Your best bet would be to find a picture/s of DNA, then use it as reference. You could make some Splines in the shape of the DNA to start with, then use the Rail Extrude tool to make the strands and the rest of the shapes. If you don't know how to use the Rail Extrude tool, these are the basics: 1. Make a spline in the first layer. 2. In the second layer, create a flat disk centered on one of the end point of the spline. 3. Use the Rail Extrude tool. You may need to flip the disk in order for it to follow your spline properly.