Regulation of Gene Expression
The chromosomes of every type of cell carry two major kinds of genetic information: DNA sequences that ultimately specify the amino acid sequence of proteins, and sequences that regulate the expression of those protein-coding sequences. Since DNA is first transcribed into RNA sequences, and then some of these RNA sequences are translated into proteins, cells control the expression of their genes at several levels, including the rate at which specific RNA sequences are made, the ways in which those RNA sequences are modified, the rate at which modified RNA sequences are translated into protein chains, and the changes that occur during and after protein chains are synthesized, which includes such changes as folding, protein degradation, and chemical modifications, such as the addition of phosphate groups for example. Regulation depends on complex networks of receptors, signals, and enzymes that allow cells to adapt to the requirements of life in particular environments.

Analysis of Gene Expression Using Microarrays
Analysis of the presence or absence and amounts of RNA----in particular cells, at specific times in development, or under particular conditions----is one of the important ways to examine control of gene expression in a systematic way. Microarrays----which are produced by robotic equipment-- provide a rapid, high-throughput method for analyzing the expression of RNA in large numbers of genes and large numbers of cells. Microarrays carry hundreds or thousands of specific spots, and each spot can be used to analyze a particular gene product, which is usually a sequence of RNA. Typically, each spot on the microarray will contain a short stretch of DNA that can specifically detect a particular RNA gene product. The quantity is detected by some sort of indicator----often a fluorescent tag-- that can be rapidly detected in an automated reader.