Restriction Endonucleases

NUCLEASES

The nucleases have the ability to cleave the DNA molecules and there are two types of nucleases: exonucleases which cleave the nucleotide at the end of the DNA/RNA molecule and endonucleases which cleave the internal phosphodiester bond in the DNA/RNA molecules. Of all the endonulceases, restriction endonucleases are unique as they are able to recognize a specific sequence within a DNA molecule and can make double-stranded cuts in the DNA either within or near the specific sequence.  Due to this we can precisely know the cuts in the DNA molecule and pieces of DNA can be excised from a bigger molecule. Due to this property, restriction endonucleases have been used in recombinant DNA technology where DNA fragments of known sequence are required.

The discovery of restriction endonucleases was made in the early 1950s after an initial observation that some strains of bacteria are immune to bacteriophage infection and the phenomenon was then called as host-controlled restriction. It took 20 years to understand the phenomenon of restriction. Restriction occurs in the bacteria as bacteria produces enzymes that degrade the incoming phage DNA and prevents it to replicate and make copies. All the cleavage sites within the bacterial DNA must be protected from the action of its own restriction enzymes. This protection is done by methylation of cytosine residues in the recognition sequences of the restriction enzymes within its own DNA. Methylation occurs just after the replication of DNA by the methylases of the organism itself.

The proper restriction endonucleases were discovered in 1970 by Hamilton Smith and Daniel Nathans. Both of them shared Noble Prize in Physiology or Medicine in 1986 with Warner Arber, pioneer scientist who led to discovery of restriction endonucleases.