# DBT JRF 2020 — Question 36

Exam: DBT JRF 2020 · Section: A · Topic: Molecular Biology · Difficulty: Easy

## Question

The role of DNA ligase in DNA replication is

## Options

- **A.** base pairing of the template and the newly formed DNA strand
- **B.** formation of a phosphodiester bond between the 3'-OH of one Okazaki fragment and the 5'-phosphate of the next on the lagging strand
- **C.** addition of new nucleotides to the lagging strand
- **D.** addition of new nucleotides to the leading strand

## Answer

Option B

## Explanation

DNA ligase plays a critical role in joining Okazaki fragments on the lagging strand during DNA replication. Specifically, after RNase H removes the RNA primer and DNA Polymerase I fills the gap with DNA, a nick remains between the 3'-OH of the newly synthesised DNA and the 5'-phosphate of the adjacent Okazaki fragment. DNA ligase seals this nick by forming a phosphodiester bond, using NAD⁺ (bacteria) or ATP (eukaryotes) as an energy source. This sealing step completes lagging strand synthesis and produces an uninterrupted daughter strand.

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