Prokaryote Reproduction and Gene Exchange - Online Tutor, Practice Problems & Exam Prep

Prokaryotes reproduce asexually through binary fission, resulting in two identical daughter cells. Despite this, genetic variation can occur via mutations due to short generation times and large populations. Additionally, prokaryotes can introduce genetic variation through transformation, where they uptake external DNA, and transduction, where viruses transfer DNA between cells. Conjugation involves direct gene transfer between linked cells using a pilus, often involving F plasmids that can confer traits like antibiotic resistance. These mechanisms highlight the adaptability and evolutionary potential of prokaryotic organisms.

Binary Fission, Transformation, and Transduction

Hi. In this video, we'll be talking about how prokaryotes reproduce and exchange genetic information. Now prokaryotes reproduce asexually, and they can actually grow at an exponential rate if provided the right conditions and the right nutrients. And what you can see here is a chart of the growth of a colony of bacteria, and we have this burst here of exponential growth. Right? That's called the exponential phase. And you know, theoretically this could go on forever, but generally speaking, the bacteria will exhaust the nutrients or they will grow to the point where there's not enough space left in the environment, you know, something will cause their growth to actually level off and eventually, without nutrients and whatever, they'll start to die. We don't really need to worry too much about this. The main point that I want to make is that bacterial growth can get exponential. And the reason for this is that bacteria reproduce asexually through a process called binary fission. Right? So one cell will produce 2 cells, those 2 cells will each produce 2 more cells. So, starting with 1 cell, you'll get like 1, then 2, 4, 8, 16, 32, 64, and so on. It gets very large very quickly. Now, this process of binary fission results in 2 identical daughter cells. So right here we have our 2 daughter cells. Sorry, it's a really ugly 'u'. There we go, daughter cells. And basically, this parental cell, if you will, is going to replicate its genetic information, grow, and divide into these 2 identical cells that will each receive a copy, an identical copy of the chromosome and whatever plasmids might be present in the parent cell. So the point is, through binary fission, you do not really get genetic variation. Now that's sort of the theory. In actuality, bacterial genes undergo a pretty significant frequency of mutation. Meaning, that the rate at which mutations are introduced, during replication, which again is through binary fission, is actually significant enough to impact evolution. And the reason for this, the reason that the frequency of mutation has a significant impact, is due to the short generation times, and large populations of bacteria. So essentially, because the cells can divide and those daughter cells will get to the point where they can reproduce in such a short window of time that the generation time, the time it takes to produce a new generation, is so small. And also the populations of bacteria that exist are so large, there are so many individuals in the population that we actually do see a significant frequency of mutations, even though there are no mechanisms in place during binary fission that allow for genetic variation, like there are in sexual reproduction. Right? That's really the basis of comparison here. In sexual reproduction, like we talked about, back in the genetics section, you can have a lot of variation due to things like crossover, for example.

Now, there are ways in which prokaryotes can introduce genetic variation. It just doesn't happen through the reproduction process. It doesn't happen through binary fission. There are alternative ways. The main thing I was trying to say is that through binary fission alone, there actually is a decent amount of genetic change over time. And it is again, due to the high frequency of mutation, which is a result of those large populations and short generation times.

Now, a really kind of crazy if you think about it, way that bacteria or prokaryotes in general, can introduce genetic variation is through a process called transformation and this is essentially when a cell alters its genetics by incorporating exogenous DNA, meaning DNA found outside the cell. So the cell will literally take up DNA from outside the cell, like literally could just take up a piece of DNA that's just lying around. It'll take it into the cell and incorporate it into its own genome or plasmid or whatever. And this will result in genetic variation in the cell. Right? So we have a little example of this going on right here. You can see this cell is going to lose or introduce the environment this little piece of DNA and then this other cell here is going to take it up and incorporate it into its plasmid. Right? So, this cell right here is transformed by this piece of DNA over here. And what's pretty cool about transformation is not only will this occur between the same species, but it can occur between different species. It actually, you know, the, it can cross some pretty far species boundaries in actuality. So it's pretty awesome and kind of crazy if you think about it, you know, just picking up random DNA from the environment, pretty crazy way of introducing genetic variation.

