In today's video, we're going to run through everything you need to know about binary fission, which is the process by which prokaryotic organisms, like bacteria, divide and reproduce. So we'll cover how binary fission works, and then how to do calculations to figure out how quickly bacterial populations can grow. Now, just before we start, I want to make clear that binary fission.
is not the same as mitosis or meiosis. These are processes that happen in eukaryotic cells, whereas binary fission is for prokaryotic cells, like bacteria. So if we start off with a single bacterial cell, we can see that they have a cell wall, a cell membrane, cytoplasm, a large circular strand of DNA that contains all of their important genes, and then a whole bunch of little plasmas, which are also circles of DNA, but contain non-essential genes, that only sometimes come in handy.
Some bacteria also have a flagellum, which is like a tail that they can use to move around, but not all bacteria have one. So as we said at the beginning, binary fission is the process by which one bacterial cell, like this one, divides into two. And the important point to understand here is that because bacteria are single-celled organisms, as well as being a type of cell division, binary fission is also a type of reproduction, because the one organism up the top has to split into two organisms.
So you might sometimes hear binary fission referred to as asexual reproduction. Now if we think about how binary fission actually works, Before a bacterial cell can divide, it needs to do two main things. First, it has to grow a bit, so that it's big enough to give rise to two new cells. And then secondly, it has to replicate all of its genetic material, so that there's enough for each new cell.
So it has to replicate its large circular strand of DNA, and then also replicate all of its little plasmids. Once this is done, the two large circular strands move to either sides of the cell, so that the offspring cells will end up getting one each. The plasmids though are arranged pretty randomly, so one offspring cell might end up getting more than the other. Then once this is done and is ready to divide, it starts to grow a new cell wall down the middle of the cell, which when it's complete allows the two halves to pull apart. And voila!
We have two new bacterial cells! The next thing we need to look at is how quickly bacterial populations can grow. Because binary fission is a fairly simple process, it doesn't take very long. In fact, given the right conditions, some bacteria can divide once every 20 minutes. And because the population doubles with every division, one cell becomes two, two becomes four, four becomes eight, and so on.
So the population as a whole grows really quickly. For example, if these bacteria kept on doubling every 20 minutes for the next 5 hours, we'd end up with a colony of over 250,000 bacteria. When it comes to your exams, you might be asked to calculate something like this for yourself.
So let's see how we'd do it for this question here. A bacterial cell has a mean division time of 30 minutes. How many cells would it produce after 3 hours? And just to point out here, that mean division time just means how long it takes to divide on average. So it's saying that they can divide every 30 minutes.
So to work this out, the first thing we need to do is figure out how many division cycles there will be in 3 hours. Which we can find by dividing the total time of 3 hours, or 180 minutes, by the mean division time of 30 minutes. which is 6. So the bacteria will be able to undergo 6 rounds of division in the 3 hours.
Next, to figure out how many cells that would make, we just take our original cell, of which there is only 1, and then multiply it by 2, or double it 6 times, which equals 64. So there will be 64 cells after 3 hours. You could also have done it a bit more simply by multiplying the 1 by 2 to the power of 6, which is the same thing as multiplying it by 2 six times. Something to point out here though is that the mean division time will depend on the species of bacteria and the conditions that the bacteria are in.
For optimal growth, bacteria like somewhere that's warm, moist, and has plenty of nutrients. If they don't have these things, then they will divide much more slowly. or maybe not at all.
Let's do one more question very quickly before we finish. A Petri dish contains 1000 bacteria. Each cell divides once every 20 minutes.
How many cells will there be after 4 hours? So this time we're to do the total time of 4 hours, or 240 minutes, divided by the division time of 20 minutes. which would give us 12 division cycles. And then because we started with 1,000 bacteria this time, we'd multiply 1,000 by 2 12 times, or just multiply it by 2 to the power of 12, which in either case gives us 4,096,000 bacterial cells.
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