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#1
08-10-2012
 USMLE Forums Guru Steps History: 1+CK+CS+3 Posts: 304 Threads: 48 Thanked 291 Times in 106 Posts Reputation: 301
Bacterial Genetic Recombination (HFR Donor)

The table below lists the observed frequency of the appearance of wild-type recombinations for four gene markers using an Hfr donor strain.

Marker Frequency in F– Recipient
His+ --------Lac+---------- Ilv+-------- Arg+
5.0%-------- 30.0% --------80.0%------ 0.03%

Which of the following provides the correct location and direction of transfer of genes in the Hfr bacterial chromosome?

A) The F factor has integrated between Arg and Ilv with Ilv transferred first

B) The F factor has integrated between His and Arg with Arg transferred first

C) The F factor has integrated between His and Arg with His transferred first

D) The F factor has integrated between Ilv and Lac with Ilv transferred first

E) The F factor has integrated between Ilv and Lac with Lac transferred first

#2
08-11-2012
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 331 Threads: 39 Thanked 308 Times in 117 Posts Reputation: 318

I think the answer is A, because Ilv marker shows the highest recombination frequency and the Arg the lowest.
#3
08-11-2012
 USMLE Forums Scout Steps History: 1+CK+CS+3 Posts: 26 Threads: 7 Thanked 86 Times in 13 Posts Reputation: 96
the key is the percentages, then draw a circle.

In Hfr conjugation, the whole chromosome attempts to transfer. But this is usually not completed. The genes at the start point get transferred first and they will be reflected most in the progeny.

First step in answering this question is to note the percentages. This tells you who is transferred first (the largest percentage).

Then draw a circle, the gene transferred first and the one transferred last will sit adjacent to each other.

#4
08-11-2012
 USMLE Forums Guru Steps History: 1+CK+CS Posts: 307 Threads: 36 Thanked 127 Times in 61 Posts Reputation: 137

Agree with you two. Should be A.

 Tags Microbiology-, Step-1-Questions

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