is a collection of microscopic DNA spots attached to a solid surface. Scientists use DNA microarrays to measure the expression levels of large numbers of genes simultaneously or to genotype multiple regions of a genome.
Type of DNA microarray :-
Spotted arrays :-
In 1996 Derisi et. al. published a method which allowed very high-density DNA arrays to be made on glass substrates.
By using slotted pins (similar to fountain pens in design) a single dip of a pin in DNA solution could spot multiple slides.
Spotting onto glass, allowed one to fluorescently label the sample. Fluorescent detection provided several advantages relative to the radioactive or chemilluminescent labels common to filter based arrays.
fluorescent detection is quite sensitive and has a fairly large dynamic range. Second, fluorescent labeling is generally less expensive and less complicated than radioactive or chemilluminescent labeling.
In-situ, Synthesized arrays :-
In 1991 Fodor et.al. published a method for light directed, spatially addressable chemical synthesis 1994,
Fodor et.al. at the recently formed company of Affymetrix demonstrated the ability to use this technology to generate DNA arrays consisting of 256 different octa-nucleotides.
Affymetrix arrays were being used to detect mutations in the reverse transcriptase and protease genes of the highly polymorphic HIV-1 genome(Lipshutz et al., 1995) and to measure variation in the human mitochondrial genome.
Applications of microarrays :-
1) Gene expression analysis :-
The predominate application of DNA microarrays has been to measure gene expression levels. In this application, RNA is extracted from the cells of interest and either, labeled directly, converted to a labeled cDNA or converted to a T7 RNA promoter tailed cDNA which is further converted to cRNA through the Eberwine amplification process (Van Gelder et al., 1990).
2) Transcription factor binding analysis :-
Microarrays have also been used in combination with chromatin immunoprecipitation (Solomon et al., 1988) to determine the binding sites of transcription factors.
3) Genotyping :-
Microarrays have been widely used as single-nucleotide-polymorphism (SNP) genotyping platforms. Several alternative approaches have been used to detect SNP’s but the most commonly used are allele discrimination.
4) Data standards and data exchange :-
With the exception of DNA sequencing, microarrays were perhaps the earliest technology that allowed biologists to vast amounts of complex digital data.
As the technology came into use, it rapidly became apparent that in order for others to be able to reproduce a given microarray experiment a detailed description of the array, the sample, the protocols and the data analysis methods needed to be available.
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