= Solution
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 pair>[base pairs] per helical turn. The tightly packed-cylinder upper <density> is therefore
$$
\boxed{\frac{2}{\pi(1\ \mathrm{nm})^2(0.34\ \mathrm{nm})}\simeq1.9\times10^{27}\ \mathrm{bits\,m^{-3}}.}
$$
A haploid human genome has about $6\times10^9$ 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 \i[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.
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