The first restriction endonuclease isolated was

The correct answer is Option (3) → Hind II

Restriction endonucleases cut DNA at specific recognition sequences. The first one to be isolated was Hind II. It always cuts DNA at a fixed point by recognising a particular sequence of six base pairs.

Option (1) EcoRI — Incorrect. EcoRI from E. coli RY13 is the enzyme the book uses to explain sticky ends, but it was not the first one isolated.

Option (2) pBR 322 — Incorrect. pBR322 is not an enzyme at all. It is a plasmid cloning vector of E. coli.

Option (3) Hind II — Correct. It is the first restriction endonuclease that was isolated and characterised.

Option (4) Bam HI — Incorrect. BamHI comes from Bacillus amyloliquefaciens. It is a restriction site on pBR322, not the first enzyme isolated.

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NCERT Extract:
Restriction Enzymes – “The first restriction endonuclease–Hind II, whose functioning depended on a specific DNA nucleotide sequence was isolated and characterised five years later. It was found that Hind II always cut DNA molecules at a particular point by recognising a specific sequence of six base pairs. This specific base sequence is known as the recognition sequence for Hind II.”