Microarrays a relatively new Analytical technique

Microarrays is an analytical technique of recent origin (about ten years old). It is used in the study of the pattern of gene expression of thousands of genes at the same time. Genes are regions of the genome that code for a specific protein. Different genes are expressed at different stages of growth and can also be expressed in response to diferent environmental conditions. Biologists are often interested in understanding which genes are expressed under which conditions; for instance, under aerobic, anaerobic conditions, in the presence of an antibiotic for those bacteria that are resistant and so on. This would help in understanding diseases at a molecular level thereby aid in developing better drugs.

The microarray technique involves spotting tens of thousands of DNA/RNA molecules on a slide with each spot uniquely identifying a unique region of the genome. These molecules can be small (~25 bps) or even somewhat larger. In some experiments molecules upto 500 bps have been used. This spotting on a slide is generally done commercially by well established companies, Affimetrix is one of them, refer to their website at:

http://www.affimetrix.com/corporate/media/genechip_essentials/gene_expression/How_the_Array_Works.affx

In order to figure out which genes are expressed under a given set of conditions, cells in that conditions are taken along with cells in the control conditions. mRNA from these cells are extracted (mRNA represents the genes that are turned on in these cells) and then labelled. The labels normaly incorporated are green and red, the labelling technique differs based on the method used. One of the methods is to use a labelled poly T tail as a primer and synthesize the cDNA using PCR. The labelled cDNA from both samples( the sample and control) are mixed together and washed on the micro array slide, when hybridization occurs. Each spot on the slide is now one of four colours: green, red, yellow or black. The colors correspond to the expression of the gene under different conditions. Spots that are only green are highly expressed in the control while spots that are red are highly expressed in the sample. Spots that are yellow are equally expressed in both sample and control and black spots are genes that are not expressed in either sample are control.

An animated version of this techniqe can be seen in the website :

http://www.bio.davidson.edu/courses/genomics/chip/chip.html

http:/www.imagecyte.com/array2.html

For more information and guidance contact the author through: mpeeran@gmail.com