Xeroderma pigmentosa is due to:
Base excision defect
Nucleotide excision defect
Sos repair defect
Cross linking defect
Ans: b (Nucleotide excision defect) Ref: Vasudevan, 4th ed, p. 403; 5th ed, p. 421Steps in nucleotide excision repair:Repair: Excision endonuclease makes nick on both sides of thymine dimer and removes defective DNA. DNA polymerase fill the gap by synthesising DNA in 5'-3' direction. DNA Ligase seals the nick in the repaired strand .Most DNA repair occurs in G1 phase. Mismatch repair occurs in G2 phase.
Molecular Genetics