Meiosis: Study with Video Lessons, Practice Problems & Examples

Meiosis consists of two rounds of cell division: meiosis 1, a reductional division that transforms a diploid germ cell into two haploid cells, and meiosis 2, an equational division that produces four genetically diverse haploid gametes. Key processes contributing to genetic diversity include crossing over during prophase 1, where homologous chromosomes exchange genetic material, and independent assortment during metaphase 1, where chromosomes align randomly. These mechanisms ensure a vast array of genetic combinations, essential for evolution and adaptation.

Introduction to Meiosis

In this video, we're going to begin our introduction to meiosis. 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. Even before meiosis takes place, interphase still needs to occur. If we take a look at our image down below, notice that the orange region is representing interphase as we discussed in our previous lesson videos when talking about the cell cycle. Even before meiosis takes place, interphase first needs to happen. Interphase is going to have a G₁ phase, an S phase, and a G₂ phase, and of course, the S phase is where the DNA is going to get replicated. Even before meiosis can take place, interphase takes place and DNA replication occurs. Notice that meiosis is indicated by this region of our image. When we look at the process of meiosis, it's actually broken down into two parts, meiosis I followed by cytokinesis, and meiosis II followed by cytokinesis.

As we move forward in our course, we'll talk more about this process of meiosis and break it down further. But 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. Instead of forming a full circle that starts and ends at the same place, with this process of meiosis, it creates a linear fashion that is not going to be cyclic and regenerate the same types of cells. The biggest takeaway about this process of meiosis is that it's going to start with a diploid germ cell, and it's going to end with four genetically diverse haploid cells or more specifically haploid gametes. Recall that gametes are just the sex cells, either sperm or eggs.

This diploid germ cell is again a diploid cell, having two copies of every chromosome, and it acts as the precursor for making gametes. Meiosis starts with the germ cell, and the end of meiosis results in making these gametes, either sperm or eggs. At the end of meiosis, notice that we have four gamete cells that are going to be haploid, represented by n, and they are going to be genetically diverse. Each cell would be genetically different from the others. 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 the cell cycle. As we move forward in our course, we're going to talk more about meiosis. I'll see you all in our next video.

Meiosis Example 1

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.

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.

Meiosis I & Meiosis II

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 n2 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, 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 going to 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 represents 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 n2 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 males or eggs in females. 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.

In Meiosis II, ________ cells are divided into 4 ___________ daughter cells.

• A. Diploid; Haploid.
• B. Haploid; Diploid.
• C. Haploid; Haploid.
• D. Diploid; Diploid.

Meiosis I

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 G1, the S, and the G2 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 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, the events of prophase 1 and telophase 1 are going to be very similar to the events that occurred in mitosis.

Meiosis 1 is going to differ significantly in both metaphase 1 as well as anaphase 1. Because really, it's just metaphase 1 and anaphase 1 that differ significantly, this is where we're going to put most of our focus. 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. In metaphase 1, the 'M' in metaphase represents the 'M' of the middle. 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. This is the big difference here: these chromosomes are aligning themselves in 2 rows instead of aligning themselves in one row like what we saw in mitosis.

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. Notice we have one row of chromosomes right here and the second row of chromosomes is over here. And 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. This is another big difference between 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. When we take a look at anaphase 1 down below right here, notice that the homologous chromosomes line up in these pairs here in metaphase 1 and then the homologous chromosomes are separated. 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. The homologous chromosomes are separating, but the sister chromatids themselves still remain intact, and so that means that we still have replicated chromosomes here.

That is going to be a big difference between meiosis and mitosis. Later in our course, we're going to do a side-by-side comparison to 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 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. 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 2 as we move forward in our course. So I'll see you all in our next video.

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.

Meiosis II

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 upforming 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.

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 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.

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 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 mitosis. 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.

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 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.

Notice that anaphase 2 here is occurring in each of the haploid daughter cells that result from meiosis 1. So prophase 2 occurs in each of these cells. Metaphase 2 occurs in each of these cells, and so on. And the phases of meiosis 2 are practically the same as the phases of mitosis. And really the only differences that prophase, in 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.

