[xoshi64]

I have been studying the makeup of DNA and I have been attempting to create a double helix molecule but for the life of me, I do not understand why it won't twist. I do not believe Kalzium (the app), would leave this basic functionality out.  Any hints or tips as to why it is not twisting would be greatly appreciated.

Actually, Im trying to get the image to load correctly. Sorry.

[Yggdrasil]

It actually takes quite sophisticated molecular modeling programs to model the structure of DNA from scratch.  The double helical structure of DNA does not come just from interaction of atoms within the DNA, but also its interaction with the surrounding solvent molecules.  For example, DNA forms a helix with one type of geometry (B-form DNA) in water, but a helix with another geometry when dehydrated (A-form DNA).

If you're interested in playing around with the 3D structure of DNA, I'd visiting this page and some of the pages it links to: http://www.rcsb.org/pdb/101/motm.do?momID=23

[Babcock_Hall]

Just to riff on what yggdrasil said, some sequences favor the Z-DNA conformation, which is a left-handed double helix.