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In a particular strain of E. coli, it was observed that DNA polymerase could add nucleotides to a growing chain of DNA at the rate of 600 per second. If the genome of this organism is 1.1mm long wherein a base pair occupies 0.34 nm, then how much time (in minutes) would be required for the complete replication of the chromosomal DNA molecule? (Report the closest integer value.) {Source}

I solved this question using this method.

No. of base pairs: $ \frac{1.1 \times 10^{-3} \space m}{0.34 \times 10^{-9} \space m/bp} = 3.23 \times 10^{6} \space bp$

Time required for replication (in minutes by the DNA Polymerase) = $ \frac{ 3.23 \times 10^{6} \space bp}{(600 \space bp/s)\times (60 \space s/minute)} =89.86 ≈ 90 \space minutes$

The answer, however, is 45 minutes. What am I getting wrong?

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Remember that E. coli has a single circular chromosome, and that chromosome is replicated bidirectionally. Hence, your calculated value (90 minutes) is exactly twice that of the correct answer.

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