In this video, we're going to begin our introduction to meiosis. And so it's important to note that even before meiosis takes place, a diploid cell must first replicate its DNA and make proteins for cell division in a process that we know as interphase. And so really what we're saying here is that even before meiosis takes place, interphase still needs to take place. And so if we take a look at our image down below, notice that this orange region that you see over here is representing interphase as we discussed it in our previous lesson videos when we were talking about the cell cycle. And so even before meiosis takes place, interphase first needs to take place. And interphase is still going to have a g one phase, an s phase, and a g two phase. And of course the s phase is where the DNA is going to get replicated. And so even before meiosis can take place, interphase takes place and DNA replication is going to occur. And so notice that meiosis here is indicated by this region of our image. And really when, we look at the process of meiosis, it's actually broken down into 2 parts, meiosis 1 followed by cytokinesis, and meiosis 2 followed by cytokinesis. And so, ultimately, as we move forward in our course, we'll talk more and more about this process of meiosis and break it down much further. But really, what you can see here is that meiosis is not going to be a cyclic, process that regenerates the same types of cells like what mitosis does. And so notice that, instead of forming a full circle that starts and ends at the same place, you can see that with this process of meiosis, it really creates a linear type fashion here, that is not going to be cyclic and regenerate the same types of cells. And, again, as we move forward in our course, we're gonna talk more and more about meiosis. But really the biggest takeaway that you can get about this process of meiosis is that it's going to start with a diploid germ cell, and it's going to end with 4 genetically diverse haploid cells or more specifically haploid gametes. And recall gametes are just the sex cells, either sperm or eggs. And so, what is this germ cell here, this diploid germ cell? Well, the germ cell is again a diploid cell. So it's going to have 2 copies of every chromosome and it's the diploid cell that acts as the precursor for making gametes. And so the germ cell, is going to ultimately be the start of meiosis, starts with the germ cell. And the end of meiosis, ends with, making these gametes, either the sperm or the egg. And so what you can see here is that, again, the process of meiosis is right here. It starts with a deployed germ cell that has 2 copies of every chromosome. And, and meiosis 2 followed by cytokinesis, and meiosis 2 followed by cytokinesis. At the end of meiosis, notice that we have 4, gamete cells that are going to be haploid represented by n, and they are going to be genetically diverse. This cell would be genetically different from this one, which is genetically different from this one and this one. And so again, through this image here and comparing it to the cell cycle that we talked about in our previous lesson videos, you can get a better feel for how meiosis is going to be similar and yet still different from, some of these cell cycle, that we talked about previously. And, again, as we move forward in our course, we're gonna talk more and more about meiosis. So I'll see you all in our next video.
2
example
Meiosis Example 1
Video duration:
1m
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Video transcript
Alrighty. So here we have an example problem that wants us to complete the sentence using one of these 5 potential answer options down below. And it says that the process of meiosis produces, and option a notice says 2 diverse haploid gamete cells. Now of course from our last lesson video we know that meiosis is going to result in 4 cells, not 2 cells. So any option that says 2 cells, we know is going to be incorrect for meiosis. So we can eliminate both option a and option b, which both mention 2 cells. And another thing that we can note is that meiosis is going to create 4 genetically diverse cells that are not identical to each other. And so to say that it's going to create 4 identical cells is going to be incorrect. And so we can eliminate both answer option c and answer option e because they both say identical. And so it turns out that option d is going to be the correct answer just through that logic there. And so meiosis ends up producing 4 genetically diverse cells that are haploid gamete cells, meaning that they're going to be, haploid, meaning that they're going to have half the number of chromosomes or just one copy of every chromosome, and gametes, meaning that they're going to be sex cells either sperm or egg. And so answer option d is the correct answer to this example problem, and that concludes this example. So I'll see you all in our next video.
3
Problem
Problem
Which of the following steps must occur before Meiosis I in germ cells?
A
The DNA of the haploid cell is replicated.
B
The RNA of the diploid cell is replicated.
C
The DNA of the diploid cells is replicated.
D
The two cells need to be physically separated by cytokinesis.
