Meiosis - Online Tutor, Practice Problems & Exam Prep

Meiosis is a crucial process for producing haploid germ cells through two main stages: meiosis I and meiosis II. It involves DNA replication, homologous recombination, and crossing over, which enhance genetic variation. During meiosis I, homologous chromosomes separate, resulting in two haploid cells, while meiosis II separates sister chromatids into four haploid cells. Key terms include bivalent, which refers to paired homologous chromosomes, and nondisjunction, leading to aneuploidy, such as Down syndrome. Understanding these processes is essential for grasping genetic diversity and reproduction.

Overview of Meiosis

Hi. In this video, we're going to be talking about meiosis. So, meiosis, I feel like a bunch of people get really confused about it, understanding what the chromosomes are doing at certain times, and so I'm hoping to be able to explain this. Meiosis is the process of creating germ cells. These are the sex cells, and these cells are used to reproduce. So 2 germ cells from 2 different organisms come together, called fertilization, and that produces some offspring. Now, meiosis 1 involves, there are 2 steps. Meiosis 1 involves replicating the DNA, and then there are 2 cells containing a single set of chromosomes, so this is going to be a set. And then meiosis 2 involves dividing those 2 cells into 4 cells with a single set of sister chromatids, and these are also haploid.

You start with a diploid cell, you replicate that DNA, and you have 2 steps of meiosis, both of which end with haploid cells. Here's an example of what this looks like. You start with a diploid cell because you have 2 copies of every chromosome. You have this blue copy and this red copy. Here are these 2 sets of chromosomes. You have 2 copies, so this is diploid. You undergo replication, and that gets you this, where you have 2 copies of every chromosome. You can see here the homologous chromosomes that you started with, and there's also a process called crossing over that happens here, where the genetic information is switched, and we'll talk about that.

So then you undergo meiosis 1, and what happens is you get the two copies of the single homologous chromosome, so these are haploid, and I'll explain why these are haploid in clearer terms in just a minute. And then you have meiosis 2, which just gets the sister chromatid, and these are also haploid. The biotic DNA duplication and the different divisions that happen result in a lot of different chromosomal forms, and this is where people get really confused. What's the difference between sister chromatids and homologous chromosomes? Is it diploid?

So let me explain. Sister chromatids are 2 copies, so one total pair of each chromosome. If you start off with this chromosome and you copy it with your replication, these 2 are sister chromatids. Now if you start off with 2 homologous chromosomes like this, right, and then you replicate each one, let me start black. These 2 are sister, and these 2 are non-sister, and they're both chromatids. Right? And these 2 are homologous chromosomes to begin with. So homologous chromosomes are the 2 copies of maternal and paternal of both sister chromatids. Here you have homologous chromosomes, sister chromatids, and non-sister chromatids.

Now you have certain terms that all these chromosome forms are called. One of them is bivalent, and this is, when you may also see this as tetrad. Let me explain which makes more sense, but essentially, they're the same. A tetrad is when the 4 sister chromatids stick together. A bivalent is when the 2 homologous chromosomes stick together. And so you can see that in meiosis because they're replicated, it ends up being the 4th sister chromatid. So you can use either term, and these separate during meiosis 1. The term haploid means having half the chromosomes of a diploid, which has two copies of every chromosome.

So let's go through this. You start with a cell, these are homologous chromosomes. These are homologous chromosomes. They have the same genes on them. They may have different alleles of those genes, but they have the same genes on them. Now this is what you start off with. Now replication occurs, and here it's diploid. Replication occurs, and now you have 2 copies. It's still diploid because you have the same number of chromosomes. You still have a black and a blue. You just now have multiple copies of it. These 2, both the black, are sister chromatids, both the blue are sister chromatids, but if you take one black or one blue, those are non-sister chromatids.

Meiosis 1 happens, and this creates 2 haploid cells. One with both copies of the black chromosome and one with both copies of the blue chromosome. This is why this is haploid because you still only have one chromosome. You started with both black and blue, but now you have just black. You could have 40 copies of this black chromosome, but it's haploid because you are down a blue chromosome. You don't have that chromosome anymore. You could have 100 copies of the black, but it's still haploid because you only have the black. The same for this, you only have the blue chromosome. You got 2 copies of it. You could have 40,000,000,000 copies of the blue chromosome. It would still be haploid because you only have the blue chromosome.

Then meiosis 2 happens, and these sister chromatids get divided. So one gets one of the sister chromatids. So one cell gets one black, and the other cell gets the other black, and the same for the blue chromosomes here. These are haploid as well because they either contain the black or the blue, not both. If the cells contain both the black and blue, that would be a diploid cell. This is why after meiosis 1, you have haploid, and after meiosis 2, you also have haploid. Even though there are different, technically, chromosome numbers, you look at the type of chromosome. Is it black or blue and how many of those you have. With that, let's now move on.

