# DBT JRF 2022 — Question 16

Exam: DBT JRF 2022 · Section: B · Topic: Molecular Biology › DNA Sequencing & ddNTPs · Difficulty: Medium

## Question

Which one of the molecular processes utilize 2', 3' Dideoxy-ribonucleoside triphosphates?

## Options

- **A.** DNA synthesis for primers.
- **B.** Okazaki fragment formation during lagging strand synthesis.
- **C.** DNA sequencing by synthesis.
- **D.** Transcript chain elongation.

## Answer

Option C

## Explanation

2',3'-Dideoxynucleoside triphosphates (ddNTPs) — application: Structure: ddNTPs lack both the 2'-OH and 3'-OH groups on the ribose sugar. Function: When incorporated into a growing DNA chain by DNA polymerase, the ABSENCE of the 3'-OH means the next nucleotide CANNOT be added (no free 3'-OH for phosphodiester bond formation). This terminates chain elongation. Application: Sanger sequencing (chain termination method / sequencing by synthesis): 1. A template is copied by DNA polymerase in the presence of dNTPs (normal) + a small amount of ddNTPs (chain terminators labelled with fluorescent dyes). 2. Termination occurs randomly at each position where a specific base is incorporated. 3. The resulting fragments of all lengths are separated and detected to read the sequence. Distractors: • DNA synthesis for primers: primers use regular dNTPs. • Okazaki fragment formation: also uses regular dNTPs. • Transcript elongation: uses NTPs (ribonucleotides), not ddNTPs. Answer: DNA sequencing by synthesis (Option 3).

Source: https://mypyqbuddy.com/questions/which-one-of-the-molecular-processes-utilize-2-6a104cd5920734408bc0c71f