Non-specific band in positive control but negative control not
 

Dear guys,

My previous PCRs were OK but recently, I have got a problem of nonspecific band in positve control. I'm sure this is not contamination in reagents because negative control is no problem. So i tend to think may be positive DNA was contaminated. But non-specific band is about 1000kb :frusty: and target band is 123bp:frusty:

If my PCR process have problem, previous PCR results would have non-specific band. But they were not.

PCR reaction
940C for 4 min
Denaturation : 94C for 2 mins
Annealing : 68C for 2 mins 35 cycles
Extension : 72C for 1 mins
72C for 10 mins

load DNA 5ul

May be because concentration DNA was too much? It's was about 28 ng/ul:shakehead: