Lesson Video: Meiosis Science • Third Year of Preparatory School

Video Transcript

In this video, we will learn how gametes are produced through the process of meiosis. We’ll investigate how genetic variation can be introduced to these cells through the various stages of the special form of cell division, which we’ll explore step by step.

Do you have any siblings? If you’re not identical twins, why is it that you are different from each other? Meiosis is a type of cell division that produces special cells called gametes. In biological females, these gametes are called egg cells, and in biological males, these gametes are called sperm cells. Because of the special events that occur during my meiosis, nearly every egg cell and every sperm cell is genetically unique. In fact, it is estimated that because of this variability, every human couple could potentially produce around 64 trillion genetically unique children.

The differences that we observe between ourselves and someone else are result of genetic variation. And the genetic differences between all living organisms give us all unique combinations of characteristics. Genetic variation is super useful as it provides any species that use meiosis in reproduction with extra resilience to changes in their environment. Throughout this video, we will be exploring the stages of meiosis and how some of them can introduce genetic variation into the gametes it produces. But first let’s find out a bit more information about the genetic material that permits this genetic variation to be introduced.

You may recall that animal cells contain a nucleus that holds that cell’s genetic information. This genetic information is DNA, and in animals, like humans, it’s organized into chromosomes. When humans reproduce, offspring are created through a process called fertilization. This happens when a sperm cell and an egg cell fuse together and combine their genetic information in the form of chromosomes. Humans tend to have 46 chromosomes in most of their body cells, which are sometimes known as their somatic cells. When a human cell contains 46 chromosomes, it is said to be diploid, as these chromosomes come from two sets of 23 chromosomes.

Diploid cells are commonly represented as two 𝑛. One set of 23 chromosomes come from the biological mother via the egg cell. And the second set of 23 chromosomes come from the biological father via the sperm cell. As you might recall, these cells are collectively known as gametes. These gametes are described as haploid as they contain half the genetic information of a diploid body cell. They have only one single set of chromosomes, which in humans is 23 chromosomes and is often represented as 𝑛. You might have noticed that in the nucleus of this diploid cell, the mother’s chromosomes from the egg cell have been color coded in orange, while the father’s chromosomes from the sperm cell have been color coded in blue.

You might have also noticed that each of these 46 chromosomes has paired up with one from the other parent that looks very similar. Each one of these two chromosome copies in a diploid cell are referred to as homologous chromosomes, one from the egg cell and one from the sperm cell. And they’re nearly identical, which is why they’re called homologous, as the word homologous comes from the Greek word meaning “consistent.” Though this diagram does not exactly represent what the chromosomes in our cells will look like most of the time, it does give us an idea of how the chromosomes from our mother and from our father pair up.

It’s interesting to note that it’s not just humans that produce gametes. In fact, most animals mainly reproduce sexually by producing either haploid sperm cells or haploid egg cells that can then fuse with the other gamete from another organism in fertilization to form diploid body cells. Even plants can reproduce in this manner, though in plants the male gametes often referred to as pollen, while the female gametes are still called egg cells or sometimes an ovum.

What’s even more interesting is that different species often have different numbers of chromosomes in a typical diploid or haploid cell. For example, many birds, like pigeons, have 80 chromosomes in a diploid body cell and 40 chromosomes in a haploid gamete, which is many more than a human cell, while the Australian ant has only two chromosomes in a diploid cell, which means they only have one chromosome in a haploid gamete. Let’s quickly review the two main types of cell division that occur in organisms like humans, mitosis and meiosis.

You might recall that mitosis is a type of cell division that produces two genetically identical daughter cells from one single parent cell. The parent cells only go through one round of cell division. All of these cells have the same number of chromosomes and so they are all diploid cells. This is the type of cell division our body uses to repair damaged tissue or to grow new tissue. In contrast, our body uses a special type of cell division called meiosis to produce genetically different gametes in our reproductive organs.

During meiosis, the original parent cell will undergo two rounds of cell division to produce four daughter cells. As the original cell goes through two cellular divisions, the daughter cells have half the original number of chromosomes of the parent cell, and they are therefore haploid. The daughter cells that are produced are called gametes. When meiosis occurs in the testes, it produces sperm cells, and when it occurs in the ovaries, it produces egg cells. And remember in humans, this haploid number of chromosomes is 23. This is important as when these two gametes fuse together in fertilization, they’re going to make up the 46 chromosomes in total found in a diploid cell.

Let’s take a closer look at what happens during the various stages of meiosis to produce these gametes. Before meiosis can begin, a stage called interphase occurs. Before interphase occurs, each of the 23 chromosome pairs within a nucleus of a body cell exists as one single chromosome. Interphase duplicates or replicates each chromosome to make two identical DNA molecules called chromatids. This will happen to every single one of the 46 chromosomes in the cell. The two chromatids in each chromosome are joined together at a region called the centromere. And it’s important to remember that even though each single chromosome is now made up of two identical chromatids, it’s still one chromosome, but it can now be referred to as a replicated chromosome.

Now that the chromosomes have been duplicated, meiosis can begin. Let’s look at the first of the two cellular divisions in meiosis, which is called meiosis I. The first stage of meiosis I is prophase I. And here the duplicated chromosomes will condense and can now be seen with a microscope. As you can see in the diagram, at this point, the nuclear envelope that surrounds the nucleus will also break down. Another event that occurs in prophase I is specialized structures called spindle fibers start to form. We’ll learn more about the role of spindle fibers soon.

