DNA has a negative charge. When you run a gel, you start with the wells containing DNA closest to the negative electrode. Positive attracts negative so the DNA migrates through the gel towards the positive electrode. If you were to start with the DNA wells near to the positive electrode, the DNA would still move towards the positive electrode - off your gel into your buffer, which would give you no data and buffer solution full of DNA. So you always start with the wells at the negative end so the negatively charged DNA moves through the gel towards the positive electrode.hope it helped
