What is a biological function of restriction enzyme?
They are DNA cutting enzymes. Each enzyme recognises one or a few target sequences and cuts DNA at or near those sequences. When it finds its target sequence, a restriction enzyme makes a double stranded cut in the DNA molecule.
Restriction enzymes are found in many different strains of bacteria, where their biological role is to participate in cell defense. These enzymes "restrict" foreign DNA (e.g. viral) that enter the cell, by destroying it. Restriction sites in the viral genome are cleaved by the bacterium's restriction enzyme, fragmenting and destroying the DNA of invading bacteriophages before it can incorporate into the host's genome and take over them.
Restriction enzymes are essential tools for recombinant DNA technology.