The two chains of DNA are coiled in a right-handed helicle fashion. The pitch of the helix is $3.4$nm and has a diameter of $2$ nm. In the B-form of DNA, the distance between a bp in a helix is approximately $0.34$nm which means there are about $10$ nucleotides present in a complete turn. 

The total length of DNA molecule in nm = $1.1\times {10}^9$ nm.

The approximate number of base pairs = $$\begin{gathered} \raisebox{1ex}{$=1.1\times {10}^9$}\!\left/ \!\raisebox{-1ex}{$0.34$}\right. \\ =3.3\times {10}^9 \end{gathered}$$