During prophase I, the homologous chromosomes from the mother and the father pair with each other to form a tetrad. The prefix tetra- means four, which describes the number of chromatids, the two paired and duplicated chromosomes have in a tetrad. The formation of a tetrad allows a process called crossing-over to occur. Crossing-over is when a homologous chromosome pair can exchange genetic information between duplicated chromosomes. This is the first point at which genetic variation is introduced as parts of the chromosomes from the mother and father are swapped, which will eventually end up in different daughter cells.

The next stage of meiosis I is metaphase I. In metaphase I, the homologous pairs line up along the middle of the cell and attach to the fully formed spindle fibers. The middle of the cell is sometimes called the equator. This is nice and easy to remember as the planet Earth, which is another spherical structure, also has an equator along its middle. The similarities between our Earth and a cell in division do not end there however; just like the Earth has a North Pole and a South Pole, the cell also has a pole at each opposite end.

Anaphase I comes next, in which the homologous pairs are separated by the spindle fibers shortening. This pulls the homologous chromosome pairs to opposite poles of the cell. During anaphase I, the chromosomes from the mother and father are randomly mixed before they’re separated. This way, the two daughter cells that will form at the end of meiosis I will have a mix of the parental chromosomes, which can also introduce genetic variation.

The final stage of meiosis I is telophase I. In telophase I, a separate nuclear envelope reforms around each of the separated chromosome sets, and the cells can now divide. By the end of meiosis I, there are two cells with half the number of duplicated chromosomes that originally went in to prophase I. In humans, this would mean that these cells would have 23 duplicated chromosomes each, even though in this diagram we’ve just shown it as two duplicated chromosomes each. In contrast, the diploid cell that starts in meiosis I had 46 duplicated chromosomes.

In meiosis II, we will see that each of the chromatids in these duplicated chromosomes will be separated. The stages of meiosis II are very similar to those in meiosis I, but with some distinct differences. During prophase II, for example, there is no pairing of homologous chromosomes, but the newly formed nuclear envelopes do still break down and the spindle fibers will still reform in each cell. In metaphase II, instead of homologous chromosome pairs, the duplicated chromosomes are aligned to each cell’s equator. In anaphase II, the duplicated chromosomes are separated. And the two chromatids in each replicated chromosome are pulled to opposite poles of each cell.

When the cells split at the end of telophase II, each daughter cell that’s produced will contain 23 singular chromatids. Each single chromatid can now be called a chromosome. And remember that even though in this diagram we’ve only shown two chromatids in each daughter cell, human gametes will actually contain 23 chromatids, which are also called chromosomes at this point.

Let’s summarize the process we’ve just covered by looking at it occurring in human cells specifically. First, the 46 chromosomes duplicate in interphase. This single 46-chromosome diploid cell undergoes meiosis I to produce two cells that contain 23 duplicated chromosomes each. These two cells then undergo meiosis II to produce four genetically different haploid gametes, each of which contains just 23 chromosomes. Both meiosis I and meiosis II include the stages, prophase, metaphase, anaphase, and telophase.

Let’s review how much we’ve learned about meiosis by applying our knowledge to a practice question.

The diagram shows two chromosomes undergoing crossing-over. What is the advantage of this? (A) It creates completely new genes. (B) It reduces the risk of mutation. (C) It increases genetic variation. Or (D) it increases the likelihood of fertilization.

The question is asking us to work out what the advantage of a process called crossing-over is, which is shown to us in a diagram. In order to answer this question, we’re going to need to understand a little bit more about how genetic information is organized in animal cells into structures called chromosomes. The nucleus of animal cells, like this one, contains the genetic information as DNA. In animals like humans, DNA is organized into chromosomes, some of which we can see in the nucleus here.

In most human body cells, there are 46 chromosomes in total or two sets of 23. One set of these 23 comes from the mother, and the other set comes from the father. The chromosomes from the mother can be said to be homologous to the chromosomes from the father because they’re nearly identical. As there are two sets of chromosomes in most body cells, they’re called diploid cells. In gametes, like sperm cells from the father and egg cells from the mother, there is only one set of chromosomes. So, gametes can be described as haploid cells. Gametes are made through a process called meiosis.

Meiosis is a type of cell division where one parent cell can make four genetically different haploid daughter cells. The genetic variation in the gametes produced through meiosis is partly as a result of a process called crossing-over. This occurs during one of the stages of meiosis. As shown in this diagram, it includes swapping of some genetic information from each of the two homologous chromosomes in a pair.

There are four chromatids in each homologous pair of chromosomes and two chromatids in each single replicated chromosome from each parent. As each of these four chromatids will end up in different daughter cells by the end of meiosis, the swapping of sections of DNA between homologous chromosomes can increase the genetic variation of the gametes produced. This does not create new genes as sections of DNA are simply swapped between homologous chromosomes.

Crossing-over does not affect the risk of mutation of DNA, nor does it either increase or decrease the likelihood of fertilization. It simply trades a section of DNA between homologous chromosomes that may have slight differences. This can happen to many of the 23 chromosome pairs during meiosis and can increase genetic variation in the gametes. Therefore, we’ve worked out that crossing-over increases genetic variation.

Let’s summarize what we learned about meiosis by reviewing the key points from this video. Meiosis produces four haploid gametes that are genetically different to each other. This special form of cell division consists of two rounds: meiosis I and meiosis II. In both meiosis I and meiosis II, the stages include prophase, metaphase, anaphase, and telophase. Meiosis produces unique cells partly due to crossing-over, which can introduce genetic variation.