Prokaryote Reproduction and Gene Exchange - Video Tutorials & Practice Problems
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Binary Fission, Transformation, and Transduction
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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, you know, without nutrients and whatever they'll they'll start to die. We don't really need to worry too much about this. The main point that I wanna 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 you know, starting with 1 cell you'll get like 1, then 2, 4, 8, 16, 32, 64, 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, you know, parental cell, if you will, is gonna 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 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, 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, you know, divide and those daughter cells will get to the point where they can reproduce in such a short window of time that that is to say the generation time, the time it takes to produce a new generation is so small, so short. And also the populations that of bacteria that exist are so large, there's so many individuals in the population that we actually, do see a significant frequency of mutations even though, though, you know, 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, you know, like we talked about, back in in the genetics section, you can have, you know, 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, you know, lose or whatever introduced 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, you know, pretty cool about transformation is not only will this is, you know, can 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 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, introduce genes into eukaryotes, but this is that's a different process, has a different name. We're not really gonna talk about it here. I will mention, so in prokaryotes, we call it, this process transduction where DNA is transferred from 1 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 gonna 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 it's got that DNA in pink in the, capsid right there, it's gonna 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 the virus is gonna do to the cell. And it's the virus's DNA that is, 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 gonna make mistakes and sometimes they're gonna 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, a as what's called a vector. Right? They can act as a a means of moving some DNA from 1 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, you know, having a virus transfer it as a, you know, through a viral vector. Alright. Let's turn the page.
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Conjugation
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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 2 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, pili, that's the plural form of pilus. You know, Latin words are gonna have strange forms to them. Anyways, append So this is an appendage used to, connect the 2 cells that are going to be involved in conjugation and really, the way, what this all boils down to is the presence of this special plasmid called the f plasmid. And, this plasmid actually can 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 that, term f factor can both mean the F plasmid and also, the, you know, f plasma genes, having been incorporated into the chromosomal DNA. So essentially, these are just a bunch of genes required for this genetic transfer. So, it in order to have conjugation, you need what's 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 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, the 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 2 are going to be linked. The pilus is actually going to kind of pull them together. There's going to be a 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 a f+ell. Right? This one was f+ the whole time, This is newly f plus. So it's f plus old, f plus new. Whatever you wanna call it. Old donor, new donor. Right? So now, this new donor, now it can make its own pilus. Right? Because it has now it has the genes too. So this is, sort of 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, sort of special, case of conjugation that involves what's called an R plasmid. And this is just a plasmid, I shouldn't say just, but it's it's 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, like, you know, probably the most important topic in micro biology right now, because, you know, 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 just, you know, a plasmid that, can, that carries the gene that convert confers some type of antibiotic resistance. It's worth noting that not only can these, be transferred via conjugation, but our our plasmids can also be transmitted by transformation and transduction. Alright. That's all I have for this video. See you guys next time.