One of the basic tools used in biotechnology is restriction enzymes.
In natural circumstances, one of the main roles of these enzymes is to break
foreign DNA entering the cell. A restriction enzyme breaks the phosphodiester
bonds of both strands of a DNA upon appearance of a certain cleavage sequence. Each such enzyme
is characterized by a different cleavage sequence. Today there are more then
150 known different cleavage sites, namely, different nucleotide configurations
that known enzymes can digest.
