Use restriction nucleases, which cuts double stranded dna at particular sequences. Target sequences are short - 4/8 nucleotides. The longer the target sequences the less likely it is to cut in a segment in a dna. Can use different length of restriction nuclease to cleave dna into sizes that you want. Bacteria use restriction nuclease to degrade foreign dna. Dna is loaded at one end of an agarose gel (that has pores) Dna (has negative backbones) will move towards the positive side. Smaller dna move fast through the gel since they can pass through pores easier. Dna separates into different sizes depended on their size. Each size forms a band" on the cell. To see the band the dna must be labeled or stained. Stain with dye that fluoresces under uv light. Tag using radioisotopes (32p - emits particles that can activate photographic film) Can cut out the desired band for further use.