Recombinant DNA molecules are transferred into appropriate host cells (e.g. bacteria) for propagation. Normally a single recombinant DNA exists per cell but sometimes co-transformation may result in two or more recombinant DNA molecules per host cell. 
DNA libraries are a collection of clones that represent the entire genome of an organism.
Two types of libraries are known:
- Genomic library. To be representative of the entire genome, the library should be >4GE. A genome equivalent (GE) is genome size/average insert size. In humans, for a GE=1, you need 3000Mb genome size)/40 kb (insert size) = 75,000 independent clones.
- cDNA library. Takes advantage of reverse transcriptase. Usually much smaller than a genomic library. 
