Why do supposedly sharp bands sometimes appear unclear in agarose gel? - (May/24/2012 )

Dear all,

Why do my bands sometimes appear like the following picture when i run agarose gel electrophoresis?

(somehow i fail to upload the photo in this forum, so please refer to http://imageshack.us...5/37539167.png/)

These are supposedly sharp bands but they just won't separate well. Sometimes I re-run agarose gel and I got sharp bands!

You told us absolutely nothing about this gel.