Xeroderma pigmentosa is due to:

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.