I'll leave the accuracy part to you but here is how you can get a spiraling curve to start your helix. Lathe a 2D Circle on the Z grid. Give it 10 revolutions or so and an equal amount of vertical displacement. Export the spiral to the Amapi format then Import that as a Skin object. Unskin it completely. You'll end up with several spiral paths. Delete all but one of the outermost paths and you'll have a helix. Select that path and click the Center button then Rotate it 180 degrees with the Option key pressed. This will give you a duplicate copy 180 degrees offset for the other half of your double-helix. Now you can Path Extrude something along the paths to start your DNA molecule.
Here's an example of how I put one together with just two lathe objects and a bunch of cylinders between them. The setting for the lathe are on the jpg. I used a rounded rectangle to lathe but you could use any shape... After the lathe obj was made I made a copy and rotated it 180 deg. to make the other side.
Go into Reshape Mode. You'll see an outline of the shape you've lathed and its path. Grab the path in the center of the shape outline and you can move it in and out go control the center of the lathing.