A microarray is a collection of gene probes which are attached to a solid surface. It consists of thousands of DNA probes arrayed on a single glass microscope slide or silicon chip. A gene probe is a specific length of single-stranded DNA, between 20 and 40 nucleotides long (or sometimes as large as 1000 nucleotides!) which is complementary to a known sequence of DNA form a particular gene. A gene probe can measure the level of gene expression in a sample of DNA, and a microarray can screen a large number of genes at the same time. It is efficient and fast as it identifies genes which are expressed in certain individuals/breeds, while also showing those genes that are not being expressed as a comparison. Microarrays can be used when scientists are trying to discern between genes that are desirable and genes that are not. For example, if some individuals are resistant to a particular disease, they may have a unique form of a gene that scientists would like to locate and analyse. The basic sequence for creating a microarray/locating a gene is as follows:

1. A probe is a sequence of DNA which is made radioactive

2. The target for the probe is double-stranded DNA containing the sequence being studies

3. The target DNA is heat-treated to denature the strands and separate them

4. The radioactive probe is introduced to find the gene

DNA microarray technology however has allowed for scientists to determine the expression of thousands of genes at one time. In the target cell, its mRNA is extracted, reverse transcribed into DNA (now called copy DNA or cDNA) and is labelled with a fluorescent marker. The fluorescently labelled DNA is then hybridised under stringent conditions to the probes on the slide. A scanner measures the fluorescence for each DNA probe on the slide, and from this information, scientists can work out the activity of the genes in the cell. The stronger the fluorescene, the more mRNA in the original sample and therefore the greater activity of each of the genes.