Abstract
Using a magnetic tweezers (MT) apparatus and an atomic force microscope (AFM), we studied the condensation of DNA induced by the cationic gemini surfactant hexyl-alpha,omega-bisdodecyldimethylammonium bromide (C12 C6 C12 Br2). Stepwise condensation events were found for forces from 0.5 to 4 pN, with a decrease in DNA extension by approximately 100 nm in each condensation event. Applied stretching forces larger than 6 pN were found to be able to decondense the condensates in a similar intermittent stepwise manner, but with approximately 60 nm of DNA released in each decondensation event. These observations are consistent with AFM images that show beadlike structures on DNA. The results lead to a model in which, during condensation, independent beadlike structures are initially formed on DNA, and as the local density of the beads increases, they compact into higher-order structures while maintaining their independence. The condensation process is different from any previously reported condensation process, but it is somewhat similar to that of chromatin assembly.
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