Genomic library

A genomic library is a population of host bacteria, each of which carries a DNA molecule that was inserted into a cloning vector, such that the collection of cloned DNA molecules represents the entire genome of the source organism. This term also represents the collection of all of the vector molecules, each carrying a piece of the chromosomal DNA of the organism, prior to the insertion of these molecules into the host cells.

Types of vectors

1. Plasmids: Have a capacity of 15 kb.
2. Phage (lambda)s: Have a capacity of 25 kb.
3. Cosmids or Fosmids: Have a capacity of 35-45 kb.
4. Bacterial artificial chromosomes (BAC)(P-1 derived): Have a capacity of 50-300 kb.
5. Yeast artificial chromosomes (YAC): Have a capacity of 300- >1500 kb.
6. Human artificial chromosomes (HAC): Have a capacity of >2000 kb.

Creating a library

The DNA molecules of an organism of interest are isolated. The DNA molecules are then partially digested by an endonuclease restriction enzyme. Sometimes, the DNA molecules are digested for different lengths of time in order to ensure that all the DNA has been digested to manageable sizes. The digested DNA molecules are separated by size using agarose electrophoresis, and a suitable range of lengths of DNA pieces are isolated and ligated into vectors. The vectors can then be taken up by suitable hosts.The hosts are kept in liquid media and can be frozen at -80°C for a long period of time. Usually the hosts are bacteria that do not contain any plasmids, so as to be sensitive to antibiotics.

The process of subdividing genomic DNA into clonable elements and inserting them into hosts is called creating a library, a clone bank or a gene bank. A complete library of host cells will contain all of the genomic DNA of the source organism.

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