Genetic Variation

Okay. So we know that meiosis involves a lot of genetic variation. This is because DNA is reorganized and shuffled around to produce genetically distinct offspring. I'm going to talk about a few of the terms and the processes that happen during meiosis to achieve this genetic shuffling. One of them is called homologous recombination. Homologous recombination allows for chromosomes to exchange similar DNA sequences. We've talked about this before in DNA repair, but this is a little different in meiosis. In DNA repair, we talked about using identical DNA sequences to repair the DNA. But homologous recombination in meiosis actually uses non-identical, but similar sequences from non-sister chromatids in the bivalent. So, what do I mean by bivalent? Remember, a bivalent is going to be the four sister chromatids: 1, 2, 3, 4. So, the identical sequence would be from 2, but if I wanted a non-identical, but similar sequence, I could choose either from 4 or from 3. So DNA repair would always use 2, whereas meiosis is going to use the non-identical sequence from 3 or 4. That's homologous recombination, the shuffling between similar DNA sequences.

Then the second form is through crossing over. Crossing over is when maternal and paternal homologs physically swap chromosomal segments. So, what we get here is we have a maternal and a paternal chromosome. If there's a segment right here, it can come over here, and that one will replace, and they'll swap the segments. When these are connected, they're called chiasmata or chiasmata, and this holds bivalents together because in meiosis, we're dealing with a lot of genetic information. Chiasmata would form, like, here and here that would connect the bivalents together where crossing over has occurred. This is super important because it actually keeps the bivalents together and prevents them from separating early during meiosis, and we'll talk about that more in just a minute.

And then finally, we have reassortment, and this is just the random division of chromosomes in the cells. So remember, during metaphase, the chromosomes line up in the middle, and then they separate from each other during anaphase. But which side they actually line up on is kind of random. So, which chromosomes the cell gets is entirely random; that's called reassortment. Now, this doesn't necessarily have to be successful, and so nondisjunction actually describes when the homologues fail to separate. So, remember we have these chromosomes that line up in meiosis. And so, the order in which these line up and which ones get separated out to the two cells is completely random. If nondisjunction occurs and say, both of these chromosomes go here, this one goes here, and this one goes here, what we get is one pair of chromosomes here and three here. That's nondisjunction. And so this causes what we say is, aneuploidy, and that's where there are, typically eggs, which are the sex cells, with the wrong number of chromosomes. We see this in Down syndrome, also called trisomy, because there is an extra chromosome there. This is what it looks like during crossing over. You can see these maternal and paternal chromosomes. They eventually cross over; the points where they cross over are called the chiasmata. And then after the end, you can see that now they have a mixing of genetic information. So, that is a bunch of different ways that genetic shuffling happens during meiosis. So, with that, let's now move on.

Meiosis Steps

Okay. So now we're going to go through the meiosis steps. Meiosis I occurs first, and the steps are the same as mitosis. They're just given a 1 or a 2 because it happens twice: Meiosis I and Meiosis II. The first step is Prophase I, and there are some extra distinctions here that you need to know about. Unlike Prophase, there are actually five phases of Prophase I, and you're going to ask, do I actually need to know these? And you are going to need to know these five different phases. But the important one, probably one of the most important ones, is where crossing over occurs. During Prophase I, you have chromatin condensation, the formation of bivalents, crossing over, which is super important, phases where they're still attached together, and then you have nuclear envelope breakdown where the spindle forms. So, if I were to pick the top two of these five steps to know, they would be crossing over and spindle formation. Let me back up while I talk about this.

Here, we have the cell that we started with. We have DNA replication because we have two red and two green chromosomes here. You can see crossing over has begun to occur in Prophase I due to the mixing of green and red chromatids. Then, we get Metaphase I. The bivalents align at the spindle equator, in the middle of the cell. You can see the bivalents aligned with four sister chromatids, two homologous chromosomes now in the middle. Notice that this alignment differs from mitosis. During crossing over, the chiasmata, which allow for these four sister chromatids to align at the middle, form. Without the chiasmata, they would break apart causing genetic issues. In Anaphase I, the homologous chromosomes separate, resulting in a full set of chromosomes on each side, so each has two sister chromatids. Telophase I and cytokinesis then produce two cells containing a single randomly sorted set of chromosomes.

Some might find it confusing and may refer to these cells as haploid, but I believe it's easier to consider them diploid. The entry into Meiosis II follows, where Prophase II is much shorter and simpler, with no DNA replication occurring. In Metaphase II, the chromosomes line up at the equator again, but this time with only two sister chromatids. You can tell if it is Metaphase I or Metaphase II by counting the number of sister chromatids aligned in the middle. During Anaphase II, the sister chromatids separate, and Telophase II in conjunction with cytokinesis results in two more cells. Each cell now contains a separated set of sister chromatids and is referred to as haploid. We now have four cells from one, each with a set of sister chromatids.

So, that is meiosis in a nutshell. So with that, let's now turn the page.

Meiosis and Animal Life Stages

Okay. So now let's talk about mitosis and animal life stages. Like I said before, not all animals use meiosis to create sex cells, so let's go over some of the organisms that don't do that. Anemophiles are classified into three groups, depending on where in their life cycle they form meiosis.

