In the living bacterial cell, these enzymes destroy the DNA of certain invading viruses (bacteriophages), thus placing a "restriction" on the number of viral strains that can cause infection; the bacterium's own DNA is protected from cleavage by methyl (-CH3) groups, which are added by enzymes at the recognition sites to mask them
Enzymes have been called the "agents of life" because all life processes are dependent on them. Enzymes are protein molecules that act as catalysts (they speed up chemical reactions without undergoing any change themselves). They can build up or break down other molecules and are responsible for regulating the many chemical reactions that occur in plants and animals. If enzymes were absent from the human body, most of its metabolic reactions would occur at a rate, too slow to support life.
Enzymes accelerate reactions by at least a million times. Molecules in the cells of solid tissues and in circulating blood are constantly being split apart and welded together again by enzymatic action. It has been estimated that a single cell, roughly one-billionth the size of a drop of water, contains about 3,000 different enzymes.