This model is one whose modeling steps are detailed in my book Modeling in SketchUp for 3D Printing.

It's basically a ladder with sides and rungs, twisted using the FredoScale plugin.

Start by printing two sides and four rungs. (If you have a dual extruder, print the rungs in two colors.) Place the rungs in one sides first, then gently attach the second side to the other side of the rungs, starting from the top and working your way to the bottom. Print several sets of rungs and sides and stack them up. Make sure you have two supports printed, if you want your DNA ladder to stand.