I thank Dr. Preston for his comments and for bringing this to my attention. I would like to clarify this point in reference to microarray experiments.
I think it is worth pointing out that there is confusion about the nomenclature that has crept into recent microarray literature for those who are familiar with conventional hybridization studies. The concept behind a microarray experiment is to know which mRNAs are present in a cell, so that we can identify which genes are being expressed in a given condition(s).
As certain genes become active, or "expressed," many copies of mRNAs corresponding to those genes are produced, and they in turn generate the necessary proteins. The term "mRNA" generally refers to the "target" or vice versa when discussing a microarray experiment. This mRNA refers to the source that is collected prior to an experiment. The mRNA is eventually labeled after being converted to cDNA prior to hybridization, which then becomes the "labeled representation of cellular mRNA." This process involves the conversion of the mRNA to labeled cDNAs by priming with oligo (dT) primers and extending the primers with fluorescently labeled nucleotides and the enzyme reverse transcriptase. The labeled cDNA becomes the target, which is then hybridized to the "probes" that are fixed to a solid support on the DNA array.
Another example of terminology confusion is the term "probe." In original hybridization literature, this term was used for the species of nucleic acid that carried the detection system, usually radioactively labeled, that is usually present in the solution phase (traditional blotting techniques). In microarray literature, "probe" often, but not invariably, refers to the DNA (usually known sequences) fixed on the solid support and that is usually not labeled.
I hope that this information is helpful. More information and details of microarray experiments can be found below.1
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