Based on their mode of action restriction enzymes are classified into Exonucleases and Endonucleases.
a. Exonucleases are enzymes which remove nucleotides one at a time from the end of a DNA molecule,
e.g. Bal 31 and Exomiclease III.
b. Endonucleases are enzymes which break the internal phosphodiester bonds within a DNA molecule,
e.g. Hind II, EcoRI, Pvul, BamHI and TaqI.