If available, please reference Genetics text: B.A. Pierce, Genetics. A Conceptua

Question

If available, please reference Genetics text: B.A. Pierce, Genetics. A Conceptual Approach, 5th ed., W.H. Freeman & Co., NY., 2014.

4. Cavalli-Sforza's discovery of Hfr cells helped geneticists ultimately work out how conjugation occurs.

How does a cell become an Hfr cell? Why does this lead to (a) a high rate of gene transfer to recipient (Fminus)

cells, and (b) transfer of many genes, but always in the same order? Why do the recipient cells

usually not become F-plus?

5. How does an F' plasmid differ from a standard F plasmid? How do F' plasmids relate to Hfr cells?

6. How are bacterial genes mapped by interrupted conjugation?

7. What does it mean for a bacterial cell to be naturally competent?

8. How does gene exchange occur by transformation?

9. How are bacterial genes mapped by cotransformation?

10. Contrast virulent and temperate bacteriophages. What is a prophage?

11. Contrast generalized and specialized transduction. Why does the latter require temperate phage?

Explanation / Answer

4. A cell becomes a Hfr cell when a conjugative plasmid like F plasmid becomes integrated with its chromosomal DNA.

a) A hfr cell is produced by homologous recombination between plasmid and the chromosome. So whenever there is a transfer of the plasmid into an F- strain apart of chromosome also gets transfrred causing higher number of recombinant cells.

b) During transfer via conjugation in Hfr bacterias, the complete chromosome cannot get transferred to the next cell due to its large size and instability of the pilus for long time, so the transfer gets interrupted. The transfer start at the OriT region of the F-factor and continues clockwise or anticlockwise depending on the position of the F-factor in the chromosome. If the conjugation is interrupted in the middle the amount of gene already transferred will have similar sequence of genes as that in the chromosome.

Because not the whole chromosome gets transffered during conjugation, the complete F factor gets nicked in the middle due to the instability of the pilus. Without the complete F-factor the F- cells cannon produce new pilus for conjugation and thus remains F-.

5. In an F' plasmid some amount of bacterial chromosome is also present. This happens due to incomplete excision of the F-fcator from the bacterial chromosome. Normal f factor do not have any chromosomal gene.

Both F' plasmid and Hfr have genes from bacterial chromosome and F factor plasmid. F' plasmid can get generated from a Hfr strain.

6. During conjugation Hfr strains try to ransfer its recombinant genome into an F- cell. But due its large size the conjugation mostly never gets completed. The transfer strats at OriT of the F-factor part and carry out in either clockwise or anticlockwise direction depending upon the position of the Ffactor. The bacterial genes always gets transferred in the same order as they are positioned with respect to the Hfr.

For interrupted mapping an Hfr and F- strain are selected where one of the cell have mutated genes like antibiotic resistance at regular intervals.Their conjugation process is physically interrupted after certain time interval and checked for the phenotype . Thats how it can be understood which gene is getting transferred first and in what order.

7. Natural competence is the property of a bacteria to genetically alter its DNA by taking up extracellular DNA naturally via transformation.

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