Restrition Enzymes

Restrition Enzymes

	Restrition Enzymes The existence of restriction enzymes was first postulated by Werner Arber in the early 1960s while studying bacteriophages. Arber found that when virus DNA entered a bacterium it was cut up into smaller pieces and destroyed. He theorized the presence of restriction enzymes that could destroy the infecting viral DNA without affecting the DNA of the host bacterium. He also proposed that the enzymes recognised and acted at specific sites on the viral DNA
Hamilton Smith and his colleagues (1970) isolated the restriction enzyme (now called Hin II) produced by the bacterium Haemoplulus influenzae. He showed that the enzyme recognized and cut the viral DNA at specific sites. All the DNA termini were pGpTpPy-3' and 5'pPupApCp, showing that they had been derived from the double stranded sequence: 5'pGpTpy \| pPupApCp-3' 3'pGpApPu \| pPypJ'pGp-5'
Thus the DNA sequences at which the restriction enzymes make breaks show two fold symmetry, with the same sequence running in opposite directions in the two strands (palindromic sequences). Daniel Nathans (1971) showed that the enzyme discovered by Smith cut the DNA of the monkey virus SV40 into II well defined pieces. In 1973 Nathans described how SV40 DNA was fragmented when two other enzymes were used. By analyzing the fragments produced by the three enzymes it was possible to map SV40 genes. This technique was later used to map the DNA of other organisms. Arber, Nathans and Smith were awarded the 1978 Nobel Prize in Physiology and Medicine for these discoveries.
Restriction enzymes serve as a means of protection against foreign DNA in many microorganisms. They Junction as a part of the restriction modification (R-M)system. A matched modification enzyme recognizes and modifies, usually by methylation, the nucleotide I sequence recognized by the restriction enzyme. The DNA modified by methylation is thus protected against cleavage by the restriction enzymes of the organism. On the other hand unmodified foreign DNA is degraded. R-M systems are widespread in bacteria where they play an important protective role in eliminating foreign DNA which may enter the cell through viruses, or as unmodified DNA. Restriction enzymes have not been reported from mammalian cells, where the immune surveillance system makes a restriction modification system unnecessary

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