Meiosis - Online Tutor, Practice Problems & Exam Prep

Meiosis is a two-part cell division process that reduces the chromosome number by half, producing haploid cells. It begins with prophase 1, where homologous chromosomes pair and undergo crossing over, leading to genetic recombination. In metaphase 1, homologous chromosomes align at the metaphase plate, followed by their separation in anaphase 1. Meiosis 2 resembles mitosis, where sister chromatids are separated, resulting in four haploid cells. Understanding the distinction between diploid and haploid states is crucial, as well as the significance of processes like nondisjunction, which can lead to genetic disorders.

Meiosis Overview

Hi. In this video, we're going to be talking about meiosis. So first, let's do an overview of meiosis. Meiosis is a type of cell division, and it creates daughter cells with half the genetic material of the starting cell because there are 2 divisions in meiosis. The first division is called a reductional division, where the genetic material is reduced by half, but there are still two copies of every chromosome. You then have a second division called an equatorial division, where each of those sister chromatids receives its own cell. Meiosis starts with a diploid cell, which has 2 copies of everything and ends with a haploid cell with one copy.

Now, there are many different chromosomal forms that occur during meiosis, and people get really confused about what we're talking about. At the beginning of meiosis, you start with homologous chromosomes. Essentially, if I were to label them, say, t and t, they have the same genes. They may have different alleles, but it's the same gene. After DNA is replicated, you get another copy of each one. So, you get 2 big T's and 2 little t's. These are sister chromatids; the 2 big T's are sister chromatids, and the two little t's are now sister chromatids. It's called bivalent when you have the pair of homologous chromosomes. You have a tetrad, referring to all 4 chromatids that form after replication, and you have a dyad, which refers to 2 sister chromatids.

Meiosis starts with homologous chromosomes. These are drawn somewhat odd, but these two are the homologous chromosomes, and the bigger ones are also homologous chromosomes. They get replicated, and when they do, that ends up with these structures we're more commonly used to seeing, right, but the homologous chromosomes stay the same. So, here we have the homologous chromosomes, these two and these two, but now there's an extra copy of each side. The first thing that happens is you divide the cell, and what you notice is you only get one copy of each homologous chromosome. And so, because you only have one copy, you have 2 sister chromatids on each one. Right? One sister chromatid and two, but you still only have one big and one little. This means that this is haploid. We have essentially tt and zz, essentially, that's what we're saying. So because you have the same chromosome, it's just multiple copies of it, that makes it haploid.

Then, after meiosis, what happens is, you get one copy. So this has tz tz tz tz. These are also haploid and they just have one chromosome. Now, people get really confused, so I want to make sure that this is clear. When you start off, you start off with homologous chromosomes. So you start off with big t and little t, and we'll just label these homologous chromosomes. These aren't representing genes, these are representing chromosomes. After replication, you get 2 big T's and 2 little t's because each homologous chromosome is replicated. After meiosis 1, what you get is you get 2 cells, 1 with 2 big T's and one with 2 little t's. This is haploid because you have the same type of chromosome. Yes, you have 2 copies of it. You could have 40,000,000,000 copies of it. Right? If you have 40,000,000,000 big T's, it's still haploid because all you have is a big T. You don't have a big T and little t, which is what you started with. People get very confused here. I get a lot of questions, you know, why after meiosis 1, why are the cells haploid? It's because you only have one type of chromosome, you just have multiple copies of it.

Then after meiosis 2, what happens is, you get 4 cells. You get big T, big T, little t, little t. And so these are also haploid but now you just have one copy of it instead of 2. This is super important. Understand how the chromosomes change and why after meiosis 1, it's haploid and not diploid because you will probably be asked that question at some point. Professors like to use it to trip students up. So after meiosis 1, the cells are haploid. So, with that, let's now turn the page and get to the meiosis steps.

