Q.The role of DNA ligase in the construction of a recombinant DNA molecule is:

The key to understanding this question is to recall exactly what DNA ligase does during recombinant DNA technology. After a restriction enzyme cuts DNA at specific sites, it often leaves short, single-stranded overhangs called sticky ends. These sticky ends can base-pair with complementary sequences on another DNA fragment through hydrogen bonds, but that pairing is temporary and not covalent.

DNA ligase's job is to seal the backbone. It catalyzes the formation of a phosphodiester bond between the 3'-hydroxyl end of one nucleotide and the 5'-phosphate end of the adjacent nucleotide. This permanently joins the two DNA fragments into a single, continuous molecule. …

To understand what DNA ligase does, you first need to picture the moment after a restriction enzyme has cut two different DNA molecules. A restriction enzyme like EcoRI cuts DNA at a specific sequence, leaving short, single-stranded overhangs — these are called "sticky ends." When you mix a fragment of human DNA (carrying a gene of interest) with a cut plasmid vector, their complementary sticky ends come together by hydrogen bonding. But that pairing is only temporary; it holds the fragments in place but does not join them covalently.

This is where DNA ligase enters the picture. The enzyme's job is to repair the nick in the DNA backbone — specifically, it catalyses the formation of a phosphodiester bond between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of the adjacent nucleotide. Once that bond is made, the two fragments are covalently linked into a single, continuous DNA molecule. That is the recombinant DNA.

Alternative Approach: Trace the Historical/Etymological Origin of the Name

Instead of listing which statements are individually true, trace exactly where the English

word "restriction" came from in the discovery history — this pins down a single answer.

Step 1: Recall the discovery context.

In 1963, two enzymes were isolated in E. coli that were responsible for restricting

(preventing) the successful growth of an invading bacteriophage. One enzyme protected the

host's own DNA by methylating it; the other cut the invading phage DNA.

Step 2: Name the phenomenon as the discoverers themselves named it.

The host bacterium was said to "restrict" the phage's growth — this phage-restricting

role is exactly why the phage-DNA-cutting enzyme was called a "restriction" endonuclease.

Step 3: Separate the NAME's origin from the enzyme's MECHANISM.

Options (a) (cleaving the phosphodiester bond) and (b) (cutting at a specific position)

are true facts about HOW the enzyme works — but many enzymes cut DNA at specific

positions without being called "restriction" enzymes. These options describe mechanism,

not the reason for the name.

Step 4: Match the definitional question to the definitional option.

Only option (c) — preventing bacteriophage multiplication in the host — is the actual …