This here really concludes our brief introduction to meiosis 2, the second half of meiosis, and how really it's going to result 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.

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.

Genetic Variation During Meiosis

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 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 synapsis and chiasma. So synapsis refers to the alignment between homologous pairs of chromosomes. 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 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 also 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, 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. 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 two possibilities of that random and independent alignment. And we have just used four chromosomes and, of course, humans would have 46. So this is a very simplistic version, but it gives you a good idea of 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 two maternal chromosomes have ended up on the left. Let me fix that right there really quick.

Alright. And our two paternal chromosomes have ended up on the right. And then in possibility number 2, we have one 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. 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 two 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 four 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 four chromosomes and showing two possibilities of those combinations, and 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.

Meiosis Example 5

Okay. So this one asks: Crossing over involves each of the following except what? Let's run through them and see what we have.

Option A reads the transfer of DNA between two non-sister chromatids and that definitely happens during crossing over. That is, like, the key event of crossing over; when we get the genetic swap or that kind of DNA exchange between two non-sister chromatids. We're going to cross out A because that definitely does happen.

Option B reads the transfer of DNA between two 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.

Option 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.

Option 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. There you go.

During which of the following processes does independent assortment of chromosomes occur?

• A. In meiosis I only.
• B. In meiosis II only.
• C. In mitosis and meiosis I.
• D. In mitosis I and meiosis II.

Here’s what students ask on this topic:

What are the main stages of meiosis and their key events?

Meiosis consists of two main stages: meiosis 1 and meiosis 2. Meiosis 1 includes prophase 1, metaphase 1, anaphase 1, and telophase 1. During prophase 1, homologous chromosomes pair up and exchange genetic material through crossing over. In metaphase 1, these pairs align in the middle of the cell in two rows. Anaphase 1 involves the separation of homologous chromosomes, while sister chromatids remain attached. Telophase 1 and cytokinesis result in two haploid cells. Meiosis 2 resembles mitosis and includes prophase 2, metaphase 2, anaphase 2, and telophase 2. In metaphase 2, chromosomes align in a single row. Anaphase 2 separates sister chromatids, and telophase 2 and cytokinesis produce four genetically diverse haploid gametes.

How does crossing over contribute to genetic diversity?

Crossing over occurs during prophase 1 of meiosis 1, where homologous chromosomes exchange genetic material. This process involves the formation of a chiasma, where the chromosomes physically overlap and swap segments of DNA. As a result, new combinations of alleles are created on each chromosome, leading to non-sister chromatids that are genetically unique. This genetic recombination increases the variability in the offspring, contributing to genetic diversity, which is crucial for evolution and adaptation.

What is the difference between meiosis 1 and meiosis 2?

Meiosis 1 is known as reductional division because it reduces the chromosome number by half, transforming a diploid germ cell into two haploid cells. Key events include the pairing and separation of homologous chromosomes. Meiosis 2, on the other hand, is called equational division because it maintains the haploid state. It involves the separation of sister chromatids, similar to mitosis, resulting in four genetically diverse haploid gametes. The main difference lies in the type of chromosomes being separated: homologous chromosomes in meiosis 1 and sister chromatids in meiosis 2.

What is independent assortment and how does it affect genetic variation?

Independent assortment occurs during metaphase 1 of meiosis 1, where homologous chromosome pairs align randomly at the cell's equator. This random alignment means that the distribution of maternal and paternal chromosomes into the daughter cells is independent of other chromosome pairs. As a result, the combination of chromosomes that each gamete receives is unique, contributing to genetic variation. This process, combined with crossing over, ensures a vast array of genetic combinations, enhancing the genetic diversity of offspring.

Why is meiosis important for sexual reproduction?

Meiosis is crucial for sexual reproduction because it produces haploid gametes (sperm and eggs) with half the chromosome number of the parent cell. This reduction is essential to maintain the species' chromosome number across generations. During fertilization, the fusion of two haploid gametes restores the diploid state in the zygote. Additionally, meiosis introduces genetic diversity through crossing over and independent assortment, which are vital for evolution and adaptation, allowing populations to better respond to environmental changes.