Q38. Multiple RNA polymerase transcribes a DNA template, unwinding about 1.5 turns of DNA template per transcription bubble. From the structural information of classical B-DNA, how many transcription bubbles are possible for a 180 base pair DNA molecule?
B-DNA has 10 base pairs per helical turn. A 180 bp DNA molecule thus contains 18 total turns (180 / 10 = 18). Each transcription bubble unwinds 1.5 turns, so the maximum number of non-overlapping bubbles is 18 / 1.5 = 12.
Solution Breakdown
The key is calculating total helical turns in B-DNA and dividing by turns per bubble. Classical B-DNA structure features ~10 bp/turn (or 0.34 nm/bp, 3.4 nm/turn). For 180 bp: total length = 180 bp × 0.34 nm/bp = 61.2 nm; total turns = 61.2 nm / 3.4 nm/turn = 18 turns exactly (or directly 180 bp / 10 bp/turn = 18). Given 1.5 turns/bubble, maximum bubbles = 18 / 1.5 = 12.
Option Analysis
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(A) 12: Correct. Matches 18 total turns ÷ 1.5 turns/bubble = 12 bubbles.
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(B) 27: Incorrect. Possibly from confusing bp/turn (10) with turns/bubble (1.5): 180 / (10 / 1.5) ≈ 27, but ignores total turns calculation.
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(C) 6: Incorrect. Half of correct value; might assume 3 bp/turn or double-count bubble overlap unrealistically.
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(D) 270: Incorrect. Likely 180 bp / (10 bp/turn × 1.5), but misapplies bubble size in bp (~15 bp/bubble) without helical turns.
In molecular biology, understanding transcription bubbles in 180 base pair DNA is crucial for GATE Life Sciences aspirants tackling questions on RNA polymerase and B-DNA helical structure. This GATE question tests helical turns: B-DNA has 10 base pairs per turn, so a 180 base pair DNA molecule spans 18 turns total. Each bubble unwinds ~1.5 turns (matching ~12-15 bp observations), allowing 12 non-overlapping bubbles max (18 ÷ 1.5 = 12).
B-DNA Helical Parameters
Classical B-DNA features:
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10 bp per turn.
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0.34 nm per bp; 3.4 nm per turn.
For 180 base pair DNA, turns = 180 / 10 = 18.
Transcription Bubble Mechanics
RNA polymerase creates a bubble by unwinding ~1.5 turns (~12-14 bp) for template access. Multiple polymerases enable processive transcription without full reannealing.
Why 12 Bubbles?
Total turns (18) ÷ bubble turns (1.5) = 12. This assumes minimal overlap for maximal packing, ideal for the query.
GATE Tip: Focus on B-DNA turns per transcription bubble—common in exams.
