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When proceeding to a DNA microarray analysis, I saw that the molecule that's taken from the cell is the RNA, which is afterwards converted to DNA by a reverse polymerase. But why not taking directly the DNA out of the cell?

EDIT : still about the DNA microarray method, I'd have another question: how can one be sure the DNA strands of the matrix are going to hybridize with the sample that one want to test?

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closed as off-topic by David, theforestecologist, AliceD Mar 14 '18 at 13:22

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework questions are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy." – David, theforestecologist, AliceD
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Your second question has little to do with the first one. Please consider opening another dedicated post for this second question. $\endgroup$ – Guillaume Mar 13 '18 at 16:34
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    $\begingroup$ Before posting a question like this, please stop and think. If you had asked yourself what you are analysing by microarray you would know the answer. I am voting to close this question as it does not show sufficient prior effort. $\endgroup$ – David Mar 13 '18 at 23:59
  • $\begingroup$ The context I've heard of that method was exclusively DNA sequencing.I didn't know it finds applications in analysis of RNA - because I'm no professional, you know, which is why I post a question here. Your comment is even more useless than my question. $\endgroup$ – Chewie Mar 14 '18 at 12:33
  • $\begingroup$ You guys are so stupid... You're so posh with your little conventions, you think science is a game with precise rules, and the one who's not rigorous enough has lost. Please consider the mortal people like me and their right to access to the sciences. $\endgroup$ – Chewie Mar 14 '18 at 22:34
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In each diploid cell, there are exactly 2 copies of every single gene. No matter what the cell type, no matter what you do to the cell. (Barring exceptions like some blood cells and cancer)

But RNA expression is what differs between cell types and conditions. Some cells make some RNA, some make others. Under some circumstances, a cell might make more of one RNA than at another time. Changes in RNA are what is worth measuring. DNA doesn't change.

RNA is converted to DNA because DNA, being double-stranded is more stable, less likely to degrade.

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RNA is extracted an analyzed because one possible application of a microarray is to quantify mRNA levels. As with other methods (for example RT-PCR), RNA is first reverse transcribed into DNA to make it compatible with the downstream protocol. Most procedures are indeed optimized for working with DNA, so it is easier to reverse transcribe RNA as the first step than to tweak everything else to make it work with RNA.

You can also start a microarray protocol with genomic DNA, but then you are looking at something completely different than gene expression. A possible application of a microarray in this case is genotyping.

Wikipedia has a lot of information about microarrays.

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