4
concept
Meiosis I & Meiosis II
Video duration:
4m
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Video transcript
So it turns out that meiosis is actually broken down into 2 rounds of cell division. And the first round of cell division is meiosis 1, and the second round of cell division is meiosis 2. And so in the first round of cell division, meiosis 1, it's important to know that it's also sometimes referred to as reductional division. And this is because meiosis 1 will reduce the ploidy of the cells, reducing a diploid cell into 2 haploid cells. And it does this by separating homologous chromosomes from each other. And so you can see once again that in meiosis 1, a diploid or 2n germ cell is going to divide into 2 haploid daughter cells. And so the ploidy has been reduced because we've gone from a diploid cell into 2 haploid cells. Now, the second round of cell division in meiosis is meiosis 2, which is sometimes referred to as equational division. And that's because meiosis 2 maintains equal ploidy, takes taking 2 haploid cells and forming 4 haploid cells. So you start with haploid cells and you end with haploid cells. And so that maintains equal ploidy, and that's why it's called equational, equa for equal. And so meiosis 2 maintains equiploidy by separating sister chromatids instead of separating homologous chromosomes. So that's a big difference between meiosis 1 and meiosis 2. And again, because meiosis 2 is equational division, it's gonna maintain equal ploidy. So you're going to start with the 2 haploid cells from meiosis 1, and those 2 haploid cells from meiosis 1 are each going to divide to produce a total of 4 genetically diverse still haploid gametes. And so you started with haploid cells and you still end with haploid cells, and that's why it maintains equal ploidy. And so if we take a look at our image down below, it's another representation of meiosis. And so notice that before meiosis even begins, of course interphase is going to take place with the germ cell which basically takes unreplicated chromosomes and forms replicated chromosomes as we see here. And so this rep represent the germ cell, the diploid germ cell that, is going to begin meiosis. And again, meiosis is really broken up into 2 rounds of cell division. The first round of cell division is meiosis 1. And so in meiosis 1, it's also referred to as reductional division because it takes a diploid germ cell that is 2 n and it divides the diploid germ cell into 2 cells, daughter cells that are haploid or n. And then the second round of cell division is called meiosis 2. And so meiosis 2 is also sometimes referred to as equational division because you start with haploid cells and you also end with cells that are still haploid. And so it turns out that each of these cells is going to undergo meiosis 2, and a cell division to create a total of 4 haploid gametes. And, of course, the gametes are going to be the sex cells, either sperm in male or eggs in female. And so, each of these phases, of these stages of meiosis, meiosis 1 and meiosis 2, consists of multiple phases. And we're going to break down meiosis 1 and meiosis 2 in their own separate videos as we move forward in our course. But for now this here concludes our introduction to meiosis and how meiosis is really broken down into 2 rounds of cell division in meiosis 1 and meiosis 2. And so we'll be able to get some practice as we move forward in our course, and I'll see you all in our next video.
5
Problem
Problem
In Meiosis II, ________ cells are divided into 4 ___________ daughter cells.
A
Diploid; Haploid.
B
Haploid; Diploid.
C
Haploid; Haploid.
D
Diploid; Diploid.
