DNA microarray Example of an approximately 40,000 probe spotted oligo microarray with enlarged inset to show detail. A DNA microarray is a multiplex technology used in molecular biology. It consists of an arrayed series of thousands of microscopic spots of DNA oligonucleotides, called features, each containing picomoles (10-12 moles) of a specific DNA sequence, known as probes (or reporters). These can be a short section of a gene or other DNA element that are used to hybridize a cdna or crna sample (called target) under high-stringency conditions. Probe-target hybridization is usually detected and quantified by detection of fluorophore-, silver-, or chemiluminescence-labeled targets to determine relative abundance of nucleic acid sequences in the target. Since an array can contain tens of thousands of probes, a microarray experiment can accomplish many genetic tests in parallel. Therefore arrays have dramatically accelerated many types of investigation. In standard microarrays, the probes are attached via surface engineering to a solid surface by a covalent bond to a chemical matrix (via epoxy-silane, aminosilane, lysine, polyacrylamide or others). The solid surface can be glass or a silicon chip, in which case they are colloquially known as an Affy chip when an Affymetrix chip is used. Other microarray platforms, such as Illumina, use microscopic beads, instead of the large solid support. DNA arrays are different from other types of microarray only in that they either measure DNA or use DNA as part of its detection system.
DNA microarrays can be used to measure changes in expression levels, to detect single nucleotide polymorphisms (SNPs), or to genotype or resequence mutant genomes (see uses and types section). Microarrays also differ in fabrication, workings, accuracy, efficiency, and cost (see fabrication section). Additional factors for microarray experiments are the experimental design and the methods of analyzing the data (see Bioinformatics section). History Microarray technology evolved from Southern blotting, where fragmented DNA is attached to a substrate and then probed with a known gene or fragment. Nucleic Acids Res. 1992 Apr 11;20(7):1679-84. Oligonucleotide hybridizations on glass supports: a novel linker for oligonucleotide synthesis and hybridization properties of oligonucleotides synthesised in situ. Maskos U, Southern EM. The first reported use of this approach was the analysis of 378 arrayed lysed bacterial colonies each harboring a different sequence which were assayed in multiple replicas for expression of the genes in multiple normal and tumor tissue (Augenlicht and Kobrin, Cancer Research, 42, 1088 1093, 1982). This was expanded to analysis of more than 4000 human sequences with computer driven scanning and image processing for quantitative analysis of the sequences in human colonic tumors and normal tissue (Augenlicht et al., Cancer Research, 47, 6017-6021, 1987) and then to comparison of colonic tissues at different genetic risk (Augenlicht et al., Proceedings National Academy of Sciences, USA, 88, 3286-3289, 1991). The use of a collection of distinct DNAs in arrays for expression profiling was also described in 1987, and the arrayed DNAs were used to identify genes whose expression is modulated by interferon. [1] These early gene arrays were made by spotting cdnas onto filter paper with a pin-spotting device. The use of miniaturized microarrays for gene expression profiling was first reported in 1995, [2] and a completeeukaryotic genome (Saccharomyces cerevisiae) on a microarray was published in 1997.
Principle hybridization of the target to the probe Main article: Nucleic acid hybridization For more details on this topic, see DNA microarray experiment. The core principle behind microarrays is hybridization between two DNA strands, the property of complementary nucleic acid sequences to specifically pair with each other by forming hydrogen bonds between complementary nucleotide base pairs. A high number of complementary base pairs in a nucleotide sequence means tighter noncovalent bonding between the two strands. After washing off of non-specific bonding sequences, only strongly paired strands will remain hybridized. So fluorescently labeled target sequences that bind to a probe sequence generate a signal that depends on the strength of the hybridization determined by the number of paired bases, the hybridization conditions (such as temperature), and washing after hybridization. Total strength of the signal, from a spot (feature), depends upon the amount of target sample binding to the probes present on that spot. Microarrays use relative quantization in which the intensity of a feature is compared to the intensity of the same feature under a different condition, and the identity of the feature is known by its position. An alternative to microarrays is serial analysis of gene expression, where the transcriptome is sequenced allowing an absolute measurement. The step required in a microarray experiment.
