Q.'Restriction' in Restriction enzyme refers to:

The term 'restriction' in restriction enzymes refers specifically to (c) — the prevention of bacteriophage multiplication by the host bacterium — not to the enzyme's cutting mechanism.

The real NCERT chapter itself explains the origin: "the two enzymes responsible for restricting the growth of bacteriophage in Escherichia coli were isolated... The later was called restriction endonuclease." The name comes from this phage-restricting phenomenon, not from the mechanics of cutting DNA. …

This is a naming-origin question, and the real textbook (Chapter 9, section 9.2.1) tells the story directly: in 1963, two enzymes responsible for restricting the growth of bacteriophage in Escherichia coli were isolated — one added methyl groups to the bacterium's own DNA (protecting it), and the other cut the invading phage's DNA. The second enzyme was named a restriction endonuclease precisely because of this phage-restricting role.

Let's examine each option against this history:

• (a) Cleaving of phosphodiester bond in DNA by the enzyme — true of restriction enzymes (they are endonucleases that break the DNA backbone), but this describes the chemistry of how the enzyme cuts, which is true of many enzymes that are not called "restriction" enzymes. It is not why this specific name was chosen.
• (b) Cutting of DNA at specific position only — also true (restriction enzymes recognise a specific palindromic sequence and cut there), but again this describes a feature of the enzyme's action, not the reason for its name.
• (c) Prevention of the multiplication of bacteriophage by the host bacteria — this IS the reason for the name. The host bacterium uses this enzyme to "restrict" — that is, prevent — the phage from successfully replicating inside it, by destroying the phage's incoming 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 …