You will see a band around 50bp. They usually form a more faint band. Check this pic (it's not mine, I found it online).

Also by the time you can see primer dimer a lot of primer has been removed so you will often find that if you run many samples the samples with primer dimer have amplified less well ( weaker band) than samples without primer dimer. It can often stop an amplification working completely and is often caused either by poor primer design ( homology at the 3' ends of the primers) or by leaving the reaction mixture for too long before starting cycling.

primer dimers will never be as big as Ur amplicons and if you r running a 100 bp ladder the lowest band of ladder would be 100 bp and primer dimers would always would be lesser than that lower band of  marker, to reduce primer dimers you can do touch down PCR

I think that's a good question. I really don't know if primer dimers can be separated from the primers themselves in something like 1% agarose, but I also dont know if anyone has tried to resolve them using, say, 15-20% acrylamide gels. I think its clear that ethidium bromide will stain DS oligos more effectively than SS oligos, but I always had the impression that given the amounts present, it was the SS primers that I have seen in my PCRs rather than PDs (especially when using two primers of quite different lengths, where I see two bands). So (not) Andre's pic to me doesn't (necessarily) demonstrate PDs!

Geoff

I don't think Baiq is trying to resolve ss primer from ds primer. Apparently she was only interested in the outcome of her PCR reaction and possible primer dimers.

Her question was: How do we know that there're primer dimers in electrophoresis gel result? Does it looks like some strange bands or what?

Indeed, she isn't trying to resolve anything, but my response related to what might be present, and was hoping to elicit comments from others on this thread who know more about these phenomena than I do  (and in so doing, help Baiq, (and me!) to understand whats happening.

hI Geoff,

I think that is a good point about single primers fluorescing but very often they do not even though the initial concentrations are the same. Do you think that primers coil up so can trap ETBR or that some primers self anneal( hairpin formation( and it is the double stranded part that traps the dye? It is surprising how robust some primer sets can be...I ran about 90 mapping CA repeats at Genethon years ago and we set up the reactions in plates and left them 2 days in a cld room prior to pcr with the enzyme in place and the p=d was still very low or non existant after the pcr

Why does primer dimer form in Gel electrophoresis? what are the possible causes and solutions of this problem. Please help if possible from anybody of the world.