DNA oligonucleotides are spotted onto the array. The main feature of oligonucleotide microarrays is that each gene is normally represented by more than one probe.
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Spotted microarraysIn spotted microarrays technology, oligonucleotides, cDNA or small fragments of PCR products corresponding to specific genes are spotted on the chip. A robot spotter normally does the process and one or more probes can be used for each gene.
Contrary to oligonucleotide arrays, spotted arrays are "customizable"; the user can choose the probes to be spotted according to specific experimental needs. These kinds of arrays are usually hybridized with labeled cDNA and allow double channel hybridization experiments: two cDNA samples can be labeled with two different fluorophores (e.g. Cy3 and Cy5); these are mixed together and hybridized to the same chip. After washing, the chip is scanned at different wavelengths, thus allowing the measurement of the two samples in one run.