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Auxotrophy is the inability of an organism to synthesize a particular organic compound required for its growth (as defined by IUPAC). An auxotroph is an organism that displays this characteristic, auxotrophic is the corresponding adjective. Auxotrophy is the opposite of prototrophy.
In genetics, a strain is said to be auxotrophic if it carries a mutation that renders it unable to synthesise an essential compound. For example a yeast mutant in which a gene of the uracil synthesis pathway is inactivated is a uracil auxotroph. Such a strain is unable to synthesise uracil and will only be able to grow if uracil can be taken up from the environment. This is the opposite of a uracil prototroph, or in this case a wild-type strain, which can still grow in the absence of uracil. Auxotrophic genetic markers are often used in molecular genetics.
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