restriction enzyme

Bacterial enzyme that cuts both strands of a DNA double helix at a specific point. Restriction enzymes are an important tool of molecular biology and genetic engineering.

Bacteria evolved many different kinds of restriction enzymes as part of their defence against viruses. Thus, there are specific enzymes for many different recognition sites, typically defined by around six base pairs. These sequences are often palindromic, that is both strands have the same sequence of bases, only running in opposite directions, for example:

GAATTC

CTTAAG

Escherichia coli restriction enzyme EcoR I cuts this site between G and A, creating two ‘sticky’ ends, which can easily be rejoined (using the base pair complementarity of the middle four bases) and reattached using enzymes known as ligases. By contrast, enzymes that cut both strands at locations facing each other are said to produce ‘blunt’ ends.