Figure 18.5 Constructing Libraries Intact genomic DNA is too large to be introduced into host cells. (A) A genomic library can be made by breaking the DNA into small fragments, incorporating the fragments into a vector, and then transforming host cells with the recombinant vectors. Each colony of cells contains many copies of a small part of the genome. (B) The many mRNAs in a cell can be copied into cDNAs and a library made from them. The DNA in these colonies can then be isolated for analysis.