6
concept
Meiosis I
Video duration:
7m
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Video transcript
In this video, we're going to introduce the first part of meiosis, which is meiosis 1. And so recall from our previous lesson videos that before meiosis can even begin, the process of interphase needs to occur first. And so notice over here in our image down below on the left hand side, we're first showing you that interphase needs to occur before meiosis can even begin. And recall that interphase consists of the g one, the s, and the g two phase. And during the S phase of interphase that is when the DNA gets synthesized or replicated. And so before meiosis can even begin interphase takes place and the DNA must be replicated. And then at that point after the G2 phase of interphase, the cell can enter into meiosis 1. And what you'll notice about meiosis 1 is that it actually has very similar steps to mitosis in terms of having a prophase, a metaphase, an anaphase, a telophase followed by cytokinesis. And so one thing that you'll note is different is that the phases of meiosis 1 are all followed by a 1. And so notice here we have prophase 1, metaphase 1, anaphase 1, telophase 1, indicating that these are the phases of meiosis 1. Now recall from our previous lesson videos when we were talking about my mitosis that sometimes the phase prometaphase, which would come between prophase and metaphase, is sometimes batched with the previous phase, prophase 1. And so even though we're not showing you a prometaphase here in this image, the events of prometaphase are still taking place in prophase 1. And so, really the events of prophase 1 and telophase 1 are going to be very very similar to the events that occurred in mitosis. And so, really, meiosis 1 is gonna be similar to mitosis, but meiosis 1 is going to differ significantly in both metaphase 1 as well as anaphase 1. And so because really it's just metaphase 1 and anaphase 1 that differ significantly, really this is where we're going to put most of our focus. And so notice that our image, down below has these two phases in blue here to help emphasize that these are really the two phases that differ the most from mitosis. And so, in metaphase 1, of course the m in metaphase represents the m of the middle. And so, in metaphase 1, what's going to happen is that homologous chromosomes are going to pair up, and they're going to align themselves in the middle of the cell, but they'll align themselves in the middle of the cell in 2 rows. And really this is the big difference here that these chromosomes are aligning themselves in 2 rows instead of aligning themselves in one row like what we saw in mitosis. And so if we take a look at our image down below at metaphase 1, what you'll notice is that in metaphase 1 the homologous chromosomes are pairing up with each other to create 2 rows. And so notice we have one row of chromosomes right here and the second row of chromosomes is over here. And so they're not all lining up in a single file row, they're lining up in 2 rows. And that is a big difference between meiosis 1, metaphase 1 of meiosis 1, and metaphase of mitosis. Now during anaphase 1 of meiosis 1, it's actually the homologous chromosomes that are going to separate from each other, while the sister chromatids themselves remain connected. And this is another big difference between my, meiosis 1 and mitosis because during mitosis, the sister chromatids would separate, but during meiosis 1, the sister chromatids remain connected and it's actually the homologous chromosomes themselves that are going to be separated. And so when we take a look at anaphase 1 down below right here, notice that the homologous chromosomes they line up in these pairs here in metaphase 1 and then it's the homologous chromosomes that are separated. So notice that this homologous chromosome is separating in this direction and so is this one here going towards this end of the cell while this homologous chromosome here is going towards this end of the cell and this one here is going towards this end of the cell. And so the homologous chromosomes are separating but the sister chromatids themselves still remain intact and so that means that we still have replicated chromosomes here. And so, that is going to be a big difference between, again, meiosis and, mitosis. And later in our course, we're going to do a side by side comparison further differentiate between these two and their similarities as well as their differences. But again, the main phases that are going to be significantly different in meiosis are going to be metaphase 1 and anaphase 1. And again prophase 1 and telophase 1 are going to be pretty much the same exact events that occurred during mitosis in those same phases. So after telophase 1, of course you can see here, there is going to be the process of cytokinesis and cytokinesis is the division of the cytoplasm splitting the cell into 2. And so cytokinesis during meiosis 1 is actually going to produce 2 haploid daughter cells. And then these 2 haploid daughter cells that result right over here can then begin the second half of meiosis which is meiosis 2. And so, meiosis 2 is going to begin with these 2 cells that are now haploid. And so you can see they have half the number of chromosomes in comparison to the very very beginning. And so notice that there were originally 4 chromosomes here in this image in the beginning, that would be the diploid number for this example, and at the end each cell only has 2 chromosomes, which is half of 4, meaning that these are going to be haploid. And so this here really concludes our introduction to meiosis 1 and how it differs most significantly from mitosis. And we'll be able to get some practice applying these concepts and then talk about meiosis too as we move forward in our course. So I'll see you all in our next video.
7
Problem
Problem
A daughter cell is created by meiosis I and the first round of cytokinesis. This daughter cell is just beginning meiosis II. Which of the following is an appropriate description of this daughter cell's genetic contents?
A
It has half the amount of DNA as the parent cell.
B
It has half the chromosomes but twice the DNA of the parent cell.
C
It has one-fourth the DNA and one-half the chromosomes as the parent cell.
D
It is genetically identical to the parent cell.
