Click here for a Meiosis Student Learning Guide

1. Introduction

In the previous tutorial, we talked about the basics of meiosis. In this tutorial, we’ll look at the life cycle of sexually reproducing organisms, and how the creation of haploid cells involves two cell divisions.

2. Haploid cells, Diploid cells, and the Life Cycle of Sexually Reproducing Organisms

Now let’s use this meiosis-related vocabulary to understand the haploid and diploid moments that occur in the human life cycle.”

We’ll start at the bottom left, where you see an adult female (1a) and an adult male (1b). All of their cells, except for their gametes, are diploid.

“3” represents the process of meiosis. During meiosis, germ cells in the ovaries (2a) or testes (2b) divide in a way that reduces the chromosome number from diploid (2 sets) to haploid (one set). The results are male and female gametes: sperm (4b) or egg cells (4a).

“5” represents fertilization. The sperm fuses with the egg. This creates a fertilized egg, or zygote (6). Because the zygote has received one set of chromosomes from the egg, and one set of chromosomes from the sperm, it has two sets of chromosomes, making it diploid.

“7” represents the processes of mitotic cell division, growth, and development. The result is a reproductively mature male or female, able to produce gametes and pass his or her genes on to the next generation.

Got it? Try this quiz. Note that, by design, I’ve changed the numbering scheme in the diagram. That difficulty will improve memory.

3. Quiz: The Life Cycle of Sexually Reproducing Organisms

[qwiz qrecord_id=”sciencemusicvideosMeister1961-meiosis: sexual life cycle”]

[h]The life cycle of sexually reproducing organisms

[i]

[q labels = “top”]

 

[l]adult female

[fx] No, that’s not correct. Please try again.

[f*] Excellent!

[l]adult male

[fx] No. Please try again.

[f*] Good!

[l]fertilization

[fx] No. Please try again.

[f*] Excellent!

[l]egg

[fx] No. Please try again.

[f*] Correct!

[l]growth and development

[fx] No. Please try again.

[f*] Excellent!

[l]meiosis

[fx] No. Please try again.

[f*] Great!

[l]ovary

[fx] No. Please try again.

[f*] Correct!

[l]sperm

[fx] No. Please try again.

[f*] Great!

[l]testes

[fx] No, that’s not correct. Please try again.

[f*] Correct!

[l]zygote

[fx] No, that’s not correct. Please try again.

[f*] Good!

[!!!!!!] question 1 +++++++++[/!!!!!!]

[q] In the diagram below, which number represents fertilization?

[c] I

[c*] II

[c] III

[f] No. Roman numeral I shows cells formed from ovaries and testes. Fertilization is when two gametes fuse together to become a zygote. Where in the diagram do you see two cells becoming one?

[f] Exactly. II shows a sperm and an egg cell fusing together to become a zygote.

[f] No. III shows a zygote growing and developing to become an adult organism. Fertilization is when two gametes fuse together to become a zygote. Where in the diagram do you see two cells becoming one?

[!!!!!!] question 2 +++++++++[/!!!!!!]

[q] In the diagram below, which number represents meiosis?

[c*] I

[c] II

[c] III

[f] Yes. Roman numeral I shows a sperm and an egg cell forming from testes and ovaries. This is exactly what happens in meiosis.

[f] No. II is showing an egg and a sperm cell fusing together to form a zygote, a process known as fertilization. Next time, choose a process that shows formation of egg cells or sperm cells (which is what happens during meiosis).

[f] No. III shows a zygote growing and developing to become an adult organism. Next time, choose a process that shows formation of egg cells or sperm cells (which is what happens during meiosis).

[!!!!!!] question 3 +++++++++[/!!!!!!]

[q] In the diagram below, which number represents mitosis, growth, and development?

[c] I

[c] II

[c*] III

[f] No. Roman numeral I shows a sperm and an egg cell forming from testes and ovaries. This is what happens in meiosis. Next time, look for a process that shows a change from a single cell to a multicellular adult. This change would come about through repeated rounds of mitosis, cell growth, and cell differentiation as the organism develops.

[f] No. II is showing an egg and a sperm cell fusing together to form a zygote, a process known as fertilization. Next time, look for a process that shows a change from a single cell to a multicellular adult. This change would come about through repeated rounds of mitosis, cell growth, and cell differentiation as the organism develops.

[f] Perfect. III shows a zygote growing and developing to become an adult organism. This change would come about through repeated rounds of mitosis, cell growth, and cell differentiation as the organism develops.

