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In addition, the function of a cloned piece of DNA (e.g., a gene) can be identified by transforming yeast in which the DNA is carried on a circular plas-mid. The introduced gene may either functionally replace a defective gene or cause a phenotypic defect in the cells indicating a function for that gene.

The ability to complement yeast defects with cloned pieces of DNA has been extended to mammalian genes. Recognizing that some genes have similar sequences and functions in both mammals and yeast, scientists sometimes use yeast as a tool to identify the functions of mammalian genes. Not many mammalian genes can directly substitute for a yeast gene, however. More frequently, scientists study the yeast gene itself to understand how its protein functions in the cell. The knowledge gained can often lead to an understanding of how similar genes might function in mammals. Now that the yeast genome has been completely sequenced and the results have been deposited in a public databank for all to use, rapid progress is being made in identifying all yeast genes and their functions.

An important method for studying mammalian genes in yeast is called the two-hybrid system. This system is used to determine if two proteins functionally interact with each other. Both genes are cloned into yeast plas-mids and transformed into the cells. A special detection system is used that is active only when both cloned proteins physically contact each other in the cell. When that happens, scientist can identify which proteins need to interact with each other in order to function.

Yeast are also being used in the laboratory and commercial production of important nonyeast proteins. Foreign genes are transformed into yeast and, after transcription and translation, the foreign proteins can be isolated. Because of the ease of growing large quantities of cells, yeast can produce a large amount of the protein. While similar protein production can be performed by bacteria, eukaryotic proteins often do not function when made in bacteria. This is because most eukaryotic proteins are normally altered after translation by the addition of short sugar chains, and these modifications are often required for proper function, but bacteria do not carry out these necessary post-translational modifications. Yeast, however, does permit these modifications, and is thus more likely to produce a functional protein. see also Cell, Eukaryotic; Cell Cycle; Genome; Human Genome Project; Model Organisms; Plasmid; Post-translational Control; Transformation; Transgenic Animals.

Suzanne Bradshaw transcription messenger RNA formation from a DNA sequence

Bibliography

Sherman, Fred. "Getting Started with Yeast." In Methods in Enzymology, vol. 194, Christine Guthrie and Gerald R. Fink, eds. New York: Academic Press, 1991.

Watson, James D., Michael Gilman, Jan Witowski, and Mark Zoller. Recombinant DNA. New York: Scientific American Books, 1992.

Zebrafish

The zebrafish (Brachydanio rerio) is a small tropical freshwater fish that began to be used as a genetic model system in the early 1980s. The zebrafish shares numerous anatomical and genetic similarities with higher vertebrates,

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