- Abstract One of the simple, robust and scalable methods for purification of plasmid DNA is by membrane filtration. Utilizing manipulation of pH, ionic strength, and membrane pore size, the process is designed to completely retain the plasmid DNA. The performed study shows that despite electrostatic repulsion and a significant size difference between plasmid and membrane pore, the circular double-stranded DNA plasmid is able to penetrate through UF membranes. Experiments demonstrate that the tertiary structure of DNA changes easily and is dramatically affected by variations in ionic strength and pH. The penetration is probably due to stretching of the plasmid by hydrodynamic shear. In contrast to previous reports, the transmitted DNA copies kept their integrity and infectious ability.