Question

What is the mechanism that is used to “repair Dna “ in prokaryotic bacteria, what are the steps that are followed abd does it apply to eukaryotic cells?

Answer 1

Nucleotide Excision Repair is the mechanism used to repair DNA in prokaryotes. For example in Escherichia coli, it is mediated by the uvr ABC endonuclease complex. This endonuclease complex essentially consists of four proteins : Uvr A, Uvr B, Uvr C and DNA helicase II. The steps are:

• Scanning of DNA by UvrA- Uvr B complex; Uvr A recognizes the distortions in the pyrimidine dimers.
• Recognition is then followed by leaving of Uvr A . Uvr C then joins in and binds to Uvr B monomer.
• Uvr B cleaves a phosphodiester bond 4 nucleotides downstream of damaged DNA while Uvr C cleaves 8 nucleotides upstream to create 12 nucleotides excised segment.
• DNA helicase II then cleaves the segment by breaking of hydrogen bonds of complementary base pairs.
• The gap is filled up by DNA polymerase I.

In eukaryotic cells, NER is a kind of DNA repair but is much more complex as compared to prokaryotes. Many proteins are involved such as XPA/B/C/D/E/F/G as well as CSA, CSB. Additional proteins are also present like ERCC1, RPA, RAD23A etc. The repair is of two types: global genomic NER and transcription coupled NER. Different sets of proteins are involved in each stage of repair. The main steps include dual incision, repair and ligation.