A gel for analysis of DNA samples by electrophoresis is accommodated in the channels of a wall structure (1) formed by an extruded plastics material such as that known as Correx. For electrophoresis treatment with the plastics wall structure (1) maintained vertical in an electrophoresis tank, the sample for analysis may be added to the open top of a channel partially filled with gel (6). The wall structure (1) may have a transverse removable strip (14) giving access to the gel-filled channels, for electrophoresis with the wall structure (1) horizontal.