Exact amplification in negative control
I am working with transgenic plants and use genomic DNA PCR screening to search for positive transgenic plants. I use the plasmid containing construct used for transformation as positive control , wild type untransformed plant as the negative control and transformed transgenic plant as samples. I am facing a very annoying and frustrating problem. I am seeing the exact band I want in all my samples including the negative control where it should not be seen at all. I have isolated genomic dna of the wild type plant a different day than all my transgenic samples to avoid cross contamination.I always prepare master mix and when adding template I first add the control followed by the others. I have also used bleach to wipe the work area (laminar hood) and pipettes and then UV'd it for 5-10 mins.Then I start working. I have used fresh batch of buffer,dNTPs and Taq polymerase. I am using different combination primers such gene, gene+terminator, GUS gene etc. All give the same results, exact size band in the negative control. I am close to tearing my hair out and this has been going on for close to 2 months. I am at my wit's end and dont know what to do. I have also used comntrols such as without primer and without template. Amplification seen in without template control. But I have used fresh primer stock.
Please help me identify the source of contamination and eliminate it as soon as possible. What can I do ?