[!!!!!!] question 4 +++++++++[/!!!!!!]

[q] In the diagram below, which structure or cell would be haploid?

[c] 1

[c*] 4

[c] 5

[f] No.  Number 1 is an ovary. Ovarian tissue, like all somatic tissue, is diploid. However, some cells in an ovary go through meiosis to become a haploid egg. Next time, make a different choice.

[f] Correct. Number 4 is an egg. Eggs are haploid cells that result from meiosis.

[f] No. Number 5 is a zygote. A zygote is a fertilized egg, with two sets of chromosomes, making it diploid. Next time, make a different choice.

[!!!!!!] question 5 +++++++++[/!!!!!!]

[q] In the diagram below, is the cell at number 5 haploid or diploid?

[c*] diploid

[c] haploid

[f] Yes. The cell at number 5 is a zygote. A zygote is a fertilized egg, with two sets of chromosomes, making it diploid.

[f] No. The cell at number 5 is a zygote. A zygote is a fertilized egg, with two sets of chromosomes, making it diploid.

[!!!!!!] question 6 +++++++++[/!!!!!!]

[q] In the diagram below, which structure is a zygote?

[c] 1

[c] 2

[c] 3

[c] 4

[c*] 5

[f] No. Number 1 is the ovary. A zygote is a fertilized egg, and it comes about when a sperm cell fertilizes an egg. Where in the diagram do you see the cell that forms when two cells fuse together to become one?

[f] No. Number 2 is a testis (singular for testes). A zygote is a fertilized egg, and it comes about when a sperm cell fertilizes an egg. Where in the diagram do you see the cell that forms when two cells fuse together to become one?

[f] No. Number 3 is a sperm cell. A zygote is a fertilized egg, and it comes about when a sperm cell fertilizes an egg. Where in the diagram do you see the cell that forms when two cells fuse together to become one?

[f] No. Number 4 is an egg cell. A zygote is a fertilized egg, and it comes about when a sperm cell fertilizes an egg. Where in the diagram do you see the cell that forms when two cells fuse together to become one?

[f] Yes. Number 5 is the zygote, which forms when a sperm cell fertilizes an egg.

[!!!!!!] question 7 +++++++++[/!!!!!!]

[q] In the diagram below, would the adult man and woman shown in the bottom of the diagram be diploid or haploid?

[c*] diploid

[c] haploid

[f] Yes. Adults of any animal species have bodies that are composed of diploid tissue, with two sets of chromosomes. This tissue developed from a zygote, a diploid cell that came about from the fertilization of an egg from a sperm.

[f] No. Adults of any animal species have bodies that are composed of diploid tissue, with two sets of chromosomes. This tissue developed from a zygote, a diploid cell that came about from the fertilization of an egg from a sperm. The only cells in an adult male or female that are haploid are their gametes (sperm or egg cells, depending on the sex).

[!!!!!!] question 8 +++++++++[/!!!!!!]

[q] In the diagram below, which structure is an ovary?

[c*] 1

[c] 2

[c] 3

[c] 4

[c] 5

[f] Yes. Number 1 is the ovary.

[f] No. Number 2 is a testis (singular for testes). Look for tissue in an adult that would give rise to an egg, which you can see at number 4.

[f] No. Number 3 is a sperm cell.  Look for tissue in an adult that would give rise to an egg, which you can see at number 4.

[f] No. Number 4 is an egg cell.  Look for tissue in an adult that would give rise to this egg.

[f] No. Number 5 is the zygote, which forms when a sperm cell fertilizes an egg. To identify the ovary, look for tissue in an adult that would give rise to an unfertilized egg, which you can see at number 4.

[!!!!!!] question 9 +++++++++[/!!!!!!]

[q] In the diagram below, which numbers refer to gametes?

[c] 1 and 2

[c*] 3 and 4

[c] 5

[f] No. Numbers 1 and 2 are showing the ovary (1) and the testes (at 2). Gametes are sex cells, which are produced by ovaries and testes. Which numbers refer to sex cells?

[f] Yes. Number 3 is a sperm cell, and number 4 is an egg. Egg cells and sperm cells are sex cells, or gametes.

[f] No. Number 5 is a zygote, which forms when a sperm cell fertilizes an egg. Gametes are sex cells, which fuse together to form a zygote. Which numbers refer to cells that have fused together to form a zygote?

