Lesson Explainer: Meiosis Science

In this explainer, we will learn how to describe the stages of meiosis, recall the products of meiosis, and explain how events that occur during meiosis will increase genetic variation.

Do you have any siblings? If you are not identical twins, why is it that you are different? Meiosis is a type of cell division that produces eggs and sperm cells. Because of special events that happen during meiosis, nearly every egg and 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!

You will recall that animal cells contain a nucleus to hold the cell’s genetic information. This genetic information is DNA and is organized into chromosomes. You can see this in Figure 1 below.

Key Term: Chromosome

Chromosomes are found in the nucleus of cells and contain DNA.

When humans reproduce, offspring are created in a process called fertilization. This happens when a sperm cell (male sex cell) and egg cell (female sex cell) fuse and combine their genetic material.

Humans have 46 chromosomes in their somatic (body) cells that are from two sets of 23 chromosomes. One set of 23 chromosomes comes from the mother via the egg cell and the second set of 23 chromosomes comes from the father via the sperm cell.

The 23 chromosomes from the mother and the 23 chromosomes from the father are nearly identical and are said to be homologous. The word homologous comes from the Greek homologos, which means “consistent.” You can see some of these homologous chromosomes in Figure 2 below.

Key Term: Homologous Chromosome

Homologous chromosomes are one of the two copies of a chromosome in a diploid cell. One comes from the mother and the other from the father.

Since we have two sets of chromosomes, somatic cells are also called diploid cells from the Greek word diploos, which means double.

Key Term: Diploid

A diploid cell is a cell that has two sets of chromosomes.

Reproductive or sex cells produced by the parents contain only one set, or half, of the chromosomes of somatic cells. Reproductive cells can also be called gametes. The gametes in humans are the egg (or ova) and sperm cells, while in plants the gametes are ova and pollen.

Key Term: Gamete

Gametes are an organism’s sex cells and are haploid as they contain half the genetic material of a normal body cell.

Since gametes have one set of chromosomes, they are called haploid cells from the Greek word haploos, which means single.

Key Term: Haploid

A haploid cell is a cell that only has a single set of chromosomes.

You will recall that mitosis is a type of cell division that produces two genetically identical daughter cells, from a single parent cell, all with the same number of chromosomes. 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 kind of division called meiosis to produce gametes that are produced in our reproductive organs: in the testes to produce sperm cells and in the ovaries to produce egg cells.

During meiosis, cells divide twice to produce four genetically different daughter cells with half the original number of chromosomes. In humans, this halved number of chromosomes is 23. This is important because when the egg cell (23 chromosomes) is fertilized by the sperm cell (23 chromosomes), together they make up the 46 chromosomes found in a diploid cell.

You can see the difference between mitosis and meiosis below in Figure 3.

Key Term: Meiosis

Meiosis is a special type of cell division where one cell can produce four genetically different cells with half the number of chromosomes. Meiosis is how gametes are made.

Example 1: Recalling the Definition of a Gamete

Which of the following is a defining feature of a gamete?

  1. A gamete contains double the genetic material of a normal body cell.
  2. A gamete is able to replicate and divide indefinitely.
  3. A gamete is formed by mitosis, not meiosis.
  4. A gamete contains half the genetic material of a normal body cell.

Answer

The nucleus of an animal cell contains the genetic information as DNA. DNA is organized as chromosomes in the nucleus.

In the somatic (body) cells of humans, there are 46 chromosomes, or two sets of 23 chromosomes. One set comes from the mother and one set comes from the father. Because there are two sets of chromosomes, these cells are called diploid cells.

Gametes are reproductive cells, which in humans are the egg or sperm cells, and they contain half the full amount of chromosomes in somatic cells. Since there is only one set of chromosomes, these cells are called haploid cells. When the egg and sperm combine, the resulting embryo has the complete (46) chromosomes.

Somatic cells can divide by mitosis, a kind of cell division where the parent cell can make two genetically identical diploid daughter cells. Mitosis allows somatic cells to replicate and divide almost indefinitely.

Gametes are formed by meiosis, a kind of cell division where the parent cell makes four genetically different haploid daughter cells.

You can see a comparison of mitosis and meiosis below.

