Summary of Previous Lecture: In the previous lecture we discussed an over-view of cloning, different steps and we have discussed different accessory molecule used to clone DNA fragment, enzymes such as DNA ligase, Alkaline phosphatase. Now in the today's lecture, we will discuss the restriction enzymes or “Molecular scissors”.

Restriction-Methylation System : The ability to develop resistance against the attack of foreign organism lies at different level. At cellular level, a thick cell wall may protect the organism from mechanical damage and chemical attack. At metabolism level, a suitable modification may reduce the potency of the action of cytotoxic molecules or drugs. To add in this list, at molecular level microorganisms have evolved the mechanism to degrade foreign DNA and inhibit the propogation of pathogenic organism. Since this system restrict the propogation of foreign DNA and can be able to identify his own DNA, this system is called as as restriction-methylation system. This system consists of restriction enzymes to degrade the foreign DNA and a methylation enzyme to protect self DNA from the action of restriction enzyme. As shown in Figure 14.1, A DNA molecule incubated with restriction enzyme BamH-I, the enzyme recognizes the cutting site on it and cleaves the DNA into the fragments. But, if the DNA is from the host bacteria, BamH-I methylase will label the DNA with methyl group at specific residues and as a result Bam H-I restriction enzyme will not be able to recognize the DNA and eventually will not catalyze a cleavage reaction.

Different types of restriction enzymes: Restriction enzymes differ in their structure, recognition site, cutting site and cofactor requirement. A list of differences between restriction enzyme classes is given in Table 14.1.

Nomenclature of restriction enzymes- Due to extensive search of presence of restriction enzyme in different microorganisms, a nomenclature system has been adopted. In this system, first alphabet represents name of the genus, 2^nd alphabet represents species, 3^rd alphabet gives information about strain and fourth is the order in which enzyme has been isolated from the particular micro-organism.

Forex.  EcoR-I, E=Escherichia (genus), Co=Coli (Species), R= RY13(strain), I=it is the first restriction enzyme from this bacteria.