Discussion
The origin of the Escherichia coli chromosome on the genetic map is shown below.
Bidirectional replication is a feature of this system and both replication forks move at the same rate. Which one of the following sequences of replication of the genes is correct?
Answer: B
The order of replication is AGBFCDE.
Replication starts at Ori and two forks move away from it in opposite directions at the same speed. A gene is therefore copied at a time proportional to its distance from Ori, measured along the shorter way round. Reading the map: A lies just clockwise of Ori and G just anticlockwise; B is next on the clockwise side, then F on the anticlockwise side; the clockwise fork then reaches C and D, and E, lying almost opposite Ori, is copied last.
ABCDEFG follows only one fork round the whole circle, as if replication were unidirectional. GAFBECD and GAFEBCD put G before A and bring E in far too early, although E is the gene farthest from the origin.
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