Which statement correctly describes base excision repair and nucleotide excision repair?

• A. BER removes bulky lesions via nucleotide excision complex and fills the gap; NER removes single damaged bases via glycosylases.
• B. BER removes single damaged bases via glycosylases and fills the gap; NER removes bulky lesions via nucleotide excision complex and fills.
• C. BER and NER both synthesize DNA.
• D. BER repairs double-strand breaks.

Answer: (B)

Base excision repair targets small, non-bulky base damage. A DNA glycosylase recognizes and removes the damaged base, leaving an abasic site; an AP endonuclease cuts the backbone, and a DNA polymerase with ligase fills in and seals the single-nucleotide gap. Nucleotide excision repair handles bulky, helix-distorting lesions such as UV-induced thymine dimers. A damage-recognition complex makes incisions on both sides of the lesion and removes a short DNA segment, then DNA polymerase fills the gap and ligase seals the strand. The statement aligns with this distinction: BER removes single damaged bases via glycosylases and fills the gap; NER removes bulky lesions via the nucleotide excision complex and fills. Other choices misstate which lesions each pathway targets or the nature of their repair steps, such as claiming BER repairs double-strand breaks or that both pathways simply synthesize DNA without their specific lesion-targeting roles.