With news about Dolly and embryonic stem cells the stuff of cocktail party conversation, cloning a transgenic sheep or cow seems like child's play. The recipe is simple: Insert a pet gene into the nucleus of a cultured cell, fuse it with an enucleated egg, and voilà--a cow with high-octane milk.

But incorporating genes into nuclear chromosomes isn't the only road to fame and fortune. Animals and plants have other sources of genetic information--their respiratory mitochondria and photosynthetic chloroplasts. Both are partially autonomous, with loops of DNA containing important genes whose encoded proteins are made on the organelles' own protein synthesis machinery.

The plant chloroplast houses about 120 genes--small potatoes compared to the nucleus. Though most have been identified, a few open reading frames of unknown function remain, says Jeffrey Palmer, professor of biology at Indiana University in Bloomington. In general, the genes govern two main functions: transcription and...

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