[!!!!!!] question 10 +++++++++[/!!!!!!]

[q] In the diagram below, which numbers refer to organs where meiosis would occur?

[c*] 1 and 2

[c] 3 and 4

[c] 5

[f] Yes. Numbers 1 and 2 are showing the ovary (1) and the testes (2). Within these organs, meiosis occurs, producing sex cells.

[f] No. Number 3 is a sperm cell, and number 4 is an egg. Egg cells and sperm cells are sex cells, or gametes. They are produced by meiosis, but meiosis doesn’t occur within these cells. Move backwards in the diagram to find the tissues that produced these gametes, and you’ll have found the location of meiosis.

[f] No. Number 5 is a zygote, which forms when a sperm cell fertilizes an egg. To find where meiosis occurs, find the organs that produced the sperm and egg that fused to form the zygote.

[!!!!!!] question 11 +++++++++[/!!!!!!]

[q] Based on the diagram below (and any other knowledge you may have), which of the following statements is true?

[c] Because the egg is so much larger then the sperm, more genetic information is contributed by the mother than by the father.

[c] The sperm provides most of the genetic information. The egg provides the cytoplasm that nourishes the zygote.

[c] The egg, with its larger nucleus, provides most of the genetic information. The sperm merely serves to activate the zygote, and to induce it to develop into an embryo.

[c*] Both parents, through their sperm and egg cells, contribute 1/2 of the genetic information received by the offspring.

[f] No. Despite their different sizes, both the egg and sperm have equal amounts of genetic information. Next time, chose a different answer.

[f] No. While the egg does contribute much more cytoplasm, both the egg and sperm have equal amounts of genetic information. Next time, chose a different answer.

[f] No. Despite the fact that this diagram shows a much larger  nucleus in the egg, the egg, in fact,  has no more genetic information than the sperm. Next time, chose a different answer.

[f] Correct. Half of the genetic information is contributed by the mother, and half is contributed by the father.

[!!!!!!] question 12 +++++++++[/!!!!!!]

[q] Based on the diagram below (and any other knowledge you may have), which of the following statements about fertilization is true?

[c] During fertilization, two diploid zygotes fuse to form haploid gametes.

[c] During fertilization, two diploid gametes fuse to form a haploid zygote.

[c*] Fertilization creates a diploid zygote from fusion of two haploid gametes.

[f] No. Look at the direction of the arrows in the diagram. The sperm and egg cell move forward in time to become a zygote. In addition, remember that each gamete has only one chromosome set (making it haploid), while the zygote has two.

[f] No. While you have the overall sequence correct (gametes fuse to form a zygote), you need to clarify the chromosomal condition of the gametes and the zygote. Remembering that the ‘di’ in ‘diploid’ means ‘two’ might help you the next time you see this question.

[f] Yes. That’s exactly what happens in fertilization, when two haploid gametes fuse to form a diploid zygote.

[x]
[restart]

[/qwiz]

4. Meiosis Involves Two Cell Divisions

Chromosome duplication occurs during mitosis and meiosis

As described above, meiosis takes diploid germ cells in the ovaries or testes and transforms them into haploid gametes. But there’s a fact about meiosis that can be confusing. Meiosis evolved from (or with) mitosis. As a result, meiosis (like mitosis) begins with an interphase during which chromosomes are replicated, becoming two connected sister chromatids. Because of this, meiosis involves two cell divisions. After replication (“1” below), there’s meiosis 1 (which is shown as “2” in the diagram below), followed by meiosis 2 (“3” below)

Focusing solely on homologous pairs and sister chromatids, here’s what happens during each division:

  • Meiosis 1 (“2” below): homologous pairs are separated. Diploid cells become haploid.
  • Meiosis 2 (“3” below): sister chromatids are pulled apart, creating four haploid gametes.

Got it? Take this quiz to help consolidate your understanding of the difference between meiosis 1 and 2.

5. Quiz: Meiosis 1 v. Meiosis 2

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Meiosis: meiosis 1 v meiosis 2″]

[h]Quiz: Meiosis 1 v. Meiosis 2

[i]

[!!!!!!] question 1 +++++++++[/!!!!!!]

[q] In the diagram below, which number shows separation of homologous pairs.

[c] 1

[c*] 2

[c] 3

[f] No. Notice how the homologous pairs (the chromosomes that are shown as being matched by size but not color), are still together inside the same nucleus after step one. Find a step where these homologous chromosomes have been separated into different nuclei.

