20. 5-Bromouracil is a base analog that can cause mutation when incorporated into DNA. Which of the following is the most likely change that 5-Bromouracil induces:
(1) T:A to C:G (2)T:A to A:T
(3) G:C to T:A (4) C:G to A:T
5-Bromouracil (5-BU), a thymine analog, primarily induces T:A to C:G transitions by tautomerizing to its enol form, which pairs with guanine instead of adenine during DNA replication. The correct answer is (1) T:A to C:G. This mutation arises because 5-BU incorporates opposite adenine in its keto form but mispairs with guanine in subsequent replications.
Mechanism of 5-Bromouracil Mutagenesis
5-BU mimics thymine and substitutes for it in DNA, pairing normally with adenine (A) in its common keto form. Bromine at the 5-position increases the enol tautomer frequency, enabling hydrogen bonding with guanine (G). After two replication cycles, an original T:A pair becomes C:G, a classic transition mutation.
Option Analysis
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(1) T:A to C:G: Correct. Keto 5-BU pairs with A; enol 5-BU pairs with G, leading to GC incorporation opposite 5-BU in the next round.
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(2) T:A to A:T: Incorrect. This represents no change or AT to AT, not induced by 5-BU mispairing.
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(3) G:C to T:A: Incorrect. 5-BU targets thymine sites, causing AT to GC, not the reverse; reverse requires different conditions.
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(4) C:G to A:T: Incorrect. This is the opposite transition; 5-BU induces forward AT→GC primarily, though bidirectional possible in theory.
Implications for CSIR NET Life Sciences
Understanding 5-BU helps in mutagenesis topics, linking to base analog effects in molecular genetics. It induces point mutations via substitution, not frameshifts. Exam tip: Focus on tautomerism for transition predictions.
