A DNA chip (or DNA microarray) is a set of microscopic spots consisting of DNA molecules fixed in an orderly fashion on a surface that can be glass, silicon or plastic. DNA chips are an interesting tool for measuring the expression levels of a large number of genes simultaneously in a sample, known as gene expression profile analysis. Each DNA spot is composed of a small amount of a specific DNA sequence (of the picomole order), this sequence is called a probe or oligo.
A typical microarray experiment involves the hybridization of an mRNA molecule to the DNA template from which it is derived. Many DNA samples are used to construct a template. The amount of mRNA bound to each site of the template indicates the level of expression of different genes. This number can be in the thousands. All the data is collected and a profile is generated for the expression of the genes in the cell.