Screening λ Libraries

The preceding chapters describe the construction of genomic and cDNA λ libraries. In this chapter we describe two methods for screening libraries. The first method can be used on both genomic and cDNA libraries and screens by sequence homology. To do this, the library is plated and then “lifts” are made by briefly laying a membrane, such as nitrocellulose, onto the plate. The recombinant λ phage DNA within each plaque is then denatured and fixed to the membrane. In this way an accurate representation of the plaques on the original plate is made. Specific clones are identified by hybridization with a labeled nucleic acid. The positive clones are then isolated by aligning the agar plate with the resulting autoradiograph from the hybridization. Clones are then purified by sequential rounds of plating and hybridization.