F+, F-, Hfr and F’ Cells – What is the Difference?
We talk about F+, F–, Hfr and F’ Cells when we talk about bacterial conjugation which is one of the modes of horizontal gene transfer in bacteria. And it is important that we understand what these cell types are and what is the difference.
These are the bacterial cells which contains the F plasmid. They are designated F+ simply because they have F plasmid. We know plasmid is an extrachromosomal DNA that can replicate independently. It is called F plasmid because it has F factor which is Fertility factor. This fertility factor contains the genes require for the transfer or conjugation.
F+ Cells = Cells containing F plasmid (F plasmid = Plasmid containing F factor)
Figure 1. F+ Cells
F– cells are the cells without F plasmid. These cells act as recipient cells because they don’t have F plasmid and thus they cannot donate the genetic material. They are designated as F– simply because they do not have F plasmid.
F– Cells = Cell lacking F plasmid
Figure 2. F– Cells
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Hfr Cells: High frequency of recombination
We just talked about F plasmid. This F plasmid is an episome which means it can either exist as an independent unit or it can integrate itself into chromosomal DNA. This integration is possible because F plasmid also contains the insertion sequence and via homologous recombination it can integrate itself. So when this integration happens, the resultant cells or the derivative of F+ cells is called Hfr cells.
Why are they called Hfr? – When these Hfr cells cross with F– cells; along with F plasmid some part of donor chromosomal DNA also gets transferred in the recipient cells because F plasmid is integrated into donor chromosomal DNA. Therefore the recipient cells now have got three different types of DNA – its own chromosomal DNA, some part of F plasmid along with some part of donor’s chromosomal DNA. So because crossing with Hfr cells results in high degree of recombination in recipient cells such cells are called Hfr cells.
Hfr Cells = Derivatives of F+ cells where F plasmid has integrated into host chromosomal DNA
Figure 3. Hfr Cells
We just discussed that F plasmid is an episome so once it gets integrated into host chromosomal DNA, it can also disintegration itself back from the host chromosome. Sometime this disintegration is not accurate and while disintegrating from host chromosome; F plasmid might pick up some genes next to its disintegration sites. So such cells where the F plasmid has disintegrated and have got some genes of host chromosome are called F’ cells. They are the derivatives of Hfr cells.
F’ Cells = Derivatives of Hfr cells where F plasmid has disintegrated from host chromosome and picks up some host genes next to F plasmid integration sites
Figure 4. F’ Cells
Figure 5. Summary
I hope this post helps 🙂
To understand more details, watch a video on this topic here.
