Non-intertwined strands of plasmid DNA contradicts the Watson and Crick model of DNA structure

According to Watson and Crick (W/C) model, a DNA molecule consists of two antiparallel polynucleotide chains, intertwined with each other. Although the W/C model is accepted widely, a number of researchers have raised questions against it and proposed alternative structures for DNA. In the present study, the W/C model was examined using plasmid DNA. It was hypothesized that two strands of plasmid DNA will remain intertwined and not separate from each other under denaturing conditions, if it follows the W/C model. To test this, plasmid DNA was denatured using sodium hydroxide (NaOH) and analyzed by gel electrophoresis. It was observed that addition of NaOH to pUC19 and pBR322 plasmids resulted in new form of DNA with higher electrophoretic mobility in agarose gel. DNA corresponding to higher electrophoretic mobility band of pUC19 (hmP19) was single-stranded and circular, indicating the separation of two strands of pUC19 plasmid. Next, we examined whether hmP19 DNA can re-anneal to form native pUC19 plasmid. It was observed that neutralization of NaOH resulted in the appearance of native pUC19 plasmid in denatured DNA. Native pUC19 was also formed by hmP19 DNA extracted from agarose gel and was found to be digestible with Hind III. Ability to confer ampicillin resistance in transformed Escherichia coli demonstrated the functionality of pUC19 plasmid formed by extracted hmP19 DNA. Reversible separation of two strands of plasmid into single-stranded circular DNA shows that DNA strands are not intertwined with each other and contradicts the W/C model of DNA structure.