GeneChip® expression arrays enable one to measure expression levels of transcripts both quantitatively and qualitatively. Affymetrix array technology involves the in-situ synthesis of hundreds of thousands of distinct oligonucleotide sequences onto a glass array using photolithography and combinatorial chemistry. Each 25-mer oligonucleotide is represented by millions of copies in a specific area. Each transcriptional sequence is spanned by 11-20 pairs of oligonucleotide probes randomly spaced throughout the array. All sequences designed on the array are selected from GenBank, dbEST and RefSeq. Target RNA is reverse transcribed into cDNA and in-vitro transcription is performed to generate biotin-labeled cRNA for subsequent hybridization. Hybridized target cRNA is stained with streptavidin phycoerythrin and arrays are scanned using a GeneArray Scanner at an excitation wavelength of 488nm. Light emissions at 570nm are proportional to the bound target at each oligonucleotides position on the GeneChip® array.
Arrays are currently available for many biologically relevant model organisms.
Detailed information about the chips is available from the Affymetrix web site.