The circular chromosome of the bacterium Escherichia coli has 4.6 million base pairs…

Step 1. (a) $4.6 \times 10^6 \times 0.34\ \text{nm} = 1.564 \times 10^6\ \text{nm}$. Since $10^6\ \text{nm} = 1\ \text{mm}$, the length is 1.56 mm.

Step 2. (b) $4.6 \times 10^6 \div 10 = 460000$ turns.

Step 3. (c) The cell is $2\ \mu\text{m} = 2000\ \text{nm}$ long. The ratio is $1.564 \times 10^6 \div 2000 = 782$.