Q.72 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?
(A) ABCDEFG
(B) AGBFCDE
(C) GAFBECD
(D) GAFEBCD
The correct sequence of replication of the genes is GAFEBCD (Option D).
Concept: Bidirectional replication in E. coli
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The E. coli chromosome is circular and replication starts at a single origin, oriC.
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Two replication forks move in opposite directions at the same speed, so genes equidistant from oriC on opposite sides are replicated at the same time.
Reading the given map
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In the figure, oriC lies between genes G and A on the left side of the circular chromosome.
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One fork moves clockwise (towards A → B → C → D → E → F → G), and the other moves anticlockwise (towards G → F → E → D → C → B → A).
Because forks travel at equal speeds, genes are replicated in pairs at equal distances from oriC:
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First: the regions immediately on each side of oriC, i.e. G and A.
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Next: equidistant genes further away, F and B.
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Finally: the farthest pair before termination, E and C, and last the gene directly opposite oriC, D.
This yields the sequence: G A F B E C D → GAFEBCD.
Option‑wise explanation
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Option (A) ABCDEFG
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This assumes a unidirectional fork going A → B → C → D → E → F → G, which contradicts bidirectional replication from oriC.
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Option (B) AGBFCDE
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Starts with A and G (correct first pair), but then places B before F, ignoring that F and B are at similar distance from oriC on opposite arms and should be replicated together.
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Option (C) GAFBECD
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Begins with the correct pairs G–A then F–B, but then lists E before C, even though E and C are equidistant from oriC on opposite arms and should appear together before the terminal gene D.
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Option (D) GAFEBCD
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Correctly follows the logic of symmetric, equal‑rate bidirectional forks: first G & A, then F & B, then E & C, and finally the terminus‑proximal gene D.
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SEO‑friendly introduction
Bidirectional replication in Escherichia coli is a favorite concept in GATE life sciences, because it tests both DNA replication mechanics and genetic map interpretation. When replication forks start from oriC and move at the same rate, the order in which genes are duplicated follows their symmetric positions around the circular chromosome, allowing multiple conceptual questions like the 2025 GATE gene order problem to be solved logically rather than memorized.
