Bacterial Conjugation - Online Tutor, Practice Problems & Exam Prep

Conjugation is a process where bacterial cells physically unite to exchange genetic material, enhancing genetic diversity. The F factor, a plasmid, enables this exchange, distinguishing F+ (donor) from F- (recipient) cells. Additionally, R plasmids confer antibiotic resistance, often aided by transposons, which facilitate gene transfer between different bacterial species. Understanding these mechanisms is crucial for addressing antibiotic resistance and mapping bacterial chromosomes through high-frequency recombination (Hfr) cells, which integrate the F factor into their main chromosome.

Conjugation Overview

Conjugation is the physical union of bacterial cells. So, two bacterial cells come together; they actually fuse in part, and during this fusion, they exchange genetic material. This phenomenon was first discovered in 1946 by scientists, including Leta Bergan and Tatum. In their experiment, they used E. coli and had two strains of E. coli, referred to as strain A and strain B.

Strain A only grows in media that contains specific components, which we do not need to memorize, just understand the concept. Strain B could only grow in a medium with different materials, requiring three specific amino acids and a chemical that strain A did not need. They mixed these two strains in a tube, placing strain A and strain B, and plated them onto a surface lacking any of the materials necessary for either strain to grow. Logically, no growth should occur because the essential nutrients were absent.

However, against expectations of a completely blank plate, some cells did grow. This perplexed the scientists, and they hypothesized that the strains had exchanged DNA. Strain A, being a wild type, could grow without certain chemicals, and strain B gained this ability through the DNA that Strain A donated, allowing it to grow independent of the other's specialized chemical requirements. For example, if strain A didn't need leucine to grow, it transferred the necessary gene to strain B, enabling strain B to also grow without leucine. This DNA exchange between the two strains facilitated their growth under otherwise inhibitory conditions.

I'll discuss further how this exchange occurred and the specifics of what was exchanged. However, I also want to mention some structures involved in this process. The structures that enable the bacteria to come together for conjugation are known as sex pili or f pili, depending on the textbook reference. The conjugation bridge, which is formed between the cells, acts as a passageway resembling a physical bridge, allowing the DNA to cross from one bacterium to another. Future videos will include images of these structures for better visualization. Thus, the existence and function of bacterial conjugation were illuminated through this pivotal experiment.

Here is a simple recap of the experiment: You have bacterial strain A, which grows in condition A, and bacterial strain B, which grows in condition B. Without conjugation, there would be no growth in a medium lacking both A and B. But with conjugation, growth still occurs because the DNA is transferred between the two strains. This was the core experiment that unveiled the mechanism of bacterial conjugation.