Now, viruses, believe it or not, can also play a role in introducing genetic variation into prokaryotes. And actually, they can also introduce genes into eukaryotes, but that's a different process, has a different name. We're not really going to talk about it here. I will mention, so in prokaryotes, we call this process transduction, where DNA is transferred from one bacteria cell to another by a virus. When DNA is transferred into eukaryotes, through a virus, it's called transfection. Right? I think I'm just going to write that down. Transfection. Kind of like infection. Right? Because you're using a virus, you're infecting cells and then they're transferring some DNA to those cells. So, transfection, remember that's what happens in eukaryotes, and again, we're not going to get into how that process works. So, looking at transduction, basically, the way this can work is, you know, let's say, this bacterial cell gets infected, here we have a nice little bacteriophage, Here's our bacteriophage and it is going to, you can see it's got that DNA in pink in the capsid right there, it's going to introduce that DNA into the cell. You can see it here in the cell, there are those pieces of viral DNA, and you might also notice that the bacterial DNA has been decomposed. Right? That's part of what the virus is going to do to the cell. And it's the virus's DNA that is going to be used to build more viruses, and that's what we see happening here. But what's crazy is sometimes, right, because viruses, they're like little simple machines, kind of, if you think about it, and and they're going to make mistakes and sometimes they're going to pick up some bacterial DNA, in the viruses that they produce. So you can see here, this is, you know, this virus right here is basically a copy of the original bacteriophage. Right? This one not only has viral DNA, it also has some bacterial DNA. And if this, if this bacteriophage right here, if this one goes on to infect a cell, like we see happening here, not only is it going to introduce that viral DNA, right? It is going to introduce some of that bacterial DNA from this other previous bacteria over here. And what can happen is that, bacterial DNA can actually, you can see, it can get incorporated into the chromosome or, you know, as what's called a vector. Right? They can act as a means of moving some DNA from one bacterial cell to some other bacterial cell. And again, this can happen between species or within the same species. So these two methods we just talked about, transformation and transduction, are ways in which bacteria can introduce genetic variation into their populations by incorporating external DNA into their cells, either by picking it up from the environment or by having a virus transfer it as a viral vector. Alright. Let's turn the page.

Conjugation

So the last way that prokaryotes can introduce genetic variation is through what's known as conjugation. And this is, unlike the other methods we discussed, a direct transfer of genetic material between two physically linked bacterial cells. And this is going to involve that appendage we previously discussed called a pilus. The plural of that, in case you're curious, is pili; that's the plural form of pilus. You know, Latin words are going to have strange forms to them. Anyway, this is an appendage used to connect the two cells that are going to be involved in conjugation and really, what this all boils down to is the presence of this special plasmid called the F plasmid. And, this plasmid can actually become incorporated into the chromosome, the actual chromosomal DNA of a cell. So really, it's probably better to just call it the F factor. Right? That term F factor can both mean the F plasmid and also, the F plasmid genes, having been incorporated into the chromosomal DNA. So essentially, these are just a bunch of genes required for this genetic transfer. In order to have conjugation, you need what is called an F-positive bacteria, F⁺. And basically, this is just a bacteria that possesses the F plasmid. You can also have this F-prime bacteria. You see that little symbol right there, that's prime. And these are bacteria that have the F factor incorporated into their bacterial chromosome. Right? So you need either F⁺ or F prime, and you also need an F-minus bacteria or a bacteria that is lacking the F factor and is going to act as a recipient of the genes. Right? So, the bacteria that has the F factor is going to pass on the genes, and bacteria that lacks it is going to receive them. So looking at our little model here, basically, you have what's called the donor and it is going to extend its pilus and guess what? The genes for that pilus are part of the F plasmid. Go figure, right? So it's going to connect to the recipient cell and from there, the two are going to be linked. The pilus is actually going to kind of pull them together. There's going to be an opening between the cells and the genetic information is going to be transferred, you can see from the donor cell into the recipient cell there. And at that point, that cell that receives the F plasmid is going to become a cell that is now capable of acting as a donor. And, let me hop out of the picture so you can see that. Yeah. So, it is now going to be an F⁺ cell. Right? This one was F⁺ the whole time, This is newly F⁺. So it's F⁺ old, F⁺ new. Whatever you want to call it. Old donor, new donor. Right? So now, this new donor, now it can make its own pilus. Right? Because now it has the genes too. So, this is a very simplified version of what goes on during conjugation. And again, the main idea is that it involves direct gene transfer from a cell that contains the F factor to a cell that does not contain the F factor. Now, there is a special case of conjugation that involves what's called an R plasmid. And this is a plasmid that carries a gene that will confer antibiotic resistance. So, that's why I say I shouldn't say just. These are actually super important. Antibiotic resistance is probably the most important topic in microbiology right now because there are many species that are becoming resistant to most forms, if not all forms, of the antibiotics that humans have developed. So, an R plasmid is a plasmid that carries the gene that confers some type of antibiotic resistance. It's worth noting that not only can these be transferred via conjugation, but our R plasmids can also be transmitted by transformation and transduction. Alright. That's all I have for this video. See you guys next time.

