RESTRICTION ENZYMES
Bacteria, as well as humans, are able to catch viral disease. One way a bacterial cell fights against viral infection is to use enzymes called restriction enzymes to cut viral DNA into bits, before viral proteins are made and the virus destroys the bacterial cell. Each of these many enzymes cuts at a specific nucleotide sequence. Most restriction enzymes recognize palindromic sequences (sequences that read the same as their complements, but in the opposite direction). In addition, most enzymes cut in the middle of their recognition sequence, leaving identical, complementary, single-stranded ends that are called "sticky ends."