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This lecture is about
Molecular Scissors: Restriction Endonucleases

These are natural enzymes of bacteria, which they use for their own protection against
viruses. The restriction enzyme cuts down the viral DNA, but does no harm to the
bacterial chromosofhe. They are called restriction enzymes because they restrict the
growth of viruses. In 1970, Hamilton O. Smith, at Johns Hopkins University, isolated the
first restriction enzyme. Bacteria produce a variety of such restriction enzymes, which
cut the DNA at very specific sites characterized by specific sequence of four or six
nucleotides arranged symmetrically in the reverse order. Such sequences are known
as palindromic sequences. So far more than 400 such enzymes have been isolated out
of which about 20 are frequently used in recombinant DNA technology.
EcoRl, a commonly used restriction enzyme, cuts double-stranded DNA when it has
this sequence of bases at the cleavage site (Fig. 23.1). Notice there is now a gap into
which a piece of foreign DNA can be placed, if it ends in bases complementary to those
exposed by the restriction enzyme. The single stranded but complementary ends of the
two DNA molecules are called “sticky ends” because they can bind by complementary
base pairing. They, therefore, facilitate the insertion of foreign DNA into vector DNA.