Each base pair consists of 2 backbone pieces and 2 matching bases. For the pictured model I printed 8 of each base and 32 backbone segments. Ideally you'd print each base in a different color. I printed them all with white ABS and tinted them with Rit dye (about 30 minutes in a solution of 3 parts hot water, 1 part acetone).
The model is under stress when built, so the backbone pieces should be glued to each base. To stabilize the hydrogen bonds I wrapped them with cellophane tape. The tower needs to be stabilized, for which I cut short lengths of acrylic doweling.