Genomic Libraries

Genomic libraries have the potential to contain a clone of every gene found in the cells of any organism. This is because the starting material in a genomic library is the DNA extracted from the cells of the organism of interest. The steps performed to make a genomic library are listed below.

1) Isolate a sample of DNA from the organisms cells

2) Cut the DNA into “gene-size” pieces with a restriction enzyme or enzymes.

3) Cut a cloning plasmid with the same restriction enzymes.

4) Add the cut plasmid and genomic DNA to the same test tube and add DNA ligase. This will make a collection of recombinant and nonrecombinant plasmids.

5) Transform the collection of plasmids into a laboratory strain of E. coli that is susceptible to antibiotics.

6) Plate the bacteria on Amp/X-gal plates. Incubate at body temperature overnight.

7) Observe the plates, select white colonies and transfer to new Amp plates.

8) Make a large collection of white colonies to form the genomic library.

Once a library is made it can be a resource used to isolate colonies that contain any gene of the organism. The problem is how can the gene cloner determine which colony has the gene they are interested in? We will discuss this in the library screening lesson. First, let’s discuss the other type of gene library that can be made, a cDNA library.

Fig.15: Genomic and cDNA libraries start with different genetic materials. (Image by D. Lee)