Here’s what students ask on this topic:

How do prokaryotes reproduce asexually?

Prokaryotes reproduce asexually through a process called binary fission. In binary fission, a single bacterial cell replicates its genetic material and then divides into two identical daughter cells. This process allows for rapid population growth, especially under optimal conditions with sufficient nutrients. The steps include DNA replication, elongation of the cell, and division into two cells. Each daughter cell receives an identical copy of the parent cell's chromosome and any plasmids present. Despite the lack of genetic variation through binary fission, mutations can occur, contributing to genetic diversity over time.

What is the role of mutations in prokaryotic genetic variation?

Mutations play a significant role in prokaryotic genetic variation. Although binary fission produces genetically identical daughter cells, the high frequency of mutations during DNA replication introduces genetic diversity. This is particularly impactful due to the short generation times and large populations of prokaryotes. Even small mutations can accumulate rapidly, leading to evolutionary changes. These mutations can result in new traits, such as antibiotic resistance, which can be advantageous for survival in changing environments.

How does transformation contribute to genetic variation in prokaryotes?

Transformation is a process where prokaryotes incorporate external DNA from their environment into their own genome. This exogenous DNA can come from dead cells or other sources. When a prokaryotic cell takes up this DNA, it can integrate it into its chromosome or plasmids, leading to genetic variation. Transformation can occur between different species, allowing for the transfer of beneficial genes, such as those conferring antibiotic resistance. This process enhances genetic diversity and adaptability in prokaryotic populations.

What is transduction in prokaryotes?

Transduction is a method of genetic exchange in prokaryotes mediated by viruses, specifically bacteriophages. During this process, a bacteriophage infects a bacterial cell and incorporates bacterial DNA into its viral particles. When the bacteriophage infects another bacterial cell, it transfers this bacterial DNA along with its own viral DNA. The recipient cell can then integrate the transferred bacterial DNA into its genome, resulting in genetic variation. Transduction can occur within the same species or between different species, facilitating the spread of genetic traits.

How does conjugation differ from other methods of genetic exchange in prokaryotes?

Conjugation is a direct method of genetic exchange between two physically connected prokaryotic cells. It involves the transfer of genetic material through a pilus, a specialized appendage. The donor cell, which contains the F plasmid (or F factor), extends the pilus to connect with the recipient cell. The genetic material, including the F plasmid, is then transferred to the recipient cell. Unlike transformation and transduction, conjugation requires direct cell-to-cell contact. This process can transfer genes that confer advantageous traits, such as antibiotic resistance, enhancing genetic diversity and adaptability.