Meiosis Steps

Okay. So, now let's talk about the steps of meiosis. Meiosis is completed in 2 parts, and luckily for us, each one of these parts is followed by either a 1 or 2, so we know what's going on. The first step is prophase 1, which occurs during meiosis 1, and there are 5 steps in prophase 1. Here they are, though I usually butcher the names, but there are 5 steps. Different things happen in each step. In the first one, the chromatin begins condensing. In the second one, a complex called the synaptonemal complex forms because the chromosomes are aligning and the homologs are actually coming together. We are often shown images of chromosomes that look perfectly lined up together, but normally in a cell, they are not condensed. They don't look like our typical representation of chromosomes. They're just kind of long strings that float around. They're not paired or anything. So in prophase 1, and especially in the zygonema stage of prophase 1, this is when they start pairing together and forming what's called the synaptonemal complex. Then, you have a third step. This is where crossing over occurs. Crossing over is an important part that we talked about a lot that happens with genetic recombination, and so crossing over happens in this step of meiosis. This will likely be a question, so know that crossing over happens here. And so what happens is that chiasmata form, and this is the structure that forms between chromosomes that are exchanging information through crossing over, and you can actually see these connections between the chromosomes at this point. Then you have the chromosomes. They're just staying here, and then in the very last step, the chromosomes begin to pull apart, but they can't really go because the chiasmata that helped in crossing over are still there, so it's keeping them together. And that's really important because if they pulled apart now, you wouldn't have proper meiosis. But because those chiasmata are keeping the chromosomes together, it's good news.

So here we have the beginning of meiosis, there has been DNA replication. You have a green and a green, so these are replicated. Red and red, replication has happened, And then you have these exchanges of materials here, where crossing over has occurred, and this occurs in this stage. So then we go to metaphase 1, so the chromosomes, or the tetrads now, right, because you have 4, because they've been replicated, are lining up at the metaphase plate, and the homologous chromosomes lie on either side. What happened is that we started off with homologous chromosomes. Right? Then they got replicated. Now when they got replicated, they're still homologous chromosomes. Right? So, if we were to label these chromosomes, it would look like this. So they have the same genes on them, they may have different alleles, but they're homologous chromosomes. So what happens is that when they line up in the middle, this is the middle. So the homologous chromosomes line up on either side of the metaphase plate, while the sister chromatids remain on the same side of the metaphase plate.

So here's the metaphase plate. These chromosomes go to this cell and these go to this cell. Now when that happens, you get one cell that has T T and one cell that has TT, and so these are haploid. Remember this. But that hasn't happened yet. We're still lined up in the middle, so then we have anaphase 1, the dyads begin to separate into one pole. This is called when the homologous chromosomes separate properly. Nondisjunction happens when the separation doesn't occur properly. So here's the metaphase plate, and we have 2 chromosomes here, and each one separates like this, that would be disjunction. But if we had chromosomes here, or like this, and these 3 separated this way, and this one separated that way, that would be nondisjunction, and it does happen. It causes a lot of different genetic disorders, so it definitely can happen. Now, separation is beginning in anaphase. Telophase, they're pretty much at opposite poles now, and the nuclear membrane is beginning to reform around them.

And then you have cytokinesis, which separates the cytoplasm and actually forms 2 cells. So here is what is pretty much a mix between anaphase and telophase, and you can see that the chromosomes have now separated to the opposite end of the pole, and cytokinesis is happening here, and we'll pinch these into 2 cells. So now we're starting with the 2 cells here, and we're on to meiosis 2. So what happens is you have prophase 2, and that lets us know we are in meiosis 2. This is really a short phase, pretty much the cells look like this here. Nothing super big happens here. It's usually very short, sometimes almost nonexistent. Then the chromosomes go to metaphase, so now the sister chromatids, like this, lined up on the metaphase plate. Right? Now, because this has gone into its own cell, so for mitosis 2, what happens is the sister chromatids line up on the metaphase plate. So this is meiosis 1, and this is meiosis 2, and this is the difference between the two.

So you can see here that now the sister chromatids are lined up in the middle. So what will happen is here's the metaphase plate. These two sister chromatids will go to this cell and these two sister chromatids will go to this cell, and the same happens for the other cell. These two chromatids go to this cell, these two chromatids go to this cell, go this way. So I don't have a great image for this, it's a little confusing. You can, if you compare the actual sister chromatids here, you'll see that they're separated, the colors are exactly the same. But essentially, anaphase 2, the sister chromatids begin separating, telophase 2, the chromosomes go to each pole, and then you have cytokinesis again, and this creates 4 haploid cells. So I've drawn 2 here, but there would be 4 with the same number of chromosomes in them, and you can say that, like I said before, these 2 sister chromatids go to 1 cell, so that would be this cell here. Whereas these 2 sister chromatids on this side of the metaphase plate go into this cell, and the same for the other cell as well. So these are haploid.

