Summary

An attempt at a shape-accurate DNA model, curved around on itself into a ring (5 helical turns for the earring, 12 for the bracelet and 6 for the spinning top).

Based on an extruded double helix generated using this script:

http://www.thingiverse.com/thing:186660

Instructions

I printed an older version of this model (shown in the picture) with a circular raft underneath, but it printed so well that I suspect it will work fine without any support.

A customisation script is also provided, which uses the path extrusion script that I previously wrote (see description).

My inspiration was from here:

http://www.shapeways.com/model/895917/dna-bracelet.html?li=related-items-int&materialId=23

But I have modified the appearance to be more consistent with "true" DNA. In particular:

10 bases per helix turn
major / minor groove
helical radius 1/3.3 * helix turn arc length
pentagonal extrusion (for ribose sugar backbone)
base pair joins perpendicular to helix direction

Additional things in the other model that were (possibly accidentally) already correct:

right-handed turn

See here for more details:

https://en.wikipedia.org/wiki/Nucleic_acid_double_helix#Helix_geometries

DNA Ring