8.11
Reparatie van nucleotide-excisie
DNA-vervorming en schade
Cellen worden regelmatig blootgesteld aan mutagenen: factoren uit de omgeving die het DNA kunnen beschadigen en mutaties kunn…
Nucleotide excision repair, or NER, fixes bulky DNA lesions caused by UV radiation and chemical carcinogens.
These bulky lesions, such as pyrimidine dimers, distort the DNA helix and interfere with DNA replication and transcription.
In E. coli, the repair proteins UvrA and UvrB form a complex that scans the genomic DNA for structural lesions.
Once it encounters a lesion, the UvrA dissociates, causing UvrB to bind tightly to the distorted DNA.
The DNA-bound UvrB then recruits UvrC, an endonuclease, which makes an incision on either side of the lesion.
Next, the UvrD helicase unwinds the DNA and excises the fragment carrying the lesion.
Following excision, DNA polymerase fills the gap with new complementary nucleotides, and the enzyme DNA ligase seals the gap between the new and old DNA, completing the repair.
Alternately, if an actively transcribing RNA polymerase encounters the bulky lesion, it can directly recruit the UvrB to the site of the lesion.
This initiates transcription-coupled nucleotide excision repair that proceeds in the same manner as regular NER to complete the repair and continue transcription.
Q1: What types of DNA damage does nucleotide excision repair fix?
Nucleotide excision repair fixes bulky DNA lesions caused by UV radiation and chemical carcinogens, such as pyrimidine dimers. These lesions distort the DNA helix and interfere with DNA replication and transcription. If left unrepaired, they can cause mutations leading to cancer or disease depending on which DNA sequences are disrupted.
Q2: How do UvrA and UvrB proteins identify damaged DNA in prokaryotes?
In E. coli, UvrA and UvrB form a complex that scans genomic DNA for structural lesions. When the complex encounters a lesion, UvrA dissociates and UvrB binds tightly to the distorted DNA region. This recognition of physical aberrations allows the repair machinery to flag the damaged area for removal and repair.
Q3: What is the role of UvrC endonuclease in nucleotide excision repair?
UvrC is an endonuclease recruited by DNA-bound UvrB that makes incisions on either side of the lesion. These cuts allow the damaged DNA fragment to be excised from the strand. Following excision, DNA polymerase fills the gap with new complementary nucleotides, and DNA ligase seals the repair.
Q4: How does transcription-coupled nucleotide excision repair differ from regular NER?
Transcription-coupled NER is initiated when an actively transcribing RNA polymerase encounters a bulky lesion and directly recruits UvrB to the damage site. This alternative pathway proceeds identically to regular NER but is triggered by transcription blockage rather than routine DNA scanning, allowing repair and transcription to resume.
Q5: Why do eukaryotes require more proteins than prokaryotes for nucleotide excision repair?
Unlike prokaryotes, which use three proteins—UvrA, UvrB, and UvrC—eukaryotes employ more than a dozen proteins to regulate nucleotide excision repair. This increased complexity reflects the greater size and organization of eukaryotic genomes and the need for more sophisticated damage recognition and coordination mechanisms.
Q6: What happens when nucleotide excision repair genes are mutated in humans?
Mutations in the NER pathway cause diseases such as Xeroderma pigmentosum, associated with a 2000-fold increase in skin cancer incidence. XP patients are highly sensitive to UV exposure, developing severe burns after brief sunlight exposure, and often show premature aging and neurological abnormalities. Without functional repair, DNA damage accumulates, leading to abnormal cell death or cancerous tumors.
Q7: What is the final step after DNA polymerase fills the gap in nucleotide excision repair?
After DNA polymerase fills the gap with new complementary nucleotides, the enzyme DNA ligase seals the gap between the new and old DNA strands. This ligation completes the nucleotide excision repair process and restores the integrity of the DNA molecule.