38. A conjugation experiment is carried out between F⁺ his⁺ leu⁺ thr⁺ pro⁺ bacteria and F^– his^– leu^– thr^– pro^– bacteria for a period of 25 minutes. At this time the mating is stopped, and the genotypes of the recipient F- bacteria are determined. The results are shown below:
Genotype                 Number of colonies
his⁺                                           0
leu⁺                                   13
thr⁺                                    26
pro⁺                                   6

What is the probable order of these genes on the bacterial chromosome?
(1) thr, leu, pro, his
(2) pro,leu, thr and the position of his cannot be determined.
(3) thr, leu, pro, and the position of his cannot be determined
(4) his, pro, leu, thr

Core Answer

The correct option is (3) thr, leu, pro, and the position of his cannot be determined. In this F+ to F- bacterial conjugation experiment interrupted at 25 minutes, genes enter the recipient sequentially from the origin of transfer, with entry frequency reflecting proximity: thr+ (26 colonies, first), leu+ (13, second), pro+ (6, third), and his+ (0, not entered or too distant).

Experiment Principle

Bacterial conjugation transfers single-stranded DNA from donor (F+ his+ leu+ thr+ pro+) to recipient (F- his- leu- thr- pro-) via a pilus, starting at the F-plasmid origin of transfer (oriT). Closer genes to oriT enter earlier and recombine more frequently into the recipient chromosome, yielding higher exoconjugant colonies on selective media. Interrupted mating at fixed times (here 25 minutes) maps relative gene order by colony counts: higher numbers indicate earlier entry.

Data Interpretation

Total colonies (26 thr+ + 13 leu+ + 6 pro+ = 45) represent successful recombinants. Thr+ shows highest frequency (26), entering first; leu+ next (13); pro+ lowest among positives (6), entering last; his+ absent (0), indicating it either lies beyond pro+ (entry time >25 min) or on the opposite chromosomal side from oriT. Gene order follows decreasing frequency: origin-thr-leu-pro-(his?).

Option Analysis

• (1) thr, leu, pro, his: Incorrect. His+ at 0 colonies cannot follow pro+ (6 colonies), as later genes yield fewer recombinants; his entered before 25 min if third.

• (2) pro, leu, thr, and position of his cannot be determined: Incorrect. Pro+ (6) lowest frequency places it last, not first (expected ~26 colonies); contradicts thr+ leading.

• (3) thr, leu, pro, and position of his cannot be determined: Correct. Matches frequencies: thr first (26), leu second (13), pro third (6); his unentered, position ambiguous without longer mating or Hfr data.

• (4) his, pro, leu, thr: Incorrect. His+ (0) cannot lead (expected highest colonies); thr+ (26) would trail with fewest if last.