F Factor and Hfr

Okay, so now let's talk about the f factor. So what is the f factor? Well, the f factor is what allows bacteria to swap DNA. It allows the bacteria to undergo conjugation. So if bacteria contain the f factor, we say that they are f+ and that allows them to donate genetic material. While bacteria that don't have it, we say are f-, and they can only accept genetic material. So not every bacteria can, like, initiate that conjugation; only bacteria with the f factor can. So what is the f factor? It's a plasmid, so remember what a plasmid is. It's that circular DNA that exists outside of the main chromosome. It's not necessary for bacterial survival, but it does give it certain advantages. This f factor is a plasmid that allows bacteria to undergo conjugation, which is a huge advantage because that allows for more genetic diversity, which is always a good thing. Now the f factor can actually be given to an f- cell, through conjugation and as well as other DNA as well. Right? That's conjugation. But it's important to realize that it has different DNA than what it started with. It's sort of a mix now between the two bacteria. So we call it a recombinant, but this is a recombinant via conjugation and not genetic recombination. And you may say, "duh, of course. It's conjugation. It's donating this DNA. It's not undergoing crossing over. The bacterial chromosomes aren't lining up and doing anything. Why are you even telling me this?" Obviously, that's the case. Well, the reason I'm telling you this is because it's going to get confusing, on the next page, with something that's a derivative of the f factor. So, I really want to get the point across that it was the f+ factor as a plasmid, that it happens through conjugation and not genetic recombination. So if we scroll down and look at what this looks like, we have bacterial cell 1, this is a donor. Bacterial cell 2, this is a recipient. So if I were to label f+ f-, which one would be plus and which one would be minus? Right. The donor would be the f+ because it's donating. It has that f plasmid, and the recipient would be f- because it doesn't. And so, here's the f plasmid here. You can see that this allows the donor to create the structure called the pili, and that structure allows for the conjugation of the two bacterial cells. You can see that it fuses and is starting to go into the other cell. And at the end, you end up with two cells with the f+ plasmid, the f factor. Now you may ask, okay, well, if it donated that factor, how come it still has it? Right? Like, it gave it away, so shouldn't it be f- now? The reason it's not, the main reason that it's not, is that usually, these cells contain multiple copies of the f factor. So if it has 30 copies, right, and I didn't draw it here for space, but if it has 30 copies and it gives one away, it's still got 29. Right? It's still f+, so it's not just completely getting rid of it, it's just spreading that f factor around. The second way is more rare, and it actually occurs in the recipient cell. But this is, again, like I said, a rare form of what happens. Most of the time, what happens is that there are multiple copies and it gives one away. Now, here we get to a little bit of confusion because there's a second type of cell called the HFR or the high frequency of recombination bacteria. Now these bacteria have the f factor, but it is not a plasmid. So at some point in this bacterium it actually integrated into the main chromosome. So it lost the plasmid. It no longer has it as a plasmid, but instead, it has it as a linear gene found in the chromosome. So now it's in the main part of the DNA. It's actually a gene. It's linear. It's completely different from the f factor, but it has the same function. Right? The f factor allows that bacteria to donate DNA. So if it doesn't have a plasmid to donate, but it's in the bacterial chromosome, it still wants to donate DNA. But if it's in the chromosome, it can't just give a copy of it away. That's not how this works. Instead, what happens is it has to happen through genetic recombination. Remember, and this is why I was telling you how if it's a plasmid, it's going through conjugation. But if it's the HFR and it's actually incorporated into the chromosome, the f+ can't be given to the f- via conjugation, because it can't just give its chromosome away or else it'll die. It needs that chromosome for life. So, how it does this is it can still make recombinants. It can still make f+ cells, but it has to do so through recombination, instead of conjugation. So what this looks like, I'm not going to go through the whole picture, but you can see here that instead of having it in a plasmid form, which is a circular form, here's this bacterial chromosome. And you can see it's actually a linear gene here in blue, is the HFR, and it still allows it to have this pili. It still can initiate conjugation, but instead of just giving that whole chromosome over to the other cell, it actually just recombines and makes a bunch of recombinants. So it has a high frequency of recombination because it wants to donate that DNA. It has that f factor, but it can't. So the only thing it can do is create those recombinations. Whereas, if it's a plasmid and it's separate, then it can undergo this normal conjugation and actually give its plasmid away. But the HFRs are actually really useful, almost even more useful than the F+ bacteria, at least in terms of humans, because we can use this HFR bacteria to map bacterial chromosomes. Now remember, the HFR is a linear gene. It's found inside the bacterial chromosome. And so how you would do this is you take an HFR cell and an f- bacterial cell. So this has the f factor in the linear, and this doesn't. So the HFR will stimulate the bacterial fusion, the bacterial conjugation, and it will start to transfer some DNA into the other cell in forms of recombination. Now the origin is the area where the first gene transfers or the first recombination areas start transferring to the other cell. Now, at any point, you can stop conjugation, or humans can, and when they stop it, it's called interrupted mating. And literally, what they do is they take bacteria who are undergoing conjugation, and they put them in a blender, like, really, like gentle blender, and that stops all conjugation. Right? Because it cuts that conjugation bridge, that pili, apart. And so now it can't finish what it's doing. But it's already started it, but it can't finish. So what happens is you actually get some of the genes that were really close to this HFR, which is where it started, will have already recombined before you interrupted it. So let's say you allowed conjugation for 5 minutes and interrupted at 5 minutes, well, the genes that are really close to the HFR will have already recombined in that 5-minute time, and the genes that are far away won't have had time to recombine in that period. So you can actually use minutes from, like, your interrupted mating from the time of conjugation starting to determine how close the genes are to that HFR. So this is what this looks like in the center of the passage. Here you have the HFR, it has the pili; it can conjugate. You can see that, it's getting transferred. Its it's undergoing, it's allowing recombination between these two bacteria. So here's the origin, which is where that started, and it always starts with that HFR. And then the genes that are really close to that HFR, if you interrupt it very quickly, soon after conjugation, will have stopped. Right? Then you can map how close those genes are to the HFR based on minutes past interruption. So with that, let's now move on.