Therefore, a gamete contains half the genetic material of a normal somatic (body) cell.

Before meiosis can begin, a stage called interphase occurs to duplicate the chromosomes. This is where each chromosome is copied to make an identical DNA molecule. This copy and the original molecule, are joined at a central part of the chromosome called the centromere. At this point, the two copies of the chromosomes are each called chromatids. Chromosomes that have this typical shape with 2 chromatids can be seen in Figure 4 below.

Now that the chromosomes are duplicated, meiosis can begin. The stages of meiosis have the same names as the stages of mitosis, except there are two divisions of meiosis. During the process of meiosis, the 23 chromosomes we received from our mother and the 23 chromosomes we received from our father are redistributed in the gamete cells that we produce.

Let’s look at the first division of meiosis, called meiosis I. The steps of meiosis I are shown in Figure 5 below.

The first stage of meiosis I is prophase I. Here, the duplicated chromosomes condense and can be seen with a microscope. The nuclear membrane also begins to break down.

During prophase I, the homologous chromosomes from the mother and father pair with each other to form a tetrad. The prefix tetra- means “four,” which is the number of chromatids the two paired and duplicated chromosomes have.

Prophase I is also when the chromosomes can “cross over” to introduce genetic variation, which we will cover in a moment.

In metaphase I, the homologous chromosome pairs are lined up along the middle, or the equator, of the cell. Then, in anaphase I, the pairs are separated by specialized structures called spindle fibers. During anaphase I, the chromosomes are randomly mixed before they are separated, and this can introduce genetic variation.

In telophase I, the nuclear membrane reforms around the separated chromosomes and the cells divide.

By the end of meiosis I, there are two cells with half the number of duplicated chromosomes. The two cells each have 23 duplicated chromosomes instead of the 46 that they started with. In meiosis II, we will see that the two chromatids in these duplicated chromosomes are separated.

Example 2: Recalling the Stages of Meiosis

The stages of the first meiotic division are listed below:

  • Prophase I
  • Metaphase I
  • Anaphase I
  • Telophase I

In which stage do the spindle fibers move the chromosomes to opposite ends of the cell?

Answer

The nucleus contains the genetic information of the cell on chromosomes. In humans, our body cells have 46 chromosomes, or 2 pairs of 23 chromosomes. One set comes from the mother and one set comes from the father. The chromosomes from the mother are homologous to the chromosomes from the father because they are nearly identical.

In our gametes, like egg or sperm cells, there is only one set of chromosomes, and they can be called haploid.

Meiosis is a kind of cell division where the parent cell (with 46 chromosomes) makes 4 genetically unique haploid cells.

Before meiosis can begin, a process called interphase occurs. This is where the DNA on the chromosomes is duplicated to form two chromatids as shown below.

These 46 duplicated chromosomes now need to be separated so that there are 23 single chromosomes in 4 cells.

There are two rounds of division during meiosis. In each division, there are 4 steps. You can see an overview of meiosis I below.

Prophase I is the first step and is where the chromosomes become visible (before this, they are not visible under a microscope) and when the nuclear membrane breaks down.

Metaphase I is the second step and is where the chromosomes line up along the equator, or middle, of the cell.

Anaphase I is the third step where specialized spindle fibers attach to the chromosomes and separate them to opposite ends of the cell.

Telophase I is the last step and is where a nucleus forms around the newly separated chromosomes and the cell splits to form two cells.

These two cells can then go on to meiosis II where the steps of prophase, metaphase, anaphase, and telophase are repeated. The result is 4 haploid cells with 23 chromosomes.

Therefore, anaphase I is the step where the chromosomes are moved to opposite ends of the cell.

During the second division, the two cells now have 23 duplicated chromosomes and these are split into 23 single chromosomes and divide again into a total of 4 cells to complete meiosis.

Let’s now look at the second division of meiosis, called meiosis II. The steps of meiosis II are shown in Figure 6 below.

In meiosis II, the steps are similar to meiosis I, but with some differences. During prophase II, there is no pairing of homologous chromosomes. Instead, the duplicated chromosomes are aligned to the cell’s equator in metaphase II and each chromatid is separated in anaphase II.

