Describe the structure of a DNA microarray and its use.

A DNA microarray is a glass plate divided into fields containing a specific DNA probe of about 20 nucleotides. The sequence of the probe differs by one nucleotide from field to field. A chip can hold more than 500,000 fields. DNA is extracted from cells and cut with one or more restriction enzymes. The fragments are fluorescence tagged, denatured, and annealed to the DNA in the array. Massive amounts of DNA can be screened for sequence changes in a relatively short period of time.