Roman numbers in the names of restriction enzymes indicate -

The correct answer is Option (3) → Order in which enzymes were isolated from that strain of bacteria

The name of a restriction enzyme tells where it came from. In EcoRI, “E” is from the genus Escherichia, “co” from the species coli, “R” from the strain RY 13, and “I” shows that it was the first enzyme isolated from that strain.

Option (1) Number of genus of the bacteria — Incorrect. The genus is shown by the first letter of the name, not by a number.

Option (2) Number of species of bacteria — Incorrect. The species is shown by the next two letters, not by a number.

Option (3) Order in which enzymes were isolated from that strain of bacteria — Correct. EcoRI was the first enzyme taken from that strain of E. coli, and EcoRV a later one.

Option (4) Position at which they cut in the DNA sequence — Incorrect. Each enzyme cuts at its own palindromic recognition sequence, such as GAATTC for EcoRI. The name does not show that sequence.

NCERT Extract:
Restriction Enzymes – …