R Plasmid

Okay. So bacteria actually contain other plasmids in addition to the F factor. So, like I said before, there are, you know, 270 different types of plasmids, and the main one that we talked about is the F factor, obviously, because of its importance in conjugation. But another really important one I talk about is the R plasmid, and this is the plasmid that allows bacteria to confer antibiotic resistance. So we hear about these superbugs and like, you know, take all of your antibiotics because you don't want superbugs, and, they're killing people all over the place, and, being a little dramatic here, but you hear about them in the news and that's how they present it. And so the R plasmid is what allows bacteria to do this. So these plasmids, if one bacterium has an R plasmid that's resistant to penicillin and there are other bacteria around, it can actually be transferred between bacterial species. It doesn't have to be the same species. It can be any bacteria that's just sort of around it. And it can say, hey, I can grow without, you know, in the presence of penicillin, and all the bacteria are like, oh my goodness, give me that plasmid. And so they do, and so all these different bacterial species now become resistant to Penicillin, which is obviously a horrible thing.

Now the R plasmids, are really effective and they're different from the F factor because they contain an extra thing on it that's super important. These things are called transposons. Now you may have heard of transposons before. If you haven't, it's totally okay. We'll talk about them. But essentially, what a transposon is, in just brief form, is it's called a jumping gene. And this gene can, sort of, cut itself out of wherever it is and sort of jump to a different part of the organism, or in this case, for the R plasmid, actually different organisms completely. So the R plasmid contains a transposon, completely. So the R plasmid contains a transposon and that transposon will allow it to cut out of the bacteria it's in and transfer into other bacteria. And so it can assist in this DNA transfer, and it's part of the reason why it allows it to be able to jump between all of these bacterial species. So this is a really important plasmid. The R plasmid does antibiotic resistance. So, with that, let's unload it on.

True or False:For conjugation to occur, bacterial cells must physically contact each other.

• A. True
• B. False

What is the name of the structure through which DNA is transferred? 

• A. DNA bridge
• B. Sex Pili
• C. Genetic Material Passageway
• D. Fertility Factor

What property does the F factor give bacteria?

• A. Antibiotic resistance
• B. The ability to swap DNA via conjugation
• C. The inability to swap DNA via conjugation
• D. The ability to integrate the DNA into the chromosome

A F⁺ bacterial cell can donate DNA to which type of bacterium?

• A. An F⁺ bacteria
• B. An F^- Bacteria
• C. An Hfr Bacteria
• D. An R⁺ bacteria

Here’s what students ask on this topic:

What is bacterial conjugation and how was it discovered?

Bacterial conjugation is the process where two bacterial cells physically unite to exchange genetic material. This was first discovered in 1946 by scientists Lederberg and Tatum. They conducted an experiment with two strains of E. coli, each requiring different nutrients to grow. When mixed and plated on a medium lacking these nutrients, some cells still grew. This unexpected result led to the hypothesis that the strains exchanged DNA, allowing them to grow without the specific nutrients. This exchange of genetic material enhances genetic diversity among bacterial populations.

What is the F factor in bacterial conjugation?

The F factor, or fertility factor, is a plasmid that enables bacterial conjugation. Bacteria with the F factor are termed F+ and can donate genetic material, while those without it are F- and can only receive genetic material. The F factor is a circular DNA molecule separate from the main chromosome. It allows the formation of a sex pilus, facilitating the transfer of DNA between bacterial cells. This process increases genetic diversity, which is beneficial for bacterial adaptation and survival.

How do R plasmids contribute to antibiotic resistance?

R plasmids are a type of plasmid that confer antibiotic resistance to bacteria. They often contain transposons, which are jumping genes that can move between different DNA molecules. This allows R plasmids to transfer antibiotic resistance genes between different bacterial species. For example, a bacterium with an R plasmid resistant to penicillin can transfer this resistance to other bacteria, leading to the spread of antibiotic-resistant infections. Understanding R plasmids is crucial for developing strategies to combat antibiotic resistance.

What is an Hfr cell and how is it used in bacterial chromosome mapping?

Hfr (High Frequency of Recombination) cells are bacteria in which the F factor has integrated into the main chromosome. Unlike F+ cells, Hfr cells transfer genetic material through recombination rather than simple conjugation. During conjugation, Hfr cells initiate DNA transfer at a specific origin point. By interrupting this process at various times, scientists can determine the order and relative distances of genes on the bacterial chromosome. This technique, known as interrupted mating, is valuable for mapping bacterial genomes.

What role do transposons play in bacterial conjugation?

Transposons, or jumping genes, are DNA sequences that can move from one location to another within a genome. In bacterial conjugation, transposons play a crucial role in the transfer of genetic material, especially in R plasmids. They facilitate the movement of antibiotic resistance genes between different bacterial species. This ability to transfer genes across species boundaries contributes to the rapid spread of antibiotic resistance, making transposons a significant factor in bacterial evolution and public health concerns.