The F-Plasmid

The function of this page are two-fold:

1. This is designed to be a beginning beginner's picture on what the F-plasmid looks like in E.coli - at least enough to give the user some mental images so that F's role in conjugation might be a little easier to understand.
2. This page is also designed to give the reader an idea of how conceptual molecular modelling takes place.

First of all, we must understand that this small piece of circular double-stranded DNA (dsDNA) contains several genes. But what are these genes? Let us see if we can make pretty good guesses as to what they do. What functions must the F-factor do?

1. It must govern the synthesis of its own brand of pili, so there must be pili genes.
2. It must have its own type of DNA-polymerase that makes ssDNA, that is to be sent to the recipient F- cell. Thus a gene govening ssDNA polymerase must be present.
3. There must be an attachment point where the polymerase starts its replication cycle. Those first bases replicated are the origin, which we will call OriF (in deference to the origin of the host chromosome, which is called OriC).
4. There must be other genes governing copy number.
5. Since F can integrate into the host chromosome in a non-allelic way, there must be a gene or genes for accomplishing the seemingly non-specific integration of the plasmid into the host chromosome to form an episome. Let's call these "X" for cross-over or chiasma.
6. Finally, the orientation of these must be that most or all of the vital "F-activities" are distal to Ori-F so that whenever F integrates and mobilizes the chromosome as in an Hfr, the vital F-functions are last to be sent into the recipient cell.

Let us "blow up" the small circle, and place "our" contrived genes on it. (Note that the actual order may differ because our conceptual model only takes into account the fact that the vital genes come after X.

We shall now take a close look at attP. That is as the attachment site for F's own special type of DNA-polymerase (which we might exhaustively name as "dsDNA dependent ssDNA polymerase". That is because the enzyme requires dsDNA as its substrate and makes ssDNA. That attachment site is oriented such that it directs the polymerase to read in the "away" direction. Notice all the arrows that specifiy directions or orientation and directions of progress. These are called "polar" sites and processes.

And this means that the first F-genes it reads and replicates as ssDNA are those after the darkened section on the diagram. This essentially makes the "arrow head" used in later diagrams. (Mnemonic: which goes into a target first - the arrow head or the tail? The head! So you have to make the head first and work backwards.) So we added the arrowhead symbol.

Here we see that the plasmid has caused the formation of a molecule of F-specific DNA polymerase (the black dot). It schematically "moves" to the arrow head, which is really where is

The polymerase has now replicated a little of the plasmid:

The polymerization continues to the end, and by that time the whole of the "arrow" has proceeded into the recipient cell. It takes only a few seconds at most for this to happen.

Once inside the recipient, the complementary strand of DNA is made and the linear fragment circularizes into a supercoiled circle of dsDNA, such as is shown in this backwards diagram!

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