There are a lot of LEGO renditions of DNA out there, indeed I chose one for the banner of this blog, but most don't really have an accurate structure. This one by SICP_DNA, though, is very good about keeping to the actual structure of DNA. In DNA, the strands on either side are pointing in opposite directions, and the white and gray backbones have the studs pointing in different directions here. Since the backbone sugars are asymmetric, the 'grooves' are different sizes. Another aspect of DNA is that two of the bases, the purines A and G, are larger than the other two, the pyrimidines T and C. The connections here are such that it only works out when A lines up with T or G lines up with C. They also got the sizes right here, so that one complete twist takes ten steps, and the width to height ratio is approximately correct.
Here you see a comparison between the 'real' structure of DNA, and a simplified depiction. BTW, the LEGO version below is from Nathan Sawaya's Building Bricks of Life. I certainly mean no disrespect, as it is a great model. But as Nathan has said elsewhere he's an artist, not a scientist, and he writes that he decided to go with this more artistic approach (as a commissioned piece for the Archon Genomics X Prize) rather than try to do a more accurate version. Someday I'd like to see him go back and do an accurate depiction of DNA. He's a great sculptor and I'm sure it would be quite cool.
BTW, since I've posted two Cuusoo projects in a row, I wanted to say a little about that. I was corresponding with another LEGO blogger the other day about these, and I agree with him that a great number of the things posted there are not actually viable LEGO sets, including this one (sorry, SICP_DNA). So if I blog something, it is not necessarily true that I am supporting it as a Cuusoo project. I just see Cuusoo as another place where LEGO builders post their creations, along with other photo sharing sites.