Am running several DNA markers and ladders on a 0.5% agarose gel at 70V. Using TAE buffer with SYBR green 1 pre-loaded in gel. I am getting one long smear for all samples. I have done this before without problems so am concerned why this is happening. Markers are only 4 weeks old and have been stored in refridgerator. Only difference from last time was I centifuged the samples breifly to get them at the bottom (they had been shaken during transit) plus I used some very old agarose while waiting for new stock.
Any help or explanations appreciated.