Q.30 Rifampicin, an inhibitor of RNA polymerase, is added to a bacterial culture.
The culture is immediately divided into three fractions, to which radioactive
thymine or uracil or methionine are added, respectively. Five minutes later
the cells are lysed and the radioactive label in all macromolecules is monitored.

What will you observe?

1. Thymine and methionine incorporated will be greater than uracil.
2. The labels will be equally incorporated.
3. Uracil incorporated is similar to methionine but less than thymine.
4. Uracil incorporated will be greater than methionine but less than thymine.

   Solving the MCQ: Rifampicin Effect on Bacterial Culture

   Rifampicin inhibits bacterial RNA polymerase, blocking transcription initiation after 2-3 nucleotides, so mRNA synthesis stops immediately. DNA replication (thymine) and protein synthesis (methionine) continue briefly using existing mRNA, while RNA synthesis (uracil) halts. In this 5-minute experiment, radioactive labels reveal incorporation differences.

   Option 1: Thymine and methionine incorporated will be greater than uracil

   • Thymine (DNA): High incorporation via ongoing replication.

   • Methionine (proteins): High incorporation using short-lived existing mRNA.

   • Uracil (RNA): Very low due to instant transcription block.

   • Verdict: Correct. Matches rifampicin’s selective RNA inhibition.

   Option 2: The labels will be equally incorporated

   • All processes don’t halt equally; RNA stops first, DNA/protein persist.

   • Verdict: Wrong. Ignores rifampicin’s specific RNA polymerase target.

   Option 3: Uracil incorporated is similar to methionine but less than thymine

   • Uracil minimal; methionine moderate but higher than uracil.

   • Verdict: Wrong. Underestimates RNA block severity.

   Option 4: Uracil incorporated will be greater than methionine but less than thymine

   • Uracil can’t exceed methionine; no new mRNA for translation.

   • Verdict: Wrong. Reverses actual hierarchy.

   Correct Answer: Option 1. Thymine/methionine > uracil reflects transcription halt vs. continued replication/translation.

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   Rifampicin RNA polymerase inhibition is a classic experiment testing selective effects on bacterial macromolecule synthesis, key for GATE Life Sciences prep. When rifampicin blocks transcription in a bacterial culture labeled with radioactive thymine (DNA), uracil (RNA), or methionine (proteins), results show thymine and methionine incorporated greater than uracil after 5 minutes. This article dissects the mechanism, options, and exam insights.

   Rifampicin Mechanism: Transcription Block

   Rifampicin binds bacterial RNA polymerase β-subunit, preventing RNA chain elongation beyond 2-3 nucleotides.

   • RNA synthesis (uracil) stops immediately.

   • DNA replication (thymine) continues independently.

   • Protein synthesis (methionine) uses stable existing mRNA briefly.

   In the experiment: Culture split post-rifampicin; labels added; lysed after 5 min. Minimal uracil in RNA vs. high thymine in DNA and methionine in proteins.

   MCQ Breakdown: Experiment Observations

   GATE-style question analyzes incorporation:

   Option	Observation	Correct?	Reason
   1	Thymine & methionine > uracil	Yes	DNA/protein ongoing; RNA halted.
   2	Equal incorporation	No	Processes unequal.
   3	Uracil ≈ methionine < thymine	No	Uracil minimal, not similar to methionine.
   4	Uracil > methionine < thymine	No	Uracil lowest.

   Thymine and methionine incorporated greater than uracil is the precise outcome.

   Exam Tips for Rifampicin Experiments

   Recall: Rifampicin targets initiation/early elongation, sparing replication/translation short-term. Common trap: Assuming total halt. Links to antibiotics like actinomycin D (different block).