Construction of Genomic library.

For preparation of genomic library, the total genomic DNA of an organism is extracted. The DNA is broken into fragments of appropriate size either by mechanical shearing this generates blunt ended fragments or sonication, or by using a suitable restriction endonuclease for partial digestion of the DNA complete digestion is avoided since it generates fragments that are too heterogenous in size For partial digestion, restriction enzymes having 4-base (thxameric) target sites, This is because a given 4-base recognition site is expected to occur every (= 4096) base pairs it is assumed here that the arrangement of the 4 bases in DNA molecules is random. Therefore, the fragments produced in partial digests with enzymes havaing 4 base recognition sites are molre likely to be of appropriate size for cloning than those generated by enzymes having 6 base recognition sites. Single or mixed digestion with the enzyme genomic libraries. The use of restrictin enzymes has the advantage that the same set of fragments are obtained from a DNA each time a specific enzyme is used, and many of the enzymes produce cohesive ends.
The partial digests of genomic DNA are subjected to agarose gel electrophoresis or sucrose gradient centrifugation for separation from the mixture of fragments of appropriate size.