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F-Mediated Gene Transfer in E. coli
(aka: conjugation or "sexduction")
Here we start off with an F+ cell. The main, or somatic, chromosome is the large circle, and the smaller ones are plasmids. E.coli, for example, can each contain many different kinds of plasmids. This cell shows two of them. The larger one we are denoting as F, and the smaller ones are something else, but with a "copy number" of two. By that we mean there are twice as many of them as there are somatic chromosomes. (For genetic engineering work, we like to have high copy numbers of the plasmids that are carrying the genes we want to use.) Anyway, here is a cell with F at a copy number of 1. ("F" stands for "fertility") It would help for you to take a closer look at a schematized "F" plasmid before continuing.
When the F+ cell encounters an F- cell, the F+'s pilus's distal end recognizes the F-'s surface,...
... and joins onto the F- cell forming a bridge. This pilus is made of proteins encoded by the F-DNA. Then a DNA polymerase begins replicating a single stranded copy of the F-plasmid. That is a DNA polymerase that is encoded also within the F-plasmid. That newly made ssDNA is directed to go through the pilus into the recipient F-cell. (Not yet known: is the ssDNA pushed through or pulled through?)
Within a few seconds, the whole of the F-plasmid has been replicated into ssDNA that has entered the F- cell.
The linear bit of F-DNA then circularizes as not self-respecting plasmid is anything other than circular.
Because the circular form is an "F" it causes the cell to sprout a pilus and become F+. (Actually F causes two pili per cell.) You will note that this cell differs from the first F+ cell because it lacks the small plasmids.
Now let us return to the first F+ with its two types of plasmids.
Here is how we go about forming an Hfr. The F-plasmid sidles up to the somatic chromosome in one of many different places...
...and a cross-over event occurs.
Taking the twisted cross-over product and straightening it out we have and Hfr,...
...which can then mate with an F- cell.
Because that mating can occur with not only F- E.coli but also with other "species" of bacteria, one is confronted with the question of what a "species" means in bacteriology!
At this point you may continue by looking at the qualitative aspects of Hfr-mating.
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