Restriction Endonuclease

Restriction endonuclease enzymes recognise short, specific sequences within DNA, and then cut the DNA. These enzymes are vital in the process of protein synthesis. The recognition sequence for restriction endonucleases are usually palindromic and symmetrical, this is because the enzymes are made up of 2 identical subunits. The cut leaves a 3’-hydroxyl end, and a 5’-phosphate end on each fragment.

These enzymes are found in bacteria, where they defend the cell against bacteriophages by digesting viral DNA. The bacteria protect their own DNA from digestion by modifying it. The method of this modification varies between bacterial species and strains, but the most common method is methylation (different from eukaryotic methylation).

Restriction endonucleases are used in molecular biology to break down DNA into fragments and recombine these fragments in different ways or to insert these fragments into plasmids for recombinant DNA techniques. Enzymes such as restriction endonucleases are found in bacteria and archaea and their purpose is to provide a defense mechanism against foreign molecules such as invading viruses. 

Restriction endonucleases are most likely to have orginated from a common ancestor and then later on become more widespread due to horizontal gene transformation. 

The number of restriction fragments generated depends on two factors:

• The restriction enzyme used (whether it cuts at 4,6 or 8 base pairs)
• The size of the genome (larger genomes produce more fragments)

For example:

• EcoRI, found in E.coli, recognises the sequence GAATTC, and cuts between the G and the A on both strands .
• BamHI - GGATCC.
• Sau3A - GATC.
• AluI - AGCT.

These essential for the genetic engineering enzymes were discovered by W.Arber, H.Smith and D.Nathans. For the discovery of restrction endonucleases they were awarded with a Nobel Prize in Physiology or Medicine in 1978.