Agarose gel electrophoresis of DNA samples sheared at different temperatures. λ DNA was sheared for 20 iterations with the PtS and separated by agarose gel electrophoresis. The samples in every lane were handled under the same conditions in each case except that the temperature at the point of shearing was as indicated. The temperature was controlled with a rectangular silicone rubber heater that was wound around the syringe, valve, and tubing. The temperature was measured with a thermocouple that was in direct contact with the tubing between the valve and the shearing assembly. (ds) Double stranded; (ss) single stranded.