Question

Function of ligase is
A) to repair single-stranded breaks that arise in double-stranded DNA
B) to join together two individual fragments of double stranded DNA
Choose the correct answer from the options given below:

• A. Both A and B are correct
• B. Both A and B are incorrect
• C. Only A is correct and B is incorrect
• D. Only B is correct and A is incorrect

Hint:

DNA ligase is the molecular glue of the cell.
It seals single-stranded nicks during in vivo DNA replication and repair (A) and joins separate DNA fragments during in vitro cloning (B).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
DNA ligase is an essential enzyme that seals breaks in the sugar-phosphate backbone of double-stranded DNA.
It catalyzes the formation of a covalent phosphodiester bond between a 3'-hydroxyl ($\text{OH}$) group and a 5'-phosphate ($\text{P}$) group of adjacent nucleotides.

Step 2: Detailed Explanation:
DNA ligase performs two primary functions in molecular biology:
1. Function A (DNA Repair): During DNA replication, recombination, and repair, single-stranded nicks can arise in the sugar-phosphate backbone of double-stranded DNA (such as between Okazaki fragments on the lagging strand).
DNA ligase seals these nicks to restore the continuous, double-stranded DNA structure. Thus, statement A is correct.
2. Function B (Cloning/Ligation): In recombinant DNA technology, restriction enzymes cut DNA to produce sticky or blunt ends.
DNA ligase is used to covalently join two separate double-stranded DNA fragments (such as an insert gene and a linearized vector plasmid) by sealing the phosphodiester bonds at their junctions. Thus, statement B is correct.
Because both statements accurately describe the biological and technological functions of DNA ligase, Option (A) is the correct choice.

Step 3: Final Answer:
Both statements A and B are correct descriptions of ligase function, which corresponds to Option (A).