Expression Cloning

In some cases, a researcher becomes interested in studying a gene not because mutations in it cause an interesting phenotype but because the protein it encodes has interesting properties. A prominent example is beta-amyloid protein, which accumulates in the brains of Alzheimer's disease patients.

Expression cloning is a method of isolating a gene by looking for the protein it encodes. If the protein of interest is an enzyme, it can be found by testing for its biochemical activity. A very common method for identifying a particular protein is by using antibodies, or immunoglobulins, that bind specifically to that protein. Expression cloning usually uses a cDNA library, in which protein-coding sequences are uninterrupted by introns. Each cDNA is inserted into an "expression vector," which contains all the necessary signals for the DNA to be transcribed into mRNA. The mRNA can then be translated into protein. Thus the host cell harboring the clone will produce the gene's protein product, and the protein can then be detected by biochemical or immunologic methods. Once the cell making the protein is found, the cDNA can be re-isolated and the gene sequenced by standard means.

Gene cloning techniques continue to advance rapidly, aided by the Human Genome Project and bioinformatics. It is likely that positional cloning will take on a secondary role, and that bioinformatics and proteomics methods will begin to contribute more, as more progress in these fields is made. see also Bioinformatics; Blotting; Chromosomes, Artificial; Cloning Organisms; Cloning: Ethical Issues; DNA Libraries; Gene; Gene Discovery; Human Genome Project; Linkage and Recombination; Marker Systems; Morgan, Thomas Hunt; Plasmid; Polymerase Chain Reaction; Recombinant DNA; Restriction Enzymes; Reverse Transcriptase; RNA Processing; Sequencing DNA; Transformation.

Paul J. Muhlrad

Restrictive enzymes (scissors) cut a gene out of its normal chromosomal position. Other enzymes insert it into a plasmid, which is then introduced into a bacterium. Only those bacteria that took up the plasmid survive on the growth medium. These bacteria can then be grown in bulk to produce many gene copies.


Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Science, 2002.

Lodish, Harvey, et al. Molecular Cell Biology, 4th ed. New York: W. H. Freeman and Company, 2000.


Micklos, David A., and Greg A. Freyer. DNA Science: A First Course in Recombinant DNA Technology. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press, 1990.

Watson, James D., et al. Recombinant DNA, 2nd ed. New York: Scientific American Books, 1992.

Cloning: Ethical Issues

Cloning is the creation of an individual that is a genetic replica of another individual. The process transfers a nucleus from a somatic nonreproductive cell into an "enucleated" fertilized egg, one that has had its own nucleus destroyed or removed. The genes in the transferred nucleus then direct the development of a complete organism from the altered fertilized egg. Two individuals who are clones have identical genes in their cell nuclei, but differ in characteristics that are acquired in other ways.

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