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(iii) Besides Hind II, more than 900 restriction enzymes have been isolated now, from over
230 strains of bacteria, each of which recognise different recognition sequences.
(iv) Naming of Restriction Enzymes
(a) The first letter is derived from the genus name and the next two letters from the species
name of the prokaryotic cell from which enzymes are extracted.
(b) The Roman numbers after name show the order in which the enzymes were isolated
from the bacterial strain.
For example, Eco RI comes from Escherichia coli RY13 and Eco RII comes from E. coli R 245,
etc.
(v) Restriction enzymes belong to a class of enzymes called nucleases.
Nucleases are of two types:
Exonucleases They remove nucleotides from the ends.
Endonucleases They cut at specific positions within the DNA.
(a) Each restriction endonuclease recognises a specific palindromic nucleotide sequences in
the DNA.
(b) Palindrome in DNA is a. sequence of base pairs that reads same on the two strands
when orientation of reading is kept same.
For example, the following sequences reads the same on the two strands in 5′ -> 3′ direction
as well as 3′ -> 5′ direction.
5′ — GAATTC — 3′
3′ — CTTAAG — 5′
(vi) Mechanism of Action of Restriction Enzymes