Question

Four Hfr strains (1, 2, 3, 4) were isolated from the same prototrophic, strF strain. In separate experiments using the technique of interrupted mating, these four Hfr strains donated the following markers to an auxotrophic, str F strain in the order shown: (only the first six genes transmitted were scored for each strain) Hfr Strain 1 Hfr Strain 2 Hfr Strain 3 Hfr Strain 4 A. What is the order of these markers on the circular chromosome of the original F*? B. Indicate on your diagram the position and orientation of transfer for each F plasmicd for each Hfr strain (1, 2, 3,4) C. Answer the following questions as true or false for Hfr Strain 1 only. All F cells which received Q from the Hfr in the chromosome transfer process must all have received A* Most M, str recombinants are likely to be Hfr cells. All Q recombinants will also be O* recombinants

Answer 1

A) There are 11 genes are present. These are- G, K, C, R, H, M, Q, A, B, F & O. In Hfr strain 1 first gene that is transferred is O. So, F factor must be remain before O & orientation of transfer is towards M. So, from Hfr strain 1 we find the gene sequence O-F-B-A-Q-M. Now, from strain 2 we find the sequence H-M-Q-A-B-F. Comparing strain 1 & 2 we find that orientation of gene transfer of 1 & 2 must be opposite. Also by comparing strain 1 & 2 we find that gene M is present between gene Q & H. So, latest gene sequence is O-F-B-A-Q-M-H. Now, from strain 3 we find the sequence A-Q-M-H-R-C. Also comparing strain 1 & 3 we find that the orientation of the gene transfer must be same. Further we find that R & C gene is present after gene H. So, latest gene sequence is O-F-B-A-Q-M-H-R-C. Now, from strain 4 we find the sequence O-G-K-C-R-H. By comparing strain 3 & 4 we find that the orientation of the gene transfer must be opposite to each other. Further as the chromosome is circular & by comparing the latest sequence with strain 4 we find that G & K gene are present between O & C. So, final gene sequence will be O-F-B-A-Q-M-H-R-C-K-G. This circular chromosome is shown in below picture.

B) For strain 1, F factor remains between O & G & orientation is anti-clockwise.

For strain 2, F factor remains between H & R & orientation is clockwise.

For strain 3, F factor remains between A & B & orientation is anti-clockwise.

For strain 4, F factor remains between O & G & orientation is clockwise.

C) All F^- cells which received Q⁺ from the Hfr in the chromosome transfer process must all have received A⁺- True.

Explanation: Due to the orientation of transfer A gene will come before Q gene. So, if F^- cells received Q⁺ it must be received A⁺.

Most M⁺, str^r recombinants are likely to be Hfr cells- False.

Explanation: str^r gene present in the F^- cells & M⁺ is present in the Hfr cells. Transfer of M⁺ gene from Hfr to F^- cells must be occur in order to presence of both M⁺ & str^r. Due to the interrupted mating all of the genes unable to transfer to F^- cells from Hfr & thus F^- cells will remain F^- & will not become F⁺. So, most of the M⁺, str^r recombinants are likely to be F^- cells.

All Q⁺ recombinants will also be O⁺ recombinants- True.

Explanation: Due to the orientation of transfer O gene will come before Q gene. So, if F^- cells received Q⁺ it must be received O⁺.