Lambda phage (λ) vectors

Plasmid vectors described in the previous section are often used for cloning DNA segments of small size (upto 10 kilobases). However, while preparing a genomic library for a eukaryote, the cloned fragments should be large enough to contain a whole gene.

This will also allow cloning of the whole genome into a number of clones, which will not be unreasonably large and therefore can be screened without serious difficulty.

The above properties and other requirements of cloning whole genome in eukaryotes are fulfilled by the phage lambda (λ) and cosmid vectors, the former permitting cloning of segments upto 20-25kb long (kb = kilobases) and the latter accommodating segments upto 45kb long. Phage lambda (λ), however, is easier and more efficient for making genomic and cDNA libraries.