| Examining Gene Expression |
Understanding the functions of genes depends on knowing when and in what cells they are each expressed. How can one measure the amount of mRNA transcribed from a gene in a particular cell type? The standard method uses a probe - a DNA sequence unique for that gene - which binds to the mRNA that has the complementary sequence. The more mRNA particular cell produces, the more mRNA that is bound to the probe, giving the probe an increased signal. Because cDNA is complementary in sequence to mRNA, it can also be used to measure the expression of a particular gene.
Organisms have so many genes in their genomes that studying the expression of all of these genes had been exceedingly difficult. Going from studying gene expression one gene at a time to examining expression patterns of a multitude of genes required new technology.
In the late 1990s the development of microarray chips allowed researchers to examine the expression of thousands of genes simultaneously. This allowed for a much broader perspective of gene expression than was possible when genes were analyzed singularly. Microarray chips are glass slides spotted with many rows containing tiny amounts of probe DNA, one for each of thousands of genes (Fig. 5). The target sample of interest, usually made from mRNA of a specific type of cell, is labeled with a fluorescent dye and added to the chip. If there is a match between the sample of interest and the DNA probe on the chip, the two molecules will bind to each other. Then, when exposed to a laser, the spot will produce a signal that will fluoresce. (Figure 5 describes this process in more detail.)
Scientists can use microarrays, a rapid and sensitive test, in a variety of experimental studies. Using microarrays, one can measure expression patterns of large numbers of genes in different cell types (such as cancer cells versus normal cells, or liver cells versus kidney cells). It can also be used to examine the changes in gene expression over time (for example, as an embryo develops), or changes in a given cell type under different environmental conditions (various temperatures, for instance).