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• Restriction enzymes belong to a larger class of enzymes called nucleases.
• Restriction enzymes are called as molecular scissors because these enzymes cut DNA at specific sites.
• The first restriction endonuclease is Hind II.
• The restriction enzymes cut DNA at specific base sequence, and these specific base sequence is known as the recognition sequence.
• The convention for naming restriction enzymes –
  (i) The first letter of the name comes from the genus.
  (ii) The second two letters come from the species of the prokaryotic cell from which they were isolated, e.g., EcoRI comes from Escherichia coli RY 13.
  (iii) In EcoRI, the letter ‘R’ is derived from the name of strain.
  (iv) Roman numbers following the names indicate the order in which the enzymes were isolated from that strain of bacteria. 
• 900 restriction enzymes that have been isolated from over 230 strains of bacteria.

These are of two kinds

1. Exonucleases

2. Endonucleases

• Exonucleases remove nucleotides from the ends of the DNA whereas, endonucleases make cuts at specific positions within the DNA.
• Each restriction endonuclease recognizes a specific palindromic nucleotide sequences in the DNA.
• The palindrome in DNA is a sequence of base pairs that reads same on the two strands when orientation of reading is kept the same.

Example- the following sequences reads the same on the two strands in 5' - 3' direction, this is also true if read in the 3' - 5' direction.

 5' —— GAATTC —— 3’

 3' —— CTTAAG —— 5'

• Restriction enzymes cut the strand of DNA a little away from the center of the palindrome sites, but between the same two bases on the opposite strands which leaves a single stranded portions at the ends and the overhanging stretches called sticky ends on each strand.
• When cut by the same restriction enzyme, the resultant DNA fragments have the same kind of ‘sticky-ends’ and, these can be joined together using DNA ligases.