- In E. coli four Hfr strains donate the following genet, t markers in the order shown below:
Strain 1: L Q W X Y
Strain 2: M T A D Y
Strain 3: E C M T A
Strain 4: W Q L E C
Which of the following depicts the correct order of the markers and the site of integration (à) of the F– factor in the four Hfr strains?
The correct answer is Option (3) because it correctly reconstructs the circular chromosome order of markers and the different F‑factor integration sites/orientations that explain all four Hfr donor sequences.
Question restatement
In E. coli, four Hfr strains donate markers in the following linear order during conjugation:
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Strain 1: L Q W X Y
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Strain 2: M T A D Y
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Strain 3: E C M T A
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Strain 4: W Q L E C
The task is to deduce:
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The correct circular order of markers on the chromosome.
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The site and orientation of F‑factor integration for each Hfr strain (indicated by arrows in options).
Step‑by‑step solution
1. Deducing a common circular gene order
Each Hfr strain donates markers starting from the F‑factor integration site and proceeds in one direction around the chromosome. The list from each strain is therefore a linear readout of a segment of one common circular map.
From the given sequences:
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Strain 1: L → Q → W → X → Y
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Strain 2: M → T → A → D → Y
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Strain 3: E → C → M → T → A
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Strain 4: W → Q → L → E → C
Now connect overlapping ends:
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Strain 3 ends in A, and Strain 2 begins with M, but Strain 3 already shows C → M → T → A.
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Strain 2 shows M → T → A → D → Y. Thus, merge them as:
E → C → M → T → A → D → Y. -
Strain 1 shows L → Q → W → X → Y. Since Y already appears at the end of the previous chain, place L, Q, W, X so that X → Y joins to Y and W → X is preserved.
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Strain 4 shows W → Q → L → E → C, which closes the circle linking back to E → C.
Putting all unique markers once in a loop gives:
L → Q → W → X → Y → D → A → T → M → C → E → (back to L)
This is the only circular order that simultaneously contains all four linear orders as consecutive blocks when read from appropriate starting points and directions.
2. Locating F‑factor integration sites and directions
Using the circular order above:
L – Q – W – X – Y – D – A – T – M – C – E – (back to L)
For each strain, the arrow in the option must:
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Start at the first marker donated in that strain.
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Proceed around the circle in the same sequence order as its list.
Check Option (3), which shows exactly this circle and four different arrow positions:
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Strain 1 arrow: begins at L, goes through Q → W → X → Y.
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Strain 2 arrow: begins at M, then T → A → D → Y.
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Strain 3 arrow: begins at E, then C → M → T → A.
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Strain 4 arrow: begins at W, then Q → L → E → C.
Thus, Option (3) simultaneously satisfies all four observed transfer orders using one coherent circular gene map and four appropriately oriented F‑factor insertions.
Therefore, Option (3) is correct.
Why the other options are incorrect
Option (1)
Option (1) presents a linear arrangement of markers (X Y D A T M C E, etc.) with arrows for different strains:
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The markers are not arranged in a closed circle, so it cannot represent a bacterial chromosome (which is circular).
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Trying to read Strain 3 (E C M T A) and Strain 4 (W Q L E C) from the depicted linear sequence either breaks the continuity or requires jumping positions, which is not allowed for Hfr transfer.
Hence, Option (1) fails to give a consistent, single circular order for all strains.
Option (2)
Option (2) does draw a circle, but the order of markers around the circle is not compatible with all the given sequences:
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When attempting to trace Strain 2 (M T A D Y) on that circle, the letter following M does not continue as T, or D does not precede Y in a single direction, breaking the necessary linearity.
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At least one of the four strains’ sequences cannot be read as a continuous run along one direction, indicating a wrong order and/or arrow placements.
Thus, Option (2) cannot account for all observed donor orders.
Option (4)
Option (4) shows a linear map C E L Q W X Y D A T M C with different arrows, but:
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The chromosome is again treated as effectively linear, and the markers are ordered so that Strain 4 (W Q L E C) cannot be read as a single unidirectional sequence.
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Some strains would require reversal in the middle of the run or skipping gene positions, which is impossible in Hfr transfer.
Therefore, Option (4) is also inconsistent with the data, leaving Option (3) as the only viable representation.
Keywords (for on‑page SEO)
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Hfr mapping in E. coli
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gene order from Hfr strains
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bacterial conjugation mapping
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F factor integration site
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SEO‑style explanation for learners
Concept overview
Hfr mapping in E. coli uses the ordered transfer of chromosomal genes from an Hfr donor to an F⁻ recipient during conjugation. The time or sequence in which markers appear in recombinants reflects their physical order on the circular chromosome, starting from the F‑factor insertion site and moving in one direction.
When multiple Hfr strains are available, each with the F factor integrated at a different site and/or orientation, combining the linear orders from all strains allows reconstruction of a single circular gene map. The correct solution is the one that can accommodate all donor sequences as continuous segments, read from different starting points but along a single circular order, without breaks or reversals.
