Ans. a. Thymidine dimmers repair (Ref: Harper 27/e p345-346; Lehning 5/e p1003)This patient is most likely have xeroderma pigmentosa, in which damage produced by UV-rays is caused due to formation of pyrimidine (mainly thymidine) dimmers.The damage produced by UV rays is caused due to formation of pyrimidine (mainly thymine) dimmersQ. The thymine dimmer prevent DNA polymerase from replicatingQ the DNA strand beyond the dimmer, thus inhibiting the repair of defective DNA.In Xeroderma Pigmentosa mechanism for repair (nucleotide excision repair)Q of damaged DNA is defective due to formation of Thymidine dimmersQ.Xeroderma Pigmentosa (AR)An autosomal recessive condition in which cells cannot repair the damaged DNA.The clinical syndrome include marked sensitivity to sun lightQ resulting inSkin cancersQPremature deathQArises because of defect in 'nucleotide excision' pathway of repair due to defect in either of three:UV specific endonuclease (MC defect)Q DNA polymerase 1QDNA IigaseQ Mechanism of DNA RepairMechanismProblemSolutionMismatch RepairCopying errorsQMethyl directed strand cutting, exonudease digestion and replacementBase Excision RepairSpontaneous or chemical, or radiation damage to a single baseQBase removal by N-glycosylase, a basic sugar removal, replacementNucleotide Excision RepairSpontaneous or chemical, or radiation damage to a single segmentQRemoval of approximately 30 nucleotide oligomer and replacementDouble Strand Break repairIonizing radiation, chemotherapy,oxidative free radicalsQSynapsis, unwinding, alignment, ligationAtaxia telangiectasia, Bloom's syndrome, Fanconi's anemia, Xeroderma pigmentosa, HNPCC and few breast cancers are due to DNA repair defectsQ.