FIGURE 1.
Model of cellular uptake of 
complexes. (a) Cationic complexes adhere to cells due to electrostatic attraction between positively charged CL-DNA complexes and negatively charged cell-surface sulfated proteoglycans (shown in expanded views) of mammalian plasma membranes. (b and c) After attachment, complexes enter through endocytosis. (d) Only those complexes with a large enough membrane charge density (σM) escape the endosome through activated fusion with endosomal membranes. (e) Released DNA inside the cell is observed by comfocal microscopy to be present primarily in the form of aggregates. The DNA aggregates must reside in the cytoplasm because oppositely charged cellular biomolecules able to condense DNA are not present in the endosome. Arrows in the expanded view of c indicate the electrostatic attraction between the positively charged membranes of the complex and the negatively charged membranes of the endosome (comprised of sulfated proteoglycans and anionic lipids), which tends to enhance adhesion and fusion. Arrows in the expanded view in d indicate that the bending of the membranes hinders fusion.