One of the most promising tools available today to researchers in life sciences is the
microarray technology. DNA chips or DNA microarrays are microscope slides (or
some other solid support) containing a large number of DNA samples. In cDNA
arrays, the samples are attached as a number of dots arranged in a regular pattern
usually forming a rectangular array. This array is subsequently probed with complementary
DNA (cDNA) obtained by reverse-transcriptase reaction. This DNA is
fluorescently labeled with a dye and a subsequent illumination with an appropriate
source of light will provide an image of the array of dots. Variants of the technique
use radioactive substances to label the mRNA. The intensity of each dot can be related
to the expression level of the particular gene corresponding to that dot. After an
image processing step is completed, the result is a large number of expression values.
The Affymetrix technology uses synthesized sequences spotted in small regions on
the chip and compares their hybridization with the hybridization of purposely mismatched
sequences. Independently of the specific technology used, the final result of
a microarray experiment is a set of numbers associated with the expression levels of
various genes or DNA fragments.