34. In a conjugation experiment between bacterial Hfr strain ‘X’ and F-cell, lac gene enters the recipient in 4 minutes, but the F-cells remain auxotrophic for Leu, Trp, Ura, Glu,Phe and Gly.
    The mating is then allowed to proceed for 20 minutes and lac⁺ exconjugants are selected. Of the lac⁺ cells,

35% are leu⁺
98% are trp⁺
10% are ura⁺
65% are glu⁺
0% are phe⁺
81% are gly⁺

Select the correct order of the genes as they enter, from the choices given below
(1) trp, gly, glu, leu. ura, phe⁺ lac⁺
(2) lact phet ura⁺, leu⁺, glu⁺, gly⁺, trp⁺
(3) phe; ura⁺, leu⁺, glu, gly, trp, Lact
(4) lac⁺,trp⁺, gly⁺, glu⁺, lau⁺, ura⁺, phe⁺

The correct order of gene transfer from the Hfr strain ‘X’ to the F- cell, based on the provided data, is:

lac+ → trp+ → gly+ → glu+ → leu+ → ura+ → phe+

This ordering matches option (1): trp, gly, glu, leu, ura, phe, lac+

Explanation:

• The lac gene enters first at 4 minutes, as given.

• Among lac+ exconjugants, 98% are trp+, 81% are gly+, 65% glu+, 35% leu+, 10% ura+, and 0% phe+. The frequency of gene entry corresponds to how long the mating proceeded after the lac gene transfer.

• Since phe+ is 0%, it enters the latest or beyond the 20-minute window.

• The order is concluded by the decreasing frequency of positives from trp to phe, fitting the classical Hfr gene transfer pattern starting with lac gene.

• Option (1) correctly lists trp, gly, glu, leu, ura, phe after lac, consistent with the data and typical Hfr mapping.

Other options do not follow this logical gene entry order based on the frequencies given in the problem.

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Introduction:
In bacterial genetics, understanding the order in which genes enter the recipient cell during Hfr conjugation is crucial for gene mapping and studying bacterial chromosome structure. This article examines a classical conjugation experiment involving the lac gene and multiple auxotrophic markers—leu, trp, ura, glu, phe, and gly—and explains the interpretation of gene transfer times and positive recombinants to deduce the precise gene order.

Option analysis:

1. trp, gly, glu, leu, ura, phe, lac+ is correct because the frequency of lac+ cells showing these markers following their transfer times fits the known order from the highest to the lowest percentages.

2. lact, phe+, ura+, leu+, glu+, gly+, trp+ is incorrect as phe+ cannot come before trp+ or gly+ given the 0% phe+ frequency.

3. phe, ura+, leu+, glu, gly, trp, lac is incorrect due to phe being the last gene transferred (0% phe+).

4. lac+, trp+, gly+, glu+, lau+, ura+, phe+ appears to have a typo with “lau+” and also does not match the gene transfer timing data.

This conjugation gene order helps in mapping bacterial chromosomes and understanding gene linkage in microbial genetics.

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If you need, I can provide a formatted detailed explanation or help with further genetic mapping questions.

This answer is based on classical bacterial genetics of Hfr conjugation and the phenotypic marker data that reflect gene entry timing and frequency of recombinants.