- In E. coli. four Hfr strains donate the genetic markers shown in the order shown:
Strain 1: F L K O Z
Strain 2: C Y A Z O
Strain 3: P D W C Y
Strain 4: P F L K O
All the Hfr strains are derived from the same F+ strain. What is the order of these markers on the chromosome of the original E. coli F+ strain?
(1) K O Z P D F L W C Y A K
(2) F L K O Z P D W C Y A F
(3) F L K O Z A Y C W D P F
(4) F L K C Y A Z O W D P F
Problem Overview
In E. coli, Hfr strains derived from the same F+ parent transfer chromosomal genes linearly during conjugation, starting from different F plasmid integration sites and directions on the circular chromosome. The transfer orders are: Strain 1 (F → L → K → O → Z), Strain 2 (C → Y → A → Z → O), Strain 3 (P → D → W → C → Y), Strain 4 (P → F → L → K → O). The task determines the original circular order matching all sequences as contiguous segments.
Mapping Process
Align overlapping sequences: Strain 4 and 1 share P-F-L-K-O-Z (extending Strain 1 backward via P). Strain 2 overlaps as Z-O from Strain 1, placing C-Y-A before Z, yielding …C-Y-A-Z-O… Strain 3 overlaps as W-C-Y from Strain 2 backward and P-D-W forward, connecting P-D-W-C-Y-A-Z-O. Strain 4’s P-F closes the circle after O, forming F-L-K-O-Z-A-Y-C-W-D-P-F. This circular order accommodates all strains: Strain 1 from F, Strain 4 from P (clockwise), Strain 2 from C (counterclockwise), Strain 3 from P (counterclockwise).
Option Analysis
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Option (1) K O Z P D F L W C Y A K: Starts at K-O-Z (Strain 1 segment) but places P-D-F-L after Z, contradicting Strain 2’s A-Z-O and Strain 4’s P-F-L; non-circular wrap fails Strain 3. Incorrect.
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Option (2) F L K O Z P D W C Y A F: Matches Strain 1 and Strain 4 early, Strain 3’s P-D-W-C-Y, but Z-P breaks Strain 2’s Y-A-Z-O continuity; no A-Y before Z. Incorrect.
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Option (3) F L K O Z A Y C W D P F: Matches Strain 1 (F-L-K-O-Z), Strain 2 (A-Y-C backward from Z-O), Strain 3 (W-C-Y-D-P backward), Strain 4 (P-F-L-K-O); circular wrap F after P confirms all. Correct.
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Option (4) F L K C Y A Z O W D P F: Places C-Y-A-Z after K, contradicting Strain 1’s K-O-Z and Strain 2’s C-Y-A-Z-O; W-D after O mismatches Strain 3. Incorrect.
Correct Answer: (3) F L K O Z A Y C W D P F. This order ensures every Hfr transfer sequence appears contiguously on the circle, vital for bacterial genetics in CSIR NET exams.
