Question

If linearized, the single circular E. coli chromosome is 1.28 mm long. Under optimal conditions this...

If linearized, the single circular E. coli chromosome is 1.28 mm long. Under optimal conditions this double stranded DNA chromosome is replicated in 40 minutes. A) What is the distance traversed by one replication fork in one minute? B) If the replicating DNA is in the B-form, how many nucleotides are incorporated in one minute at one replication fork? Some (perhaps) pertinent properties of B-form DNA:

Rise per basepair: 3.4 Å Helix diameter: 23.7 Å Basepairs per turn: 10.4 Pitch per helical turn: 35.4 Å

Answer A: The distance travelled by the replication fork in one minute would be given as below:

Distance travelled in 1 minute = Total linear length of DNA / time taken for complete replication

Distance travelled/minute = 1.28 mm / 40 or 0.032 mm or 32 micrometer.

The number of nucleotides in 32 micrometer or 32 * 10-6m of DNA would be given as:

Number of turns in the DNA = Total length / pitch per helical turn

Thus, number of turns = 32 * 10-6 / 35.4 * 10-10 or 0.9039 * 104 or 9039

Number of base pairs in 1 turn = 10.4

Thus, number of base pairs in 9039 turns = 10.4 * 9039 or 94006 base pairs.

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