Restriction enzymes - chemical scalpels, molecular scissors of Biology

Linn and Arber discovered that Escherichia coli is able to protect itself from the infection of bacteriophage  by a defence mechanism called restriction modifications system. This system has two component Restrictions enzymes and Methylase.

Restriction enzymes belong to class nuclease which recognise specfic sequence of DNA. It is also called restriction enzyme because it stop the propagation of bacteriophage in Escherichia coli. 

Methylase add methyl group to sequence of  bacterial DNA to prevent the harmful effect of restriction enzyme.

Restriction enzyme is also restriction endonuclease. The first restriction endonuclease discovered was Hind II from Haemophilus influenzae by the Smith. It was found that it cut DNA at specfic sequence of six base pairs.

Now so far more than 900 restriction enzyme have been discovered from 230 strain of bacteria.

The naming of restriction enzymes has been given by the following ways - The first letter of restriction enzymes has been taken from genus. The other two letter has been taken from the species of bacteria. The Roman number signify the order of enzyme which is extracted from bacteria and last letter belong to strain if present.

Remember 👌👌 Arber, Smith and Nathan were awarded Nobel prize in 1978 for discovery of restriction endonuclease.

There are two types of restriction enzymes - Endonuclease and exinucleaese. Exonuclease cut DNA at the ends whereas endonuclease cut molecule of DNA at specific sites called recognition sequence. Each endonuclease scan the entire length of DNA and once find the specfic sequence that it stick with recognition sequence and cut both strand of DNA helix at sugar phosphate backbone.

The recognition sequence of each restriction endonuclease is 4 to 8 nucleotide long palindromic sequence. The two strands of DNA possess the same base sequence but in opposite manner that is 5' to 3' and 3' to 5' directions.

5'...GAATTC...3' 
3'...CTTAAG...5'

Restriction enzymes attach between the same two bases on opposite strands and cause a break by cutting two Phospho diester bond. These restriction endonuclease are called as chemical scalpel or molecular scissors.

There are two group of restriction endonuclease. One group of endonuclease produce short single stranded ends which can join single stranded ends of other DNA fragments. Such ends are called staggered ends, sticky ends or cohesive ends. These ends are sensitive for DNA ligase enzyme and used in formation of recombinant DNA. 

The second group of endonuclease cut both the strand of DNA at the same place so that single stranded pieces are not left on the ends. The ends without single stranded sequences are called blunt ends. Small linkers are sometimes are added to the ends. Blunt ends are also made sticky by addition of poly A and ploy T sequences with the help of enzymes terminal transferase.