Restriction enzymes have the ability to cut DNA
molecules at very precise sequences of 4-8 base pairs called recognition
sites. They naturally occur in many bacteria as a defence mechanism against
bacteriophages (viruses that infect bacteria). The restriction enzymes in the
bacteria were able to cut the viral DNA, usually at least at several locations,
thus rendering the viral DNA inactive so it couldn't infect the bacterium.
Purified forms of these restriction enzymes are now used as tools to cut DNA.
Scientists use over 400 restriction enzymes that recognize roughly 100 recognition
sites as a means to isolate, sequence and manipulate individual genes taken
from any cell (including humans).
The sites where the fragments of DNA are cut may result in overhanging "sticky
ends" (See diagrams below.) Pieces with similar sticky ends can
be joined together using the enzyme DNA ligase.
Hybrid DNA from different sources (or species) may be produced this way and
such DNS is known as recombinant DNA.
An Example of the Use of Restriction
Enzymes to Create Recombinant DNA
1. A restriction enzyme cuts the double-stranded
DNA molecule at its specific recognition site.
2. The cuts creat a small DNA fragment with 2
"sticky ends". The larger piece of DNA also has 2 "sticky ends".
3. When 2 such fragments of DNA cut by the same
restriction enzyme are brought together, they can by joined by base-pairing
using the enzyme DNA ligase. This can allow DNA fragments from 2 different sources
(often one is a plasmid- a small, circular bacterial chromosome) to be joined
together, making a new type of (recombinant) DNA.