When the cells split during the end of telophase II, each daughter cell now contains one chromatid, which is now called a chromosome.

To summarize, a single 46-chromosome diploid cell undergoes meiosis to produce 4 genetically different daughter haploid cells, each with 23 chromosomes.

This is all summarized in Figure 7 below.

Example 3: Recalling the Stages of Meiosis

The stages of the second meiotic division are as follows:

  • Prophase II
  • Metaphase II
  • Anaphase II
  • Telophase II

What stage of meiosis II is shown in the diagram?

Answer

The nucleus contains DNA on chromosomes. In humans, our body cells have 46 chromosomes, or 2 pairs of 23 chromosomes. They are diploid: one set comes from the mother and one set comes from the father.

In our gametes that we produce in our reproductive organs, like egg or sperm cells, there is only one set of chromosomes, and they can be called haploid.

Meiosis is a kind of cell division where a body cell makes 4 genetically unique haploid cells.

Before meiosis can begin, a process called interphase occurs. This is where the DNA on the chromosomes is duplicated to form two chromatids as shown below.

These 46 duplicated chromosomes now need to be separated so that there are 23 single chromosomes in 4 cells.

There are two rounds of division during meiosis. During the first division, the 46 duplicated chromosomes are split between two cells, so each cell gets 23 duplicated chromosomes. This occurs over several steps called prophase, metaphase, anaphase, and telophase. You can see meiosis I below.

During the second division, the two cells now have 23 duplicated chromosomes and these are split into 23 single chromosomes and divide again into a total of 4 cells to complete meiosis. This is shown below.

Therefore, the image in the question corresponds to metaphase II.

One of the hallmarks of meiosis is that one cell can produce four genetically different cells. Let’s look at this in more detail.

One way to introduce genetic variation in humans is during anaphase I, when the homologous chromosome pairs are separated. During this stage, chromosomes from the mother and the father can be mixed together randomly. This way, the two daughter cells that form at the end of meiosis I can have a mix of chromosomes.

The other way to introduce genetic variability is through a phenomenon called crossing-over. This is where a homologous chromosome pair can exchange genetic information between duplicated chromosomes. This happens during prophase I. You can see crossing-over in Figure 8.

Key Term: Crossing-Over

Crossing-over is the exchange of genetic information during meiosis to increase genetic variability.

Example 4: Understanding the Role of Crossing-Over in Meiosis

The diagram shows two chromosomes undergoing crossing-over. What is the advantage of this?

  1. It creates completely new genes.
  2. It reduces the risk of mutation.
  3. It increases genetic variation.
  4. It increases the likelihood of fertilization.

Answer

The nucleus of an animal cell contains the genetic information as DNA. DNA is organized as chromosomes in the nucleus.

In the somatic (body) cells of humans, there are 46 chromosomes, or two sets of 23 chromosomes. One set comes from the mother and one set comes from the father. The chromosomes from the mother can be said to be homologous to the chromosomes from the father because they are nearly identical.

Since there are two sets of chromosomes in somatic cells, these cells are called diploid cells.

In gametes, like egg or sperm cells, there is only one set of chromosomes, so these cells are called haploid cells.

Gametes can be made by meiosis. Meiosis is a kind of cell division where the parent cell can make four genetically different haploid cells.

This genetic variation can come from crossing-over, which occurs during one of the steps of meiosis.

As shown in the image in the question, this involves swapping of segments of homologous chromosomes. This does not create new genes or mutate the DNA in any way. It simply trades a segment of DNA between homologous chromosomes, which may have slight differences. This can happen to many chromosome pairs during meiosis and can increase genetic variation in the gametes.

Therefore, crossing-over increases genetic variation.

Let’s recap some of the key points we have covered in this explainer.

Key Points

  • Gametes can be made by meiosis.
  • Meiosis is a special kind of cell division where one cell can produce four genetically different cells with half the number of chromosomes.
  • Meiosis has two rounds of division called meiosis I and meiosis II.
  • The basic steps in meiosis I and meiosis II are prophase, metaphase, anaphase, and telophase.
  • The steps of meiosis I are repeated in meiosis II.
  • Crossing-over can be used during meiosis to increase genetic variability.

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