Why can gel electrophoresis separate DNA fragments?

Because each DNA molecule is negatively charged, it can be pulled through the gel by an electric field

Ans.

Gel electrophoresis can be defined as a molecular biology technique, which is used to separate biomolecules, such as RNA, DNA, or proteins from a mixture on the basis of their size.

During separation, sample mixture, having DNA fragments of various size are placed in the wells, formed on agarose gel. The gel acts as sieve, by which DNA molecules move according to their size and get separated.

The apparatus has two poles, negative pole and positive pole. Due to phosphate groups, DNA fragments carry negatively charged, so when charged in applied in gel electrophoresis apparatus, they move toward positive pole from negative pole.

Thus, ragments get separated by the gel electrophoresis on the basis of their size as large fragments move through the agarose gel slower than the smaller ones.