# DBT JRF 2022 — Question 36

Exam: DBT JRF 2022 · Section: B · Topic: Molecular Biology › DNA Base Composition & GC Content · Difficulty: Medium

## Question

DNA isolated from four unidentified species of bacteria - A, B, C and D has been estimated to have 38%, 26%, 24% and 12% thymine, respectively. One of these four species was isolated from a hot spring (64°C). Identify the candidate which is most likely to be a thermophilic bacterium.

## Options

- **A.** A
- **B.** B
- **C.** C
- **D.** D

## Answer

Option D

## Explanation

Identifying the thermophile from % base composition: In double-stranded DNA, by Chargaff's rules: %A = %T and %G = %C. Given species D has 12% T: by Chargaff's rule, %A = 12%. Therefore %G + %C = 100% − 12% − 12% = 76%. This is the highest G:C content among the species listed (A, B, C, D). G:C base pairs (3 hydrogen bonds) confer greater thermostability than A:T (2 H-bonds). Higher G:C content → higher DNA melting temperature ( $T_m$ ) → adapted to high-temperature environments → thermophilic. Conclusion: Species D with 76% G:C content is the thermophile. Answer: Species D.

Source: https://mypyqbuddy.com/questions/dna-isolated-from-four-unidentified-species-of-bacteria-6a104cd5920734408bc0c733