DNA microarray experiment steps involved in a microarray experiment (some steps omitted)
This is an example of a DNA microarray experiment, detailing a particular case to better explain DNA microarray experiments, while enumerating possible alternatives. 1. The two samples to be compared (pairwise comparison) are grown/acquired. In this example treated sample (case) and untreated sample (control). 2. The nucleic acid of interest is purified: this can be all RNA for expression profiling, DNA for comparative hybridization, or DNA/RNA bound to a particular protein which is immunoprecipitated (ChIP-on-chip) for epigenetic or regulation studies. In this example total RNA is isolated (total as it is nuclear and cytoplasmic) by Guanidinium thiocyanate-phenol-chloroform extraction(e.g. Trizol) which isolates most RNA (whereas column methods have a cut off of 200 nucleotides) and if done correctly has a better purity. 3. The purified RNA is analysed for quality (by capillary electrophoresis) and quantity (by using a nanodrop spectrometer): if enough material (>1µg) is present the experiment can continue. 4. The labelled product is generated via reverse transcription and sometimes with an optional PCR amplification. The RNA is reverse transcribed with either polyt primers which amplify only mrna or random primers which amplify all RNA which is mostly rrna, mirna microarray ligate an oligonucleotide to the purified small RNA (isolated with a fractionator) and then RT and amplified. The label is added either in the RT step or in an additional step after amplification if present.the sense that is labelled depends on the microarray, which means that if the label is added with the RT mix, the cdnais on the template strand while the probe is on the sense strand (unless they are negative controls). The label is typically fluorescent; only one machine uses radiolabels. The labelling can be direct (not used) or indirect which requires a coupling stage. The coupling stage can occur before hybridization (two-channel arrays) using aminoallyl-utp and NHS amino-reactive dyes (likecyanine dyes) or after (single-channel arrays) using biotin and labelled streptavin. The modified nucleotides (typically a 1 aautp: 4 TTP mix) are added enzymatically at a lower rate compared to normal nucleotides, typically resulting in 1 every 60 bases (measured with a
spectrophotometer). The aadna is then purified with a column (using solution containing phosphate buffer as Tris contains amine groups). The aminoallyl group is an amine group on a long linker attached to the nucleobase, which reacts with a reactive dye. A dye flip is a type of replicate done to remove any dye effects in two-channel dyes, in one slide one same is labeled with Cy3 the other with Cy5, this is reversed in a different slide. In this example, in the presence of aminoallyl-utp added in the RT mix. 5. The labeled samples are then mixed with a propriety hybridization solution which may contain SDS, SSC, dextran sulfate, a blocking agent (such as COT1 DNA, salmon sperm DNA, calf thymus DNA, PolyA or PolyT), Denhardt's solution and formamine. 6. This mix is denatured and added to a pin hole in a microarray, which can be a gene chip (holes in the back) or a glass microarray which is bound by a cover, called a mixer, containing two pinholes and sealed with the slide at the perimeter. 7. The holes are sealed and the microarray hybridized, either in a hyb oven, where the microarray is mixed by rotation, or in a mixer, where the microarray is mixed by alternating pressure at the pinholes. 8. After an overnight hybridization, all nonspecific binding is washed off (SDS and SSC). 9. The microarray is dried and scanned in a special machine where a laser excites the dye and a detector measures its emission. 10. The image is gridded with a template and the intensities of the features (several pixels make a feature) are quantified. 11. The raw data is normalized, the simplest way is to subtract the background intensity and then divide the intensities making either the total intensity of the features on each channel equal or the intensities of a reference gene and then the t-value for all the intensities is calculated. More sophisticated methods include z-ratio, loess and lowess regression and RMA (robust multichip analysis) for Affymetrix chips (single-channel, silicon chip, in situ synthesised short oligonucleotides).