# DBT JRF 2020 — Question 8

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

## Question

Which one of the following can be extended by Klenow enzyme upon addition of dNTPs and Mg2+ ?

## Options

- **A.** single-stranded DNA
- **B.** restriction fragment with a 5' overhang
- **C.** restriction fragment with blunt ends
- **D.** restriction fragment with a 3' overhang

## Answer

Option B

## Explanation

Klenow fragment is the large C-terminal fragment of E. coli DNA Polymerase I, generated by subtilisin cleavage. It retains 5'→3' polymerase activity and 3'→5' exonuclease (proofreading) activity, but lacks the 5'→3' exonuclease activity of the intact Pol I. To extend DNA, Klenow needs a recessed 3'-OH end (a primer annealed to a template). A restriction fragment with a 5' overhang (sticky end) provides exactly this: the 3' end of the complementary strand is recessed, and Klenow can fill it in using the 5' overhanging strand as template, converting it to a blunt end. 3' overhangs cannot be extended (the 3' end is already protruding) and blunt ends have no recessed 3'-OH to extend.

Source: https://mypyqbuddy.com/questions/which-one-of-the-following-can-be-extended-6a05a8a02adafec470293220