Really cool topic. I think Wikipedians will be drawn to this, both DNA and nanotechnology are popular buzzwords. I think inherent difficulty of the material is not so hard that it would be impossible to write an article that draws people in (some very tough math and physics is).
Prose is not awful (bad grammar or the like), but it is verbose in places. Lots of places with excess words or "ands". Feels like way too many nominalizations also.
Feels like there is too much definition by blue-linking and/or bluelinking to another article when not needed. The blue bumps will slow the reader down and material is already somewhat technically difficult, so the more you can avoid colored text the better. I wonder if you are trying to pimp a bit some of the other nano articles.
I know you were asked to increase the lead, but I find the first two paras somewhat repetitive and giving us info on a pretty low value concept (this is DNA for non biology). Third para is OK. I wonder if we had second para more on different types of structures or sub-branches of the field, that would be better content.
In terms of technical level, it is not off the hook hard, but still a bit harder than desirable. Worry that you are killing reader interest in what could be a cool topic. We probably ought to be looking to be about like a Scientific American article or a tad lower in technicality. I had a hard time staying engaged with it while I read.
Just some ideas, what about moving History to the first post lead section and maybe moving Applications higher up. I just know these will be easier for lay readers to get into.
Diagrams seemed decent, although small. I do appreciate how they help make sense of the shapes and such. Maybe a diagram of the origami would be good.