Relationship between mean size of sheared DNA and number of passes through the shearing device. λ DNA was sheared by passing it through the orifice at 6.7 ml/min for 1, 3, 5, 10, 15, or 20 iterations. Each aliquot (0.5 μg) was separated by agarose gel electrophoresis and stained with ethidium bromide. The median size was determined by the darkest staining position on the gel.