Some examples of Type II restriction endonucleases

    Restriction endonucleases are used by bacteria to recognize foreign DNA and destroy (restrict) it by introducing double-stranded cuts at characteristic recognition sites. They are typically named after the bacterial species from which they were first isolated. For example, EcoRI was the (I)st  (R)estriction endonuclease isolated from E. coli: Eco + R + I. Most endonucleases used in molecular biology recognize four or six base sites ("four-cutters" and "six-cutters"); some have ten-base sites, and are called "long cutters" because the interval between sites is much greater.

    Restriction sites are molecular palindromes. A palindrome is a word or phrase that reads the same forward or backward: "Able was I ere I saw Elba", "Madame I'm Adam," or the Finnish word for a soap seller,
"Saippuakauppias". DNA palindromes read the same forward and backward when both strands are considered. In the first example above, the recognition site of EcoRI is 5'-GAATTC-3', so the paired strand is 3'-CTTAAG-5': the double-stranded 6 bp sequence will therefore read the same in either direction.

Figure © 2000 by Griffiths et al. ; text © 2012 by Steven M. Carr