A False Positive

I perhaps became a little too ambitious, overexcited, and rushed with PCR after I figured out the process. Yesterday, I attempted to both extract DNA samples from 11 snails, and run a PCR on all of them. Though that may not sound time consuming, the DNA extraction took at least two hours, DNA quantitation 30 minutes, the PCR prep took another few hours, and the little 0.2mL tubes had to cook overnight for the reaction to work. Today, when I went in, I ran 3 gels and found that all my negative snails had supposedly “positive” results. Instead of one solid band between the 300 and 400 base pair markers, I noticed a double band in almost each snail DNA sample. I had not originally made a negative control, where I excluded template DNA. When I made the negative control later, I found that one of the PCR chemicals had been contaminated. What should have shown up as  a blank space on the gel had the same double banded pattern in the same location.


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About Diana Dalbotten

Diana Dalbotten is the Director of Diversity and Broader Impacts for the National Center for Earth-surface Dynamics and the St. Anthony Falls Laboratory, University of Minnesota; and for the Geoscience Alliance, a national alliance for broadening participation of Native Americans in the Geosciences.

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