This model was created in SketchUp, and twisted using the FredoScale plugin. For every pair of sides you print, print 4 rungs to go inside. You can print out a pair of supports if you want your DNA to stand on its own. If you can print in two colors, try printing the rungs with a color switch halfway along their length.
If you want to learn how to create this model yourself, the steps are shown in Chapter 5 of the book Modeling in SketchUp for 3D Printing.