DNA microarray

DNA chip that provides information about the activity of individual genes

DNA microarrays help identify possible changes in the activity (expression) of certain genes as a function of various factors. In genetic engineering, the method is used among other things to compare expression in transgenic and non-transgenic plants.

• In a first step the mRNA (messenger RNA) of the organism under investigation is isolated. The quantity and composition of the mRNA in a cell provide an indication of which genes are expressed with what frequency. The mRNA can be used to reconstruct the DNA fragments that contain the information expressed by the mRNA. This reconstructed DNA (cDNA) is marked using a fluorescent dye.
• The DNA microarray contains all the genes of the organism under investigation on a glass plate, the ‘gene chip’, as an arrangement (or array) of single-stranded DNA. The generated cDNA is inserted onto the DNA chip and the cDNA sequences bind (hybridise) to their complementary counterpart on the array.
   

In order, for example, to compare the gene expression of two samples (see diagram), the level of fluorescence is measured. The position, intensity and colour of each spot on the chip provide information about the expression rate of a gene. A blue spot means that the gene in question is expressed more strongly in sample A than in sample B. A yellow spot indicates a stronger expression in sample B. Green spots show equal activity of the gene in the two samples.