['lizbeth]

p220




. . . the functioning of all genes in a genome can be by measuring the relative amounts of their respective messenger RNA (mRNA) products. If you are interested in the genes being expressed in, say, a human liver cell, you isolate a sample of mRNAs from liver tissue. This represents a snapshot of the mRNA population in the liver cell: Very active genes, those most heavily transcribed and that produce many mRNA molecules, will be abundantly represented, whereas genes that are rarely transcribed will contribute only a few copies to the mRNA sample.
The key to transcriptomics is a surprisingly simple invention known as a DNA microarray. Imagine a microscope slide with a grid of 35,000 tiny dot-shaped wells etched onto it. Using precise micropipetting techniques, DNA sequences from just one gene are deposited in each well so that the grid contains every gene in the human genome. Critically, the location on the microscope slide of each gene’s DNA is known. . . . Using standard biochemical techniques, you can tag your live mRNAs with a chemical marker so, like the proteins mentioned above, they will fluoresce obligingly under UV light. Then comes the step where the power and simplicity of the technique becomes wonderfully apparent: you simply dump your sample of mRNAs onto the microarray with its minuscule chessboard of 35,000 gene filled wells. The very same base-pairing bonds that hold together the two strands of the double helix will compel each mRNA molecule to pair off with the gene from which it was derived. The complementarity is precise and foolproof: The mRNAs from gene X will bond only to the very spot occupied by gene X on the microarray. The next step is merely to observe which spots have picked up the fluorescent mRNAs. One spot on the microarray may show no fluorescence, implying that implying that there was no complementary mRNA in the sample – and thus, we may infer, no active transcription of that gene in the liver cell. On the other hand, a number of spots do fluoresce, some with particular intensity; this indicates that many mRNA molecules have bound to it. Conclusion: a very active gene. Thus, with a single simple experimental assay, you have identified every one of the genes active in the liver. And such molecular panoramas have been made possible thanks to the success of the Human Genome Project and the new mind-set it has ushered into biology: we no longer need to study bits and pieces – we can now see the whole picture in all its spectacular glory.

To be continued