So to review, you start off with 2 homologous chromosomes. I'll write it like this, and I'll draw it like this. Oh, I want to use a different color. Hold on. So here we go. Then replication occurs, so then you get 2 copies. Right? Then you get meiosis 1. Right? And that creates 4 cells. So the metaphase plate is here or here. The division plate. So one cell gets this, one cell gets that, or this, this, however you like it. Then you have meiosis 2. These are haploid. Remember? Because you have one type of chromosome, even though you have 2 copies, it'd still be one type. So it's haploid, then you have meiosis 2, where you get 4 cells. So that would be big T, big T, little t, little t, or if you like the chromosomes drawn, 2 black and 2 red, which is also haploid because you either have black or red, not both together. These are diploid. So hopefully, you understand the difference between diploid and haploid and why this is how meiosis works. So, these are the steps of meiosis, so hopefully that makes sense. With that, let's now move on.

In which step of prophase I does crossing-over occur?

• A. Leptonema
• B. Zygonema
• C. Pachynema
• D. Diplonema
• E. Diakinesis

Are the two daughter cells produced from meiosis I diploid or haploid?

• A. Diploid
• B. Haploid

Are the four daughter cells produced from meiosis II diploid or haploid?

• A. Diploid
• B. Haploid

Which of the following are sources of genetic variation during meiosis?

• A. Crossing over, independent assortment
• B. Crossing over, DNA replication
• C. DNA replication, independent assortment
• D. Disjunction, crossing over

During which stage of meiosis do sister chromatids separate?

• A. Prophase I
• B. Anaphase I
• C. Prophase II
• D. Anaphase II

Here’s what students ask on this topic:

What is the difference between diploid and haploid cells in meiosis?

Diploid cells contain two complete sets of chromosomes, one from each parent, denoted as 2n. In humans, this means 46 chromosomes. Haploid cells, on the other hand, contain only one set of chromosomes, denoted as n, which in humans is 23 chromosomes. During meiosis, a diploid cell undergoes two rounds of division to produce four haploid cells. The first division (meiosis I) reduces the chromosome number by half, creating haploid cells with duplicated chromosomes. The second division (meiosis II) separates the sister chromatids, resulting in four genetically unique haploid cells. Understanding this distinction is crucial for comprehending genetic variation and inheritance.

What are the stages of prophase 1 in meiosis?

Prophase 1 of meiosis is divided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. In leptotene, chromatin condenses into visible chromosomes. During zygotene, homologous chromosomes pair up and form the synaptonemal complex. Pachytene is marked by crossing over, where homologous chromosomes exchange genetic material. In diplotene, the synaptonemal complex dissolves, but homologous chromosomes remain attached at chiasmata. Finally, in diakinesis, chromosomes condense further, and the nuclear envelope breaks down, preparing the cell for metaphase 1. These stages are crucial for genetic recombination and proper chromosome segregation.

What is crossing over and why is it important in meiosis?

Crossing over is the process where homologous chromosomes exchange genetic material during prophase 1 of meiosis. This occurs at points called chiasmata. Crossing over is crucial because it increases genetic diversity by creating new combinations of alleles. This genetic recombination ensures that offspring have a mix of traits from both parents, contributing to variation within a population. This variation is essential for evolution and adaptation, as it provides a broader range of traits that may be beneficial in changing environments.

What is nondisjunction and what are its consequences?

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis. This can occur in anaphase 1 or anaphase 2. The result is gametes with an abnormal number of chromosomes, leading to conditions such as trisomy or monosomy. For example, trisomy 21, where an individual has three copies of chromosome 21, causes Down syndrome. Nondisjunction can lead to various genetic disorders, highlighting the importance of accurate chromosome segregation during meiosis.

How does meiosis contribute to genetic diversity?

Meiosis contributes to genetic diversity through two main mechanisms: crossing over and independent assortment. During prophase 1, crossing over exchanges genetic material between homologous chromosomes, creating new allele combinations. In metaphase 1, independent assortment randomly distributes homologous chromosome pairs to daughter cells, ensuring a unique combination of chromosomes in each gamete. These processes result in genetically unique gametes, which, when combined during fertilization, produce offspring with diverse genetic traits. This diversity is crucial for evolution and adaptation.