are found in replicating DNA in both prokaryotes and eukaryotes.

TLS Online TPP Program

#Id: 2998

#Unit 3. Fundamental Processes

On the leading strand (sometimes called the forward strand) DNA synthesis can proceed continuously in the 5 ' to 3’ direction as the parental duplex is unwound.