8
concept
Meiosis II
Video duration:
6m
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Video transcript
So now that we've introduced the first half of meiosis, which is meiosis 1 in our last lesson video, In this video, we're going to introduce the second half of meiosis, which is meiosis 2. And so recall from our last lesson video of meiosis 1 that meiosis 1 starts with a diploid cell, and it ends with 2 haploid daughter cells. But these haploid daughter cells still have replicated chromosomes, which means that cell division is not yet over with. And so these 2 haploid daughter cells that result from meiosis 1 are going to transition into the second half of meiosis, meiosis 2. And so in meiosis 2, each of the haploid daughter cells produced in meiosis 1 is going to divide, and that's going to end up forming a total of 4 genetically diverse haploid gametes. Now later in our course, we'll talk about how this genetic diversity arises during meiosis, but for now, we're gonna focus on the events of meiosis 2. And in terms of the events that occur in each phase of meiosis 2, it actually turns out that meiosis 2 is almost exactly the same as mitosis. And really the only difference is that mitosis starts with a diploid cell, whereas meiosis 2 starts with haploid cells from meiosis 1. But outside of that, meiosis 2 and mitosis are practically the same. And so very, very similarly to mitosis, chromosomes are going to align in just one single file row during metaphase 2. And that's exactly what happens during metaphase of my mitosis. The chromosomes align in 1 single file row. And recall from our last lesson video that this was not the case for meiosis 1, metaphase 1. Because during meiosis 1, metaphase 1, chromosomes did not align in 1 row, The chromosomes aligned in 2 rows, and that makes a big difference. And if we take a look at our image down below of meiosis 2 right here, what you'll notice is that it also has very very similar phases to mitosis in terms of having a prophase, metaphase, anaphase, and telophase, but what you'll notice is that each of these phases is followed up by the number 2. So we have prophase 2, metaphase 2, anaphase 2, and telophase 2 to indicate that these are the phases that belong to meiosis 2. And again, really meiosis 2 is almost exactly the same as my ptosis. And so what you'll notice is the events here are going to be exactly the same. And really, even though we're not showing you a prometaphase, between prophase and metaphase, the events of prometaphase are still occurring, they're just occurring, they're being batched here with prophase. And so that's something important to keep in mind. And really, meiosis 2 is mitosis just occurring immediately after meiosis 1. That's a way to think of it. And so notice that during metaphase 2 of meiosis 2, the chromosomes are aligning in just one single file row instead of aligning in 2 rows like what we saw during meiosis 1. And, also, very very similarly to mitosis, during meiosis 2, sister chromatids are actually going to separate and be divided during anaphase 2 of meiosis 2, just like sister chromatids are separated during anaphase of mitosis. And so if we take a look at our image down below at anaphase 2, notice that these replicated chromosomes are being divided, and so the sister chromatids are being separated and divided from each other. And so notice that, this sister chromatid here is going in this direction whereas this one here is going in the opposite direction and this sister chromatid here is being pulled into this direction and this one here is being pulled into this direction over here. And so, notice that, anaphase 2 here is occurring in each of the haploid daughter cells that results from meiosis 1. So prophase 2 occurs in each of these cells. Metaphase 2 occurs in each of these cells, and so on. And so, again, the phases of meiosis 2 are practically the same as the phases of mitosis. And really the only difference is that prophase, my meiosis 2 is starting with haploid cells instead of starting with diploid cells. And, of course, because each of these cells is going to undergo a cell division, there's going to be a total of 4 cells that are going to result, and these 4 cells are all haploid. And so you start with haploid cells in meiosis 2, and you end with haploid cells as well. And so, that's why it's called equational division at times since the ploidy number remains equal. And so this here really concludes our brief introduction to meiosis 2, the second half of meiosis and how really it's going to result to, in forming 4 genetically diverse haploid gametes. And so these 4 cells here, you could imagine, would either be sperm or egg, depending on if it's male or female. And so again, this here concludes our introduction to meiosis 2, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
9
Problem
Problem
During which of the following stages of meiosis do homologous chromosomes pair up and align along the metaphase plate of the cell?
A
Metaphase I of meiosis.
B
Telophase I of meiosis.
C
Anaphase I of mitosis.
D
Metaphase II of meiosis.
