(Instead of destroying DNA by DNAses, cant we use this DNA directly for PCR)
cDNA made from rna has no introns . If we destroy all other ( huge size) genomic DNA then any amplification must come from the cDNA of interest . With the other DNA present there is always a chance of the wrong thing amplifying Perhaps not a problem if amplifying across introns but you never know if there is a pseudogene or similar sequence in the genome to confuse things so best to have as little unwanted DNA as possible
PCR with genomic DNA gives a idea about the presence or absence of the target gene but to see whether it is expressing or not ,semiquantitative or Real time PCRs are done. DNase treatment is given during RNA isolation to remove DNA ,So that whatever amplification we get is only from cDNA (which is directly drives from mRNA) and not because of genomic DNA contamination present in our samples.
The objective of RNA to DNA coversion is to analyse some specific gene and not to analyze entire genome. So during RNA isilation DNAse is used to digest genomic DNA and when RNA Is converted to DNA through reverse transcription it contain only cDNA(Complementary DNA) of all different gene. cDNA does not contain any introns..
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