Now that the sunny weather is almost here, it's timely to consider that mutations that inactivate nucleotide-excision repair (NER) have been associated with several genetic diseases, such as xeroderma pigmentosum (XP). In humans, NER is the sole repair pathway for cyclobutane pyrimidine photodimers, and thus people with XP are highly sensitive to light and are predisposed to sunlight-induced skin cancers. A) In many microorganisms there is another pathway for repairing pyrimidine photodimers. What is it? B) A consequence of the inability to repair UV-induced photodimers is stalled replication forks. What are the two ways to repair (or bypass) stalled/collapsed replication forks that were mentioned in class? C) In a laboratory experiment cultures of fibroblasts from healthy individuals and from patients with type G XP (XPG) were irradiated with UV light. After the DNA was isolated and denatured, it was found that the samples from the normal, healthy patients showed a significant reduction in the average molecular weight of ssDNA after UV exposure, but the XPG fibroblasts did not show the same reduction in average ssDNA molecular weight. Explain this observation. D) It has also been noted that XPG cells replicate normally in the absence of UV-induced DNA damage. Which step in the NER pathway is presumably defective in XPG patients?
A) in bacteria thymidine dimer formed by UV light is repaired by photo reactivation mechanism. In this mechanism after formation of dimer , photolyase enzyme binds with thymidine dimer. This enzyme has cofactor NAD. This cofactor absorb light and use this light energy to cleave in between dimer.
B) these stalled replication fork can be repair by either using lesion skipping or by TLS ( trans lesion patway ).
Special type of DNA polymerases which could tolerate bulky lesions in the template at their active site is used during TLS. DNA polymerase IV is involved in trans lesion synthesis of DNA. At the place of stall normal DNA polymerase is replaced with DNA polymerase IV . Which randomly insert bases mostaly T at staples places which is template independent.
Innlesion skipping , lesion is skipped and later missed part is restored by process of homologous revombination.
C) in healthy individuals, normal NER mechanism operate which correct the damage done by UV which lead to decreased ssDNA molecular weight.
While in XPG fibroblast, due to defect in XPG NER mechanism not occur . So damage is not repaired and ssDNA is not decreased.
D) XPG is an endonuclease, which cut DNA stand near to dimer.
Now that the sunny weather is almost here, it's timely to consider that mutations that inactivate...
Now that the sunny weather is almost here, it's timely to consider that mutations that inactivate nucleotide-excision repair (NER) have been associated with several genetic diseases, such as xeroderma pigmentosum (XP). In humans, NER is the sole repair pathway for cyclobutane pyrimidine photodimers, and thus people with XP are highly sensitive to light and are predisposed to sunlight-induced skin cancers. A) In many microorganisms there is another pathway for repairing pyrimidine photodimers. What is it? B) A consequence of the...