Restriction enzymes (also known as restriction endonucleases) are proteins which cut DNA up at specific sequences in the genome. For example, the commonly used restriction endonuclease
EcoRI recognizes every point in DNA with the sequence GAATTC, and cuts at the point between the Guanine and Adenine. Interestingly, the recognition sequences for most restriction endonucleases are genetic
palindromes, e.g., the sequence reads exactly the same backwards on the complementary strand. In the case
of EcoRI, the two complementary DNA strands for the recognition sequence are:
5'--GAATTC ---3'
3'--CTTAAG--5'