Past exam of the short options paper shared by Prelims and Parts A and B of the physics course of the University of Oxford / 2016 / S12 / 2 / 3

A base pair has four possible sequence identities, hence two bits, not four bits from treating complementary strands as independent. B-DNA has diameter approximately 2 nm, axial rise 0.34 nm per base pair and about ten base pairs per helical turn. The tightly packed-cylinder upper density is thereforeA haploid human genome has about $6\times 10^9$ raw bits; in a nucleus of several hundred cubic micrometres its bulk density is order $10^{25}$ bits m${}^{-3}$, far lower. Cellular DNA must allow hydration, packaging, regulatory access and copying/repair machinery, and cannot fill a cell as parallel bare cylinders. Diploidy and repeated or constrained sequence also reduce independent functional information compared with the raw four-letter capacity; those concepts should not be confused with geometric storage density.

The original molecular figure in question 3 shows the approximate helical dimensions.