Sunday, March 22, 2015
On the handedness of DNA
This is not how DNA is actually built since there's never a time inside the cell when the DNA forms a ladder-like structure that's not helical, but you get the picture. [The Three-Dimensional Structure of DNA]
The final form of double-stranded DNA on the right is a cartoon used to illustrate certain features. I've deliberately drawn it with about 10-11 base pairs per turn so you can see the shape of the helix.
There are rare forms of DNA (e.g. Z-DNA) that are left-handed. The normal form of DNA is B-DNA.
When you see the two forms side-by-side it's easy to see that they are different but when you see a single depiction of DNA it's pretty hard for most of us to tell whether it's the correct form or the incorrect form. I'm not very good at it. In fact, I almost published the incorrect form in my first textbook. A graduate student/science writer named John Challice saved me from embarrassment by pointing out the error. (He's now a bigwig at Oxford University Press in New York.)
If you know this stuff then you can check published drawings of DNA to see if they are correct. If they are not then you have a right to be skeptical about the quality of the work that's attached to the drawing. If it's a book about DNA or genomes and the author gets the structure of DNA wrong then caveat emptor.
This came up recently in comments about one of my recent posts [How the genome lost its junk according to John Parrington]. I've pasted in a copy of the cover of the book so you all can practice your new knowledge of DNA structure.