set of restriction sites. Thus a given restriction enzyme will cut the DNA from a particular source into a reproducible set of fragments called restriction fragments. Restriction enzymes have been purified from several hundred different species of bacteria, allowing DNA molecules to be cut at a large number of different sequences corresponding to the recognition sites of these enzymes (Table 9-1).

Inserting DNA Fragments into Vectors DNA fragments with either sticky ends or blunt ends can be inserted into vec tor DNA with the aid of DNA ligases. During normal DNA replication, DNA ligase catalyzes the end-to-end joining (lig-ation) of short fragments of DNA, called Okazaki fragments. For purposes of DNA cloning, purified DNA ligase is used to covalently join the ends of a restriction fragment and vector DNA that have complementary ends (Figure 9-11). The vector DNA and restriction fragment are covalently ligated together through the standard 3' ^ 5' phosphodiester bonds of DNA. In addition to ligating complementary sticky ends, the DNA ligase from bacteriophage T4 can ligate any two

Genomic DNA fragments (a)

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