Enzymes are invaluable tools in molecular biology since several decades. They opened the field of molecular biology and enabled biotechnology and gene technology, which now have a strong influence on biological research or medicine.
During the 50's and 60's of last century the major breakthroughs in DNA enzymology occurred. The DNA polymerase was isolated by Arthur Kornberg and colleagues (1955), Weiss and Richardson isolated DNA ligase (1966), and Smith, Wilcox, and Kelley characterized the first sequence specific restriction nuclease Hind II (1968). Today almost 1000 restriction enzymes, some sequence specific and some not, have been isolated from over 230 strains of bacteria since the initial discovery of Hind II.
See more general information at:
Nuclease @ wikipedia
Restriction Enzymes @ Access Excellence
Kimball's Biology Pages