Abstract: Apparatus and method of sequencing nucleic acid molecules, including DNA and RNA molecules. Several copies of the molecules are labeled with a radioactive tracer and divided into separate groups. The groups of molecules are then each chemically cleaved to produce labeled fragments of varying length which terminate in predetermined bases. The groups of fragments are then introduced into separate columns of electrophoresis gel and voltage is applied across the gel to cause subgroups of the fragments to propagate along the columns at a velocity which is a function of the length of the fragments in the subgroup. Radiation detectors are used to detect the subgroups of fragments as they pass a predetermined position along the gel column, and data corresponding to the order in which the subgroups of fragments are detected is stored in a memory. Such data is then used to determine the bases sequence of the molecules.