The grid you can see in this microarray slide is actually composed of 30,000 individual DNA dots, each targeted to match a specific human gene. In order to work out which genes are being expressed, messenger RNA is first extracted from the cell sample and copied back to DNA using an enzyme. This DNA, called cDNA, is complementary to the target gene, so we'll associate with it, or hybridize with it, on the slide. After labeling with a fluorescent dye, the cDNA is washed over the slide. The genes currently active in the cell can then be identified by the level of the fluorescence and the color of the spots.
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Igor Dawid and Thomas Sargent explain how they developed subtractive mRNA hybrization to find genes expressed by different cell types. Pat Brown and Steve Fodor show how genomes can be screened with DNA arrays and GeneChipsï¿½