DNA microarrays consist of a collection of gene sequences immobilized on a solid support. The solid support can be glass, silicon chip (in which case they are commonly called “gene chip”), or microscopic beads. Each unique gene sequence forms a tiny feature called “spot” or “target” on the solid surface. Spot sizes vary depending on the way the microarray was manufactured, but they are usually less than 200 µm in diameter with each spot containing picomoles of a specific DNA sequence.
A single microarray can contain up to tens of thousands of spots. The spots themselves can be DNA, cDNA, or oligonucleotides. Microarrays can be manufactured in different ways, depending on the number of spots/targets, costs, customization requirements, and the type of scientific question being asked. Arrays may have as few as 10 targets to up to 2.1 million µm-scale targets. They can be fabricated using a variety of technologies, such as printing with fine-pointed pins onto glass slides, photolithography using pre-made masks, photolithography using dynamic micromirror devices, ink-jet printing, or electrochemistry on microelectrode arrays.
Once the microarray is ready, the targets can hybridize with complementary cDNA and cRNA probes derived from experimental or clinical samples, which have been labeled. Probe-target hybridization is usually detected and quantified by detection of fluorophore-, silver-, or chemiluminescence-labeled targets to determine relative abundance of nucleic acid sequences in the target.
One of the main applications of microarray technology is in studying many different diseases, such as heart disease, mental illness and infectious diseases, to name a few. It is also used in drug discovery for target identification and toxigenomics.
[Note: “Probe” and “target” are sometimes used interchangeably. In this text, the immobilized nucleic acid sequence is referred to as the “target”, and the labeled sample is the “probe”.]
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