Six wells sit at the cathode (−) end. DNA's phosphate backbone is negative — the bands migrate toward the red anode. Each sample can be loaded once per gel.
DNA ladder: Known fragments (150–3000 bp) — the ruler of the gel. Every unknown is sized by comparing band distances against this lane.
Evidence (crime scene): DNA recovered at the scene, cut with the same restriction enzymes as the suspects’ samples (RFLP profiling).
Suspect 1: Reference sample from suspect 1.
Suspect 2: Reference sample from suspect 2.
Suspect 3: Reference sample from suspect 3.
Field strength sets the pace — at 100 V a 150 bp fragment crosses the 80 mm gel in ~30 s. Above 130 V the gel runs hot and bands smear.
While running you see only the blue dye front — the DNA bands appear under UV after you stop. Bands that reach the far end run off the gel and are lost.