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 built 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 Stephan 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 is produced by the cell. This is like a mirror image of a particular stretch of DNA Matching pieces of mRNA, and then the cell contents are added to the microarray. The mRNA strands will stick to any matching DNA sequences and dye will show them up. The microarray has already proved to be powerful tool for learning about many different illnesses, from heart disease to cancer.