The first group is called gametic or terminal meiosis. These are the organisms that we are most familiar with. They use meiosis to produce gametes or sex cells. This is going to be us, including humans, most living organisms. They say that meiosis, this type of meiosis, is complete after because you get two haploid cells coming together, an egg or sperm forming a diploid cell. This is the one we are most familiar with.

The second group is zygotic or initial meiosis. These are organisms that use meiosis after fertilization. Meiosis is used to perform to create spores that are haploid. The unique thing about this is it's kind of different. Diploid cells are actually the gametes, and haploid cells are the normal organism. So that is the unusual part of that. These produce haploid spores and the diploid spores of the gametes.

Finally, we have the third group, which is sporic or intermediate meiosis. In this case, meiosis actually has nothing to do with gamete formation or fertilization at all in any way. What you get is sporophytes, these are diploid cells that undergo mitosis. Then you have sporogenesis, and meiosis produces these cells called gametophytes, and gametophytes are produced gametes through mitosis. So you get sporogenesis, where meiosis forms gametophytes, and then gametophytes produce gametes through mitosis. Meiosis has nothing to do with the production of gametes, which occurs through mitosis.

Here's an example of sporic intermediate meiosis. You see here, you have the sporophyte, it undergoes meiosis, which produces these spores. These spores then undergo mitosis to produce the gametophyte. The gametophyte then produces gametes through more mitosis. The gametes come together, fuse, form a zygote, there's more mitosis. Then the sporophyte can produce spores via meiosis. So you can see that the gametes are all the way over here, and meiosis is all the way over here, so they really have nothing to do with each other. And that's kind of the example of sporadic intermediate meiosis. So there are probably some unique ways that meiosis is handled in different organisms. So with that, let's now move on.

In which of the following phases does crossing over occur?

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

In which of the following steps of prophase I does crossing over occur?

• A. Leptoene
• B. Zygotene
• C. Pachytene
• D. Diplotene
• E. Diakinesis

The cells produced from meiosis I and meiosis II are both haploid?

• A. True
• B. False

Fill in the blanks. In anaphase I the _____________ separate, while in anaphase II the ____________ separate.

• A. Sister chromatids, homologous chromosomes
• B. Homologous chromosomes, sister chromatids
• C. Homologous chromosomes, Homologous chromosomes
• D. sister chromatids, sister chromatids

Which of the following life cycles uses meiosis after fertilization?

• A. Gametic meiosis
• B. Zygotic meiosis
• C. Sporic meiosis

Here’s what students ask on this topic:

What is the difference between meiosis I and meiosis II?

Meiosis I and meiosis II are the two main stages of meiosis, each with distinct processes and outcomes. In meiosis I, homologous chromosomes pair up and undergo crossing over, exchanging genetic material. This stage results in two haploid cells, each with one chromosome from each homologous pair. Meiosis II resembles mitosis, where the sister chromatids of each chromosome are separated into four haploid cells. The key difference is that meiosis I reduces the chromosome number by half, while meiosis II separates the sister chromatids without further reducing the chromosome number.

How does crossing over contribute to genetic variation?

Crossing over occurs during prophase I of meiosis I, where homologous chromosomes exchange segments of genetic material. This process creates new combinations of alleles on each chromosome, contributing to genetic variation in offspring. By shuffling the genetic information between maternal and paternal chromosomes, crossing over ensures that each gamete contains a unique set of genetic instructions, increasing the diversity of traits in a population and enhancing the potential for adaptation and evolution.

What is nondisjunction and how does it lead to aneuploidy?

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis. This error can occur in either meiosis I or meiosis II. When nondisjunction happens, it results in gametes with an abnormal number of chromosomes, a condition known as aneuploidy. For example, if a gamete with an extra chromosome (trisomy) or a missing chromosome (monosomy) fertilizes a normal gamete, it can lead to genetic disorders such as Down syndrome, which is caused by an extra copy of chromosome 21.

What are the stages of meiosis and their key events?

Meiosis consists of two stages: meiosis I and meiosis II. Meiosis I includes prophase I (crossing over occurs), metaphase I (homologous chromosomes align at the equator), anaphase I (homologous chromosomes separate), and telophase I (two haploid cells form). Meiosis II includes prophase II (chromosomes condense), metaphase II (chromosomes align at the equator), anaphase II (sister chromatids separate), and telophase II (four haploid cells form). Each stage is crucial for reducing the chromosome number and ensuring genetic diversity through processes like crossing over and independent assortment.

What is the role of homologous recombination in meiosis?

Homologous recombination is a process during meiosis where homologous chromosomes exchange similar DNA sequences. This occurs during prophase I and involves the pairing of homologous chromosomes, followed by the exchange of genetic material between non-sister chromatids. This recombination increases genetic diversity by creating new combinations of alleles, which are then passed on to the offspring. It is a key mechanism for generating genetic variation and plays a crucial role in evolution and adaptation.