Q.46 Adenine can undergo a spontaneous change to hypoxanthine in a cell, leading to
a DNA base pair mismatch. The CORRECT combination of enzymes that are
involved in repairing this damage is
(A) Nuclease, DNA polymerase, DNA ligase
(B) Nuclease, DNA ligase, helicase
(C) Primase, DNA polymerase, DNA ligase
(D) Primase, helicase, DNA polymerase
Adenine deamination to hypoxanthine causes a mutagenic DNA mismatch repaired primarily through base excision repair (BER). The correct enzyme combination is nuclease, DNA polymerase, and DNA ligase.
Repair Mechanism Overview
Hypoxanthine from adenine deamination pairs with cytosine instead of thymine, risking AT-to-GC mutations during replication. BER initiates with a DNA glycosylase (a nuclease) excising the damaged base, creating an apurinic site processed by endonucleases. DNA polymerase then fills the gap with correct nucleotides, and DNA ligase seals the backbone.
Correct Answer: Option (A)
Option (A) nuclease, DNA polymerase, DNA ligase matches BER steps precisely. Nuclease (e.g., AlkA glycosylase or endonuclease like EndoV) removes hypoxanthine. DNA polymerase synthesizes the replacement strand, and DNA ligase joins the phosphodiester bonds.
Why Other Options Fail
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Option (B) Nuclease, DNA ligase, helicase: Lacks DNA polymerase for gap filling; helicase unwinds DNA but skips synthesis.
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Option (C) Primase, DNA polymerase, DNA ligase: Primase synthesizes RNA primers for replication, irrelevant to mismatch excision repair.
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Option (D) Primase, helicase, DNA polymerase: Missing nuclease for damage removal; primase and helicase suit replication, not repair.
