# DBT JRF 2022 — Question 32

Exam: DBT JRF 2022 · Section: A · Topic: Molecular Biology › DNA Structure & Melting Temperature · Difficulty: Medium

## Question

The melting temperature ( $t_m$ ) of DNA is higher when the content of:

## Options

- **A.** A:T base pairs is higher.
- **B.** G:C base pairs is higher.
- **C.** A:T base pairs is same as G:C base pairs.
- **D.** G:C base pairs is lower.

## Answer

Option B

## Explanation

DNA melting temperature ( $T_m$ ) and G:C content: The melting temperature ( $T_m$ ) is the temperature at which 50% of double-stranded DNA is denatured. Approximate formula for short sequences: $T_m \approx 4 \times (G + C) + 2 \times (A + T)$ (in °C, for oligonucleotides) Reason G:C raises $T_m$ : • G:C base pair: stabilised by 3 hydrogen bonds • A:T base pair: stabilised by only 2 hydrogen bonds More G:C → more hydrogen bonds → greater thermal energy required to separate strands → higher $T_m$ . Answer: G:C base pair content is higher (Option 2).

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