The genome microarray, also known as a deoxyribonucleic acid (DNA) microarray, is a type of genetics technology that allows scientists to determine levels of gene expression. In living organisms, a gene is expressed when the DNA of that gene is decoded through a series of processes into a protein, which serves a specific function in a cell. By measuring levels of gene expression in a given sample, researchers can find out which genes are most active. Microarray technology is used particularly in medicine for learning about the genetic aspects of diseases such as cancer.
When a gene is expressed in an organism, DNA is decoded into a protein through a series of steps. Segments of the gene are transcribed onto a messenger ribonucleic acid (mRNA) template, a single stranded molecule that is complementary to one strand of the original DNA molecule. This mRNA carries the genetic information out of the nucleus of the cell to the site of protein synthesis. A genome microarray reveals which genes are generating the most mRNA — and, by extension, which genes are operating at the highest level of expression.
The genome microarray is a glass or silicon chip with a series of microscopic dots of DNA attached to its surface. The specific sequences of DNA, called probes, are chosen based on the genes that researchers wish to study. A whole-genome microarray contains sequences from across the entire genome, while a focused microarray contains DNA from only certain genes.
In disease research, the microarray would be used in the following way. First, a sample of healthy tissue and diseased tissue would be taken from the subject. MRNA from both samples would be isolated through a series of chemical techniques. Each sample would be combined with a different labeling solution made up of subunits known as nucleotides, modified to include fluorescence, which would then bind to the mRNA molecules to create fluorescent complementary DNA (cDNA). For example, the diseased sample might be labeled with red fluorescence and the healthy sample with green fluorescence.
When each sample is flushed onto a genome microarray, some of the cDNA from the samples hybridize, or bond, with the DNA on the chip. This causes different colors and levels of fluorescence to appear. If a gene in a sample were highly active, for example, it would produce many mRNA, which would appear on the microarray as a strong fluorescent color. By merging the visual sample data using a scanner, researchers can determine whether a particular gene is expressed more in the diseased tissue or the healthy tissue.
In the example above, a green dot would indicate that the gene was expressed more in healthy tissue, since the dominance of the green fluorescence indicates that the healthy sample mRNAs were more numerous than the unhealthy ones. A red dot indicates that the gene was producing more mRNAs in the diseased tissue and was more active under disease conditions. Yellow dots would mean that the gene is expressed about equally in both healthy and diseased tissue. Researchers can use this information to determine which genes are more active in a diseased cell and how such changes affect other genes in the cell. Genome microarrays can be used not only to research and diagnose diseases such as cancer and heart disease but also potentially to learn how to treat them through targeted therapy.