Brown and his associates recently developed a machine called a DNA microarrayer, with which they hope to make a big dent in this ignorance. Naturally, since the yeast genome was the first to be completed, the new microarrayer was used first with yeast. But the technology is spreading to the genes of many other organisms and is even being used with fragments of the human genome.

A microarrayer is basically a robot whose pointed tip can deliver up to 20,000 bits of DNA, each representing a different gene, to specific spots on microscope slides. These "printed" DNA strands remain fixed on the slides. When samples of fluorescently tagged DNA made from a cell's active genes are washed over the glass slides, any sample that complements a fixed strand of DNA will stick to it (the chemical base adenine [A] can pair only with thymine [T], while the chemical base guanine [G] pairs only with cytosine [C]), making it glow. The location of the glowing spots thus reveals which genes are active in the cell at that time.

The method works because the tagged DNA is a copy of messenger RNA (mRNA), which is produced only when a gene is "expressed" and actively making protein. The glowing spots therefore represent only the genes that are turned on in the cell at that particular moment. (While every cell contains all the genes in the genome, only some of the genes are turned on at any time, depending on the type of cell and its circumstances.)

The neatest part of this technique is that it enables researchers to compare the activity of genes in two different cells simply by tagging their DNA with two different types of fluorescent dye, which glow in different colors. The two sets of tagged DNA are mixed before being spread over the DNA that is printed on the glass slides. The spots where fragments of tagged DNA stick to the fixed DNA are illuminated, and scientists measure the fluorescence of each dye separately with the aid of an optical scanner; then they measure the ratio of one color to another. "The brighter green a dot glows, for instance, the higher the level of expression of a particular gene from the cell whose DNA is labeled green," Brown explains. "The brighter red, the lower that gene's level of expression compared to an identical gene from the other cell." When the two genes are expressed to the same extent, the dots glow in yellow.

— Maya Pines

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How a Microarrayer Works

	A Microarrayer Shows How Genes in a Yeast Cell Respond to Different Types of Stress

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