# DBT JRF 2020 — Question 149

Exam: DBT JRF 2020 · Section: B · Topic: Molecular Biology · Difficulty: Hard

## Question

A covalently closed circular DNA was in a relaxed state in water at 30 °C. What will happen if the water temperature increases to 60 °C or decreases to 10 °C?

## Options

- **A.** DNA will be relaxed at 60 °C and positively supercoiled at 10 °C
- **B.** DNA will undergo positive supercoiling at 60 °C and negative supercoiling at 10 °C
- **C.** DNA will be negatively supercoiled at both 60 °C and 10 °C
- **D.** DNA will undergo negative supercoiling at 60 °C and positive supercoiling at 10 °C

## Answer

Option B

## Explanation

Covalently closed circular (ccc) DNA topology: The linking number $Lk = Tw + Wr$ (Lk = linking number, Tw = twist, Wr = writhe) is FIXED for ccc DNA. At 30°C relaxed: Lk = Tw $_0$ (corresponding to ~10.5 bp/turn B-DNA), Wr = 0. Effect of INCREASED temperature (60°C): DNA becomes more underwound (partial melting/unwinding); Tw decreases (fewer bp per turn). Since Lk is fixed, Wr must increase positively to compensate: $\Delta Wr = -\Delta Tw > 0$ → POSITIVE SUPERCOILING. Effect of DECREASED temperature (10°C): DNA winds more tightly (increased bp per turn — like A-DNA-like packing); Tw increases. Since Lk fixed, Wr must decrease: $\Delta Wr = -\Delta Tw < 0$ → NEGATIVE SUPERCOILING. Result: 60°C → positive supercoiling; 10°C → negative supercoiling.

Source: https://mypyqbuddy.com/questions/a-covalently-closed-circular-dna-was-in-a-6a05a8a02adafec4702932ad