One of the newest additions in the arsenal of molecular biology techniques is the use of DNA microarrays, also known as gene chips or DNA chips. One of the main uses of a DNA microarrays is to study patterns of gene expression. Developed in the 1990s, DNA microarrays have provided a more rapid method of screening genomes for genes that are expressed under specific environmental conditions. The technology of a DNA microarray is relatively simple. Using a process called photolithography, which was adapted from a technology used by the electronics industry in the preparation of integrated circuits, a small piece of glass or plastic is spotted with fragments of DNA. These fragments may be genomic DNA that has been fragmented using restriction enzymes. However, most often the DNA is generated using PCR reactions so that the sequence content of each spot on the plate is precisely known. This technology allows researchers to place specific DNA sequences in specific places on the array. Thus, the sequence content of each location on the microarray is known in advance. For example, the first spot on the plate may hold the DNA sequence of an insulin gene, while the next location contains the DNA sequence of a muscle fiber gene. In the early years of development, researchers were able to place several thousand DNA sequences on a single array. To use a DNA microarray a researcher must first prepare a sample of mRNA. When examining patterns of gene expression a researcher willoften prepare mRNA from two different samples. However, some of the newer technologies can spot up to 1 million DNA sequences in a square centimeter area.
