Construction and Screening of Genomic and cDNA Libraries - In order to isolate one or more related genes from a genome, we like to prepare a mixture of clones each carrying DNA derived either from the genomic DNA or from cDNA (derived from the mRNA isolated from a specific metabolically active tissue of an organism).

This mixture may contain thousands of clones, which when derived directly from the genomic DNA are collectively called a genomic library. Similarly when these clones are derived from cDNA, they are collectively called a cDNA library. Construction and use of these libraries will be discussed in this section.