Sooo, it seems like there was an interesting little dustup over the last couple of hours over which one of these duplicate question threads should be closed:
I wrote the accepted answer for the first question, and @David wrote the answer for the second question. It seems like the mods decided to close the thread with David's answer on the basis that it a couple of years younger.
I'm not crazy about the text of David's answer, but his illustration is pretty awesome. I think he really hit the nail on the head when it comes to giving a viewer an intuitive sense of how the geometry of the individual bases give rise to the particular grooves in the 3D helix. I'd like to merge the two answers (with David's permission, of course).
What's the SE policy about this kind of answer merging? Should the final result be a community wiki or something?