Conjugation: Hfr & F' Cells: Videos & Practice Problems

HFR cells, or high frequency of recombination cells, have an F plasmid integrated into their chromosome, allowing them to transfer chromosomal DNA during conjugation. Unlike F+ cells, HFR cells do not convert the recipient F- cell into F+; instead, the recipient remains F- after receiving only small portions of the donor's chromosomal DNA. In contrast, F' cells, formed through error-prone excision of the F plasmid, transfer both the entire F plasmid and chromosomal DNA, converting the recipient into an F' cell. This process exemplifies horizontal gene transfer and genetic recombination.

Hfr Cell Conjugation

This video, we're going to begin our lesson on the conjugation of HFR cells as well as the conjugation of F' cells. However, in this video we're only going to focus on the conjugation of HFR cells. And then later in a different video we'll focus on the conjugation of F' cells. And so, in order to understand HFR cell conjugation, it's first important to highlight an important fact. And that is that F plasmids have the reversible ability to integrate themselves into the host chromosome, and it can also excise or remove itself from the host chromosome.

An HFR cell, where the HFR really stands for high frequency of recombination, these HFR or high frequency of recombination cells are simply cells that have an F plasmid integrated into the host chromosome. If we take a look at our image down below, specifically on the left-hand side over here, notice that this box is focused on the reversible HFR cell formation. Notice that the top cell that you see here is actually the same F⁺ cell that we talked about in our previous lesson videos because it's a cell that contains the entire F plasmid and therefore it forms this pilus that you see over here. This F plasmid has the reversible ability to integrate itself into the chromosome. When it does integrate, represented by this blue arrow down below, that is what forms the HFR cell.

The integration of this F plasmid is reversible, which means that the F plasmid can excise itself or remove itself to become the F plasmid again. What we can see is that F⁺ cells and HFR cells can be converted into each other depending on the integration status of the F plasmid. If the F plasmid is integrated, it's an HFR cell, whereas if the F plasmid is not integrated, it's an F⁺ cell. In our last lesson video, we were able to talk about the conjugation of these F⁺ cells. In this video, we're going to be focusing on the conjugation of these HFR cells. During the conjugation of the HFR cells, these HFR cells are going to serve as the donor cells and the transfer of chromosomal DNA to the recipient cell via conjugation.

Instead of only transferring the F plasmid, an HFR cell conjugation, part of the donor cell's chromosomal DNA is going to be transferred, and that marks a big difference from what we saw in our previous videos. There are also some similarities between the conjugation of HFR cells' chromosomal DNA, and the conjugation of the F plasmid in E. coli. However, there are also some key differences. One such key difference is that HFR cells, they are going to be making the F pilus to conjugate with the F^- cell. However, the entire integrated F plasmid is not going to be transferred to the recipient, which means that at the end of conjugation the recipient is still going to remain F^-. The recipient stays F^- from the beginning to the end of the conjugation process. This is very different than what we've seen in our previous videos where both cells were F⁺ at the end of conjugation.

Only a really small portion, only small portions of the donor cells' chromosomal DNA as well as small portions of the F plasmid are going to be transferred to the recipient. The transferred DNA that the F^- recipient cell receives is either going to integrate into the recipient cell's host chromosome or it will be degraded and have no effect. Over here on the left, we're showing you an HFR cell, which once again is a cell that has the F plasmid integrated into the chromosome. In this chromosome, just for the sake of an example, we've included three genes: gene A, gene B, and gene C. This HFR cell because it has an integrated F plasmid is capable of forming the F pilus and it's going to be the donor cell in this conjugation process.

The other cell that's over here, this is going to be an F^- cell, the recipient that does not have the F plasmid at all. Notice that over here we've also added, in the recipient cell's chromosome, these specific genes, lowercase a prime, lowercase b prime, and lowercase c prime so that you can distinguish the F^- cell's recipient chromosomal genes from the HFR cell's chromosomal genes. The HFR cell is going to initiate conjugation and forms an F pilus, which can bind to the F^- cell as we see right here.

Only parts of the integrated F plasmid are going to be transferred and only parts of the host chromosome's genes are going to be transferred. So in this image, only genes A and B are being transferred to the recipient, but gene C is not being transferred to the recipient. Because the recipient only receives portions, small portions of the integrated F plasmid and small portions of the host chromosomal DNA, it is going to remain F^-. In the next image that we have down here, the transferred DNA is going to have the ability to integrate into the chromosome. You can see that there's a small portion of the F plasmid, and there are only portions of the chromosomal genes being transferred, only genes A and B, but not gene C.

At the very end of this process, after this portion integrates via homologous or combination, after integration of genes A and B, it replaces the DNA and that DNA that's replaced is going to be degraded. The cell at the end is going to remain, once again, F^- because it does not have the entire F plasmid and so it only receives a small portion of the F plasmid. The initial donor cell is still going to remain HFR. What has happened in this process is that the HFR cell has transferred over some of its genes, in this case, gene A and gene B over to the recipient, and it is also transferred over only a portion, small portion of the F plasmid. The recipient remains F^- which means it's not capable of initiating conjugation but it has received genes from its neighbor through horizontal gene transfer. This here concludes our brief introduction to HFR cell conjugation, and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Hfr strains of bacteria:

• A. Do not have an F plasmid.
• B. Have an F plasmid.
• C. Have an F factor integrated in the bacterial chromosome.
• D. Have a partial F plasmid as a linear fragment in the cytoplasm.

