"By being able to see... all the genes, all the genetic variation, we can readily pick out answers."
Eric Lander, a human genome project leader
There are about 30,000 different genes in human DNA. Different cells in the body, although having identical DNA, switch on and off different genes, depending on what is needed to build that particular cell. Studying which genes are active in a cell is a useful way to find out what makes it function, and helps identify what has gone wrong when it is not functioning properly.
In 1989 U.S. scientist Stephen Fodor presented a technique that was to revolutionize DNA analysis. He created a DNA microarray—a glass slide with up to 500,000 different strands of DNA attached to it. When a gene is switched on in a cell, a complementary copy of that gene's information (called messenger ribonucleicacid, or mRNA) is produced by the cell. This is like a mirror image of a particular stretch of DNA. Matching pieces of mRNA and DNA will stick together. To find the active genes, a cell is treated with a dye that attaches itself to mRNA, and then the cell contents are added to the microarray. The mRNA strands will stickto any matching DNA sequences, and the dye will show them up. The microarray has already proved to be a powerful tool for learning about many different illnesses, from heart disease to cancer.
