For example, if I have Ct = 18 for GAPDH and Ct = 20 for the gene of interest, is it possible to conclude that there are 4 times more transcripts of GAPDH than transcripts of the gene of interest?
Hi,
if both PCRs have the same efficiency and in case the efficiency is near to 100% the answer is yes.
Simple answer: no, that is not possible (in contrast to Sven's answer; not even when the efficiencies are the same!)
If you really need the ratio of different sequences, then you have to quantify them using external standards.
Jochen Wilhelm Well he does only want to know if the abundance is 4 times higher. This is something I would say is possible even without an abaolute quantification. I agree if you want to know the total copies you need to have the standards
Sven
Sven Reister, I quite agree with you regarding the detection of large differences. However, 4 times are about two cycles, and I yould not consider this difference large enough to reliably indicate a higher (or lower) expression. If you design two different primer pairs for the very same transcript sequence, you easily find 1-3 cycles difference between them, although they do quantify the same transcript.
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