Answer: (C)

What is transferred between two bacterial cells in Hfr conjugation?

• A. A small portion of the integrated F plasmid from the Hfr donor cell.
• B. A small, random portion of the Hfr donor's cells chromosome.
• C. A small portion of the Hfr donor cell's chromosome and integrated F plasmid.

Answer: (C)

F’ Cell Conjugation

In this video, we're going to begin our lesson on F' cell conjugation, where the little apostrophe here represents a prime. First, we need to recall from our previous lesson videos that F plasmid integration into the host chromosome is a reversible process that can regenerate F+ cells when the entire F plasmid is excised. However, the excision process of this integrated F plasmid is error-prone. On some occasions, donor DNA can be excised with the F plasmid creating what we call F' cells. F' cells are cells that have an excised F plasmid that contains a small fragment of the cell's chromosomal DNA.

If we take a look at our image down below, we can get a better understanding of these F' cells. We're going to focus specifically on the top half of this image, which is showing you F' cell formation. In this image, on the far left, we're showing you the original F+ cell, which is a cell that contains the entire F plasmid, allowing it to form a pilus. Recall that the F plasmid has the reversible ability to integrate itself into the host chromosome, forming the HFR cell. The HFR cell contains the integrated F plasmid. This is a reversible process, so the integrated F plasmid can also excise itself back out to form the F+ cell again.

Notice we have arrows going forward and backward to represent this reversible process. However, on occasion, when the integrated F plasmid excises itself, it is an error-prone process, and occasionally, some donor DNA is excised with the F plasmid to create the F' cell. In this part of the image, the integrated F plasmid in the HFR cell can excise itself and remove part of the chromosomal DNA along with it, generating the F' cell. The F prime cell contains the entire excised F plasmid as well as a small fragment of the chromosomal DNA.

Regarding the conjugation of an F' cell, it's important to note that the F' cells serve as the donor cells in conjugations, and they are able to conjugate with F- recipient cells. When they conjugate with F- recipient cells, the F' cells transfer the chromosomal DNA that was excised as well as the entire F plasmid. This means that the recipient cell, the F- recipient cell, is going to end up becoming an F' cell as well.

If we take a look at our image down below, we can get a better understanding of F' cell conjugation. Notice on the far left over here, we're showing you the F' cell, which serves as the donor cell in conjugation. The F' cell once again contains the excised plasmid that has some chromosomal DNA and the entire F plasmid. The donor F' cell is able to conjugate with the recipient F- cell and transfer this entire F' plasmid. When it does so, both cells end up being F'. The donor cell is F', and the recipient cell goes from F- to F'. This is the conclusion of our brief introduction to F' cell conjugation, and we'll be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.

Which of the following statements about bacterial cells and plasmids is false?

• A. F- cells do not possess any form of a plasmid.
• B. F+ cells possess an F plasmid which is separate from the bacterial chromosome.
• C. Hfr cells possess a plasmid that replaces the bacterial chromosome entirely.
• D. F' cells possess an F' plasmid which contains plasmid DNA and some bacterial chromosomal DNA.

Answer: (C)

Here’s what students ask on this topic:

What is the difference between HFR and F+ cells in bacterial conjugation?

HFR (High Frequency of Recombination) cells have an F plasmid integrated into their chromosome, allowing them to transfer chromosomal DNA during conjugation. In contrast, F+ cells contain the F plasmid as an independent circular DNA molecule. During conjugation, HFR cells transfer portions of their chromosomal DNA to the recipient, which remains F- because it does not receive the entire F plasmid. On the other hand, F+ cells transfer the entire F plasmid to the recipient, converting it into an F+ cell.

How does the conjugation process differ between HFR and F' cells?

In HFR cell conjugation, the donor cell transfers portions of its chromosomal DNA along with a small part of the F plasmid to the recipient, which remains F-. In F' cell conjugation, the donor cell transfers the entire F plasmid along with a small fragment of chromosomal DNA that was excised with the F plasmid. This results in the recipient cell becoming an F' cell, containing both the F plasmid and the additional chromosomal DNA.

What happens to the recipient cell after conjugation with an HFR cell?

After conjugation with an HFR cell, the recipient cell remains F- because it does not receive the entire F plasmid. Instead, it receives small portions of the donor's chromosomal DNA, which may integrate into its own chromosome through homologous recombination or be degraded. This process allows for genetic recombination and horizontal gene transfer but does not convert the recipient into an F+ cell.

What is an F' cell and how is it formed?

An F' cell is formed when an F plasmid excises itself from the host chromosome in an error-prone manner, taking along a small fragment of the host's chromosomal DNA. This results in an F plasmid that contains additional chromosomal genes. During conjugation, an F' cell can transfer this modified F plasmid to a recipient F- cell, converting it into an F' cell as well.

Why does the recipient cell remain F- after conjugation with an HFR cell?

The recipient cell remains F- after conjugation with an HFR cell because it does not receive the entire F plasmid. Only small portions of the donor's chromosomal DNA and a small part of the F plasmid are transferred. Since the complete F plasmid is necessary for the recipient to become F+, the recipient remains F-.