DNA fragments generated by the restriction endonucleases in a chemical reaction can be separated by

The different sized fragments generated by restriction digestion of a long DNA molecule are separated by a size-based separation technique called gel electrophoresis. Separation is size based and it is achieved by the sieving effect of agarose gel, as the negatively charged DNA molecules migrate towards anode through the gel matrix.

(2) Restriction mapping involves mapping of an unknown segment of DNA by breaking it into pieces and identifying the locations of the break points. 

(3) Centrifugation is a density-based separation technique widely used in molecular biology. In centrifugation,
molecules having different densities get separated by spinning them in solution around an axis (in a centrifuge rotor) at high speed. 

(4) Polymerase chain reaction is a powerful laboratory technique for rapid amplification of a specific segment
of DNA molecule to obtain billions of copies using a thermocycler.