help PCR

[Julian Robinson]

      Iam trying to amplify a ~450bp fragment with two 20bp primers. 
It was working perfectly before I went on holiday. However when I 
came back I begtan to have problems. The problem was smears in all 
the lanes, including the control lane. It wasn't the template DNA 
that was the problem (it oocured in the -ve control), and it is'nt 
contamination (new everything has been used). 
We are leaning towards primer dimers as the answer at the moment (as 
there is some complementarity? between the primers), but the real 
question is why has it just started to occur, when it was perfect 
(i.e. no smears and clean bright product) before I went on holiday?

many thanks 
julian
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julian.robinson at plant-sciences.oxford.ac.uk