conjugation - okt3

[ArchivalUser]

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

[ArchivalUser]

A?

[ArchivalUser]

The correct answer is A.
The mention of an Hfr donor strain and recombinations with wild-type strains should tell you that this is an Hfr × F“ cross and therefore a question about conjugation. An Hfr cell is a donor cell that contains its fertility factor in episome form (stabilized within the chromosome by site-specific recombination).
Because the tra operon (the genes that cooperate to make conjugation possible) is found in the chromosome of an Hfr cell, the Hfr cell is capable of donating chromosomal genes across the conjugal bridge that it forms with its F“ partner. If the transfer is followed by homologous recombination in the F“ cell, then the newly transferred DNA can become a part of the genome of the recipient F“ cell.
Because conjugal crosses of this type always follow the order of oriT (origin of transfer) first, followed in order by chromosomal genes and last (if it were possible) by the tra operon, we can use the frequency of observed recombinations as a measure of genetic distance.We are told in the data that the recombinant that has the highest frequency is Ilv+ (80%), next is Lac+ (30%), next His+ (5%), and last Arg+ (0.03%). This then should have been the order of these loci on the donor chromosome.
Because the greatest number of recombinants successfully acquired Ilv+, this was transferred first and was therefore the closest locus to oriT on the original Hfr chromosome. Because the bacterial chromosome is circular and we know that oriT is closest to Ilv+, the remainder of the episome has to be found next to oriT: The fertility plasmid inserted itself between Ilv+ and Arg+.