Construction of a genomic library involves the isolation of genomic DNA, purification of the genomic DNA and fragmentation of genomic DNA into desired size , then cloning of the fragmented DNA using a suitable vector. Just like a library is a place for sheltering a large collection of books on different subjects for public use, a DNA library involves a collection of DNA sequences from various organisms serving different purposes. A genomic library holds the DNA sequences derived from genomic DNA. It is slightly different from a cDNA library which hold sequences generated from mRNA.