Q.45 A 5250 base–pair long plasmid with 10 negative supercoils would have a linking
number of ____________, considering 10.5 base pairs per turn for B DNA.
(Answer in integer)
The linking number for a 5250 base-pair plasmid with 10 negative supercoils is 490, using 10.5 base pairs per turn for B-DNA.
DNA Topology Basics
DNA topology describes how the double helix twists and coils in closed circular molecules like plasmids. The linking number (Lk) is a fixed topological invariant equal to twist (Tw) plus writhe (Wr): Lk = Tw + Wr.
For relaxed B-DNA, writhe is zero (Wr = 0), so Lk₀ = Tw₀ = N/h, where N is base pairs (5250) and h is base pairs per turn (10.5). Negative supercoils mean Wr < 0, reducing Lk below Lk₀.
Step-by-Step Calculation
- First, compute relaxed turns:
Tw₀ = 5250/10.5 = 500. Thus,Lk₀ = 500. - With 10 negative supercoils, Wr = -10 (assuming Tw remains ~500, as supercoiling partitions mostly into writhe).
- Therefore,
Lk = 500 + (-10) = 490.
Superhelical Density
Superhelical density σ = (Lk - Lk₀)/Lk₀ = -10/500 = -0.02, confirming mild negative supercoiling typical for plasmids.
Introduction to Plasmid Linking Number Calculation
Plasmid linking number calculation is essential for understanding DNA supercoiling in molecular biology, especially for competitive exams like CSIR NET. This guide solves a 5250 base-pair plasmid with 10 negative supercoils at 10.5 base pairs per turn for B-DNA, yielding Lk=490.
Key Concepts in DNA Supercoiling
- Linking Number (Lk): Total times strands link:
Lk = Tw + Wr. - Twist (Tw): Helical turns, ~10.5 bp/turn in relaxed B-DNA.
- Writhe (Wr): Superhelical coiling; negative for underwound DNA.
- Negative supercoils (Wr=-10) reduce Lk, aiding replication/transcription.
Detailed Plasmid Linking Number Calculation
- Relaxed turns:
Lk₀ = 5250/10.5 = 500. - Add supercoils:
Lk = Lk₀ + Wr = 500 - 10 = 490. - This integer matches exam expectations for CSIR NET Life Sciences.
Applications in CSIR NET and Biotech
- Mastering plasmid linking number calculation helps in questions on topoisomerases, gyrase, and topology.
- Negative supercoiling (~-0.06 density in vivo) compacts DNA and facilitates biological processes.
- Essential for understanding plasmid behavior in genetic engineering and molecular cloning.
CSIR NET Exam Tip
No multiple-choice options needed – direct integer answer: 490. Remember the formula Lk = Lk₀ + Wr where Wr = – (number of negative supercoils).
