Maybe a little too cute for some, but Kimberly Chapman has posted a guide to knitting your own model of the Double Helix. Scroll to the bottom of the page for the instructions.

From her guide: "Crafts Knit-Dna2This is a knitted model of DNA, complete with GC/TA base pairs represented by orange-green bars with a pointed join and blue-yellow bars with a stepped join (because there weren't enough stitches to make a wave or curve), replicating the standard simplified DNA model. As you can see from the pictures, it also makes a good toy insofar as it holds its shape while squished or stretched, because it is stuffed firmly with cotton balls."