Construction of a genomic library involves creating many recombinant DNA molecules. An organisms genomic DNA is extracted and then digested with a restriction enzyme. For organisms with very small genomes (~10 kb)', the digested fragments can be separated by gel electrophoresis. The separated fragments can then be excised and cloned into the vector separately. However, when a large genome is digested with a restriction enzyme, there are far too many fragments to excise individually. The entire set of fragments must be cloned together with the vector, and separation of clones can occur after. In either case, the fragments are ligated into a vector that has been digested with the same restriction enzyme. The vector containing the inserted fragments of genomic DNA can then be introduced into a host organism.
Below are the steps for creating a genomic library from a large genome.
# Extract and purify DNA.
# Digest the DNA with a restriction enzyme. This creates fragments that are similar in size, each containing one or more genes.
# Insert the fragments of DNA into vectors that were cut with the same restriction enzyme. Use the enzyme DNA ligase to seal the DNA fragments into the vector. This creates a large pool of recombinant molecules.
# These recombinant molecules are taken up by a host bacterium by transformation, creating a DNA library.
Below is a diagram of the above outlined steps.