10
concept
Genetic Variation During Meiosis
Video duration:
5m
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Video transcript
In this video, we're going to talk about how meiosis leads to such incredible genetic diversity and variability. That's mainly due to 2 events, and those are crossing over and independent assortment. So crossing over is the process in which pairs of homologous chromosomes exchange genetic material. So, basically, the homologous chromosomes are going to be swapping DNA and which will create non sister chromatids and this occurs during prophase 1 of meiosis 1. So it's one of the first events to occur in meiosis. And when you're learning about this you may hear the terms synopsis and chiasma. So synopsis refers to the alignment between homologous pairs of chromosome. More specifically, it's those homologous chromosomes aligning their DNA sequence at similar alleles. So if you look at our image here, we have this pair of homologous chromosomes and so each of these is comprised of identical sister chromatids. So this one and this one are identical, and this one and this one are identical. And these homologous chromosomes are going to be similar in size, similar in shape, they're going to have similar genes, but they can have different variations or different alleles of those genes. So I'm going to represent synapsis by just drawing a line in between them to show, the alignment between those DNA sequences there and that will kind of set the stage for crossing over to take place. Now next up we have our chiasma, which is the actual site of crossing over, and you can see the chiasma represented here where those chromosomes are literally crossing over and touching and making this kind of x shape and the result of that is going to be what we see over here where now our blue chromosome has a little pink part and our pink chromosome has a little blue part. So they have exchanged that genetic material. And as you can see now we have non sister chromatids. So now this one and this one are no longer identical as are this one and this one, and as you can imagine that results in an enormous amount of possible genetic combinations there. So that is crossing over. Now next up we have independent assortment, and independent assortment is when pairs of homologous chromosomes are independently and randomly aligned, and this will happen during metaphase 1 of meiosis 1. And this can result in an enormous amount of possible genetic combinations. So just to give you an idea of that, we have created a little graph here showing 2, possibilities of that random and independent alignment. And we have just used 4 chromosomes and, of course, humans would have 46. So this is a very simplistic version, but it gives you a good idea of how, how much diversity it can create. So here in this first row, we are looking at metaphase 1 of meiosis 1, and you can see in our first possibility here, our 2 maternal chromosomes have ended up on the left. Let me fix that a right there really quick. Alright. And our 2 paternal chromosomes have ended up there on the right. And then in possibility number 2, we have 1 maternal chromosome here on the top left, a paternal one here on the top right, and then a paternal one here on the bottom left and a maternal one here on the bottom right. K. So that is the result of that, independent assortment. Now in this middle row here, what we are looking at are the haploid daughter cells that would have resulted from meiosis 1. So you can see these are all completely different. So this has resulted in this cell over here is comprised completely of the maternal chromosomes, this one is comprised of the paternal chromosomes, and then these 2 over here are comprised of different versions of those maternal and paternal chromosomes. And then finally looking at this bottom row here we are looking at the haploid daughter cells that resulted from meiosis 2. And so each of these represents a different combination. So we ended up with 4 different combinations in total. I will go ahead and number those 1, 2, 3, and 4. Okay. And, again, of course, this is a very simplistic example using only 4 chromosomes and showing 2 possibilities of those combinations, In real life, it would be much more complex. You can imagine how this in combination with that crossing over process is going to result in nearly infinite combinations, of genetic variability. Alright. So that is genetic variation during meiosis and I will see you guys in the next one. Bye bye.
11
example
Meiosis Example 5
Video duration:
1m
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Video transcript
Okay. So this one asks us crossing over involves each of the following except what? So let's run through them and see what we have. So a reads the transfer of DNA between 2 non sister chromatids and that definitely happens during crossing over. That is, like, the key event of crossing over is when we get the genetic swap or that kind of DNA exchange between 2 non sister chromatids. We're going to cross out a because that definitely does happen. B reads the transfer of DNA between 2 sister chromatids. Now that doesn't really need to happen because sister chromatids are identical so they don't need to be transferring DNA to each other it's going to be the exact same DNA. So B is looking pretty promising, let's circle B. C reads the alignment of homologous chromosomes and again that is a key feature of crossing over that will kind of set the stage for that genetic swap to be taking place, so c does happen, and then d reads all of the above, which we know is not true because b does not take place. So our answer here is B, crossing over involves each of the following except the transfer of DNA between sister chromatids, and there you go.
12
Problem
Problem
During which of the following processes does independent assortment of chromosomes occur?