UVA Virtual Lab: DNA (close-up & do-it-yourself)
© 2003-Present J.C. Bean
Here are two repetitions of the DNA backbone unit. You can identify the sugar and phosphate components by clicking the corresponding button, and move the model with your mouse.

The phosphate group is of particular interest. One of the P-O bonds is actually a double bond (a sigma bond plus a pi bond). By clicking the "Toggle Bonds" button you can see how the pi bond is actually shared by two different O atoms.

The phosphate group′s bonding gives it a net single negative charge. That means there is a line of negative charge along both helical backbones of DNA. This tries to force the backbones apart (and must be countered by the attractive force of base pair hydrogen bonding). It also means that DNA backbones attract positive ions. This has hugely complicated nanoscience experiments aimed at using DNA as an electrical conductor.
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