To isolate a specific gene, it is often necessary to produce a genomic
library, since the number of genes present in a genome can be very
large. Generally, a genomic library is composed of many self-replicating
plasmid vectors, each containing a genetic insert from the genome of interest.
The four most common cloning vectors used to represent the haploid human
genome are listed below:
Modified forms of bacteriophage l are generally
used for library construction, since they can accomodate inserts averaging
20,000 base pairs (1/5 as many copies needed as compared to plasmid vectors),
and efficiency of recombination of E. coli is much greater via transfection
(phage infection) than by artificial transformation.
||Average Insert (bp)
||Clones to Represent Genome
|Yeast Artificial Chromosome (YAC)
Cosmid (combination l/plasmid)
vectors and YAC vectors are also being used
more commonly, since each can accomodate even larger inserts and require
fewer copies to represent the entire genome.