13. 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?
a. The labels will be equally incorporated
b. Uracil incorporated is similar to methionine but less than thymine
c. Uracil incorporated will be greater than methionine but less than thymine
d. Thymine and methionine incorporated will be greater than uracil
Rifampicin specifically inhibits bacterial RNA polymerase, blocking transcription while DNA replication and translation continue unaffected for 5 minutes. Thymine (DNA precursor) and methionine (protein precursor) show high radioactive incorporation, but uracil (RNA precursor) incorporation drops sharply. Correct answer: d. Thymine and methionine incorporated will be greater than uracil.
Experiment Setup
Bacterial culture receives rifampicin, then splits into fractions with radioactive thymine, uracil, or methionine added. After 5 minutes, lysis reveals label distribution in macromolecules. Rifampicin binds bacterial RNA polymerase β-subunit, preventing chain elongation beyond 2-3 nucleotides, halting mRNA synthesis. DNA polymerase and ribosomes function normally during this brief window.
Macromolecule Pathways
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Thymine → DNA: DNA replication proceeds via DNA polymerase, incorporating thymine into replicating chromosomes.
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Uracil → RNA: Transcription halts immediately; minimal uracil enters short aborted transcripts or stable RNAs.
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Methionine → Proteins: Translation uses existing mRNA pools, incorporating methionine into nascent polypeptides.
Option Analysis
| Option | Statement | Correct? | Reason |
|---|---|---|---|
| a | Labels equally incorporated | ❌ | Ignores rifampicin’s RNA-specific block; uracil sharply reduced |
| b | Uracil ≈ methionine < thymine | ❌ | Methionine unaffected by transcription block; uracil << both others |
| c | Uracil > methionine < thymine | ❌ | Reverses reality: protein synthesis outpaces blocked RNA synthesis |
| d | Thymine + methionine > uracil | ✅ | Matches selective RNA inhibition; DNA/protein synthesis continue |
This classic experiment demonstrates central dogma dependency: transcription feeds translation, but short-term mRNA pools sustain protein synthesis post-rifampicin. Key for CSIR NET molecular biology.
