Why do DNA fragments migrate through the gel from the negatively charged pole to the positively charged pole?

DNA fragments are negatively charged due to presence of phosphate groups.

Explanation:

Each deoxyribonucleotide of DNA has one nitrogenous base, a pentose sugar and a phosphate group. The phosphate group is negatively charged at pH=7. Therefore, DNA fragments with a sugar-phosphate backbone are negatively charged. The presence of negative charge allows the DNA molecules to interact with positively charged histones. These negatively charged DNA fragments migrate towards the positively charged terminal of the gel during the process of gel electrophoresis.