[f] Exactly. Step 2 shows homologous pairs being separated into different daughter cells.

[f] No. In step three, the homologous pairs have already been separated. Three shows sister chromatids being pulled apart. Next time, choose an earlier step in the process.

[!!!!!!] question 2 +++++++++[/!!!!!!]

[q] In the diagram below, which number shows separation of sister chromatids?

[c] 1

[c] 2

[c*] 3

[f] No. In this first step, you can see single chromosomes becoming doubled chromosomes, which are composed of sister chromatids. Next time, try to identify a step where these sister chromatids are pulled apart.

[f] No. Step 2 shows homologous pairs being separated into different daughter cells.

[f] Correct. In step three, sister chromatids are being pulled apart.

[!!!!!!] question 3 +++++++++[/!!!!!!]

[q] In the diagram below, which number shows duplication of chromosomes?

[c*] 1

[c] 2

[c] 3

[f] Yes. In this first step, you can see single chromosomes becoming doubled chromosomes, each of which is composed of sister chromatids.

[f] No. Step 2 shows homologous pairs being separated into different daughter cells. Find a step where single chromosomes, which look like thick lines, become doubled chromosomes (which look somewhat like an ‘X’).

[f] No. In step three, sister chromatids are being pulled apart. What you’re looking for is actually the opposite. Find a step where single chromosomes, which look like thick lines, become doubled chromosomes (which look somewhat like an ‘X’).

[!!!!!!] question 4 +++++++++[/!!!!!!]

[q] In the diagram below, which number shows where the cell changes from diploid to haploid?

[c] 1

[c*] 2

[c] 3

[f] No. In this first step, you can see single chromosomes becoming doubled chromosomes, each of which is composed of sister chromatids. You’re looking for the stage where the matched homologous pairs get sent to different daughter cells.

[f] Perfect. Step 2 shows homologous pairs being separated into different daughter cells. As that happens, the cell is transformed from having two chromosome sets to having one. Another way of saying this is that the cell is changing from being diploid to being haploid.

[f] No. In step three, sister chromatids are being pulled apart. By this point, the cells are already haploid (but their chromosomes are still doubled). Next time, choose an earlier phase in the process.

[!!!!!!] question 5 +++++++++[/!!!!!!]

[q] In the diagram below, which number shows a process that results in haploid gametes with single chromosomes.

[c] 1

[c] 2

[c*] 3

[f] No. In this first step, you can see single chromosomes becoming doubled chromosomes, each of which is composed of two sister chromatids. You’re looking for a later stage in the process.

[f] No. Step 2 shows homologous pairs being separated into different daughter cells. As that happens, the cell is transformed from having two chromosome sets to having one. Another way of saying this is that the cell is changing from being diploid to being haploid. However, the chromosomes are still doubled, Look for a phase where the daughter cells are single rods, as opposed to ‘X’s.

[f] Nice. In step three, sister chromatids are being pulled apart, so the doubled chromosomes are becoming single chromosomes. By this point, the cells are already haploid. The result, in other words, is four haploid gametes with single chromosomes.

[!!!!!!] question 6 +++++++++[/!!!!!!]

[q] Which process separates homologous pairs?

[c*] meiosis 1

[c] meiosis 2

[f] That’s right. Meiosis 1 separates homologous pairs.

[f] No. Meiosis 2 separates sister chromatids.

[!!!!!!] question 7 +++++++++[/!!!!!!]

[q] Which process reduces chromosome number from diploid to haploid?

[c*] meiosis 1

[c] meiosis 2

[f] That’s right. Meiosis 1 separates homologous pairs. As it does, chromosome number is reduced from two sets (one from the mother, one from the father), to one set.

[f] No. Meiosis 2 separates sister chromatids. The cells are already haploid.

[!!!!!!] question 8 +++++++++[/!!!!!!]

[q] Which process results in two haploid cells with doubled chromosomes?

[c*] meiosis 1

[c] meiosis 2

[f] That’s right. Meiosis 1 separates homologous pairs. As it does, chromosome number goes from diploid to haploid. Each chromosome, however, is still doubled.

[f] No. Meiosis 2 separates sister chromatids. The result is four haploid daughter cells, each of which has single chromosomes.

[!!!!!!] question 9 +++++++++[/!!!!!!]

[q] Which process separates sister chromatids?

[c] meiosis 1

[c*] meiosis 2

[f] No. Meiosis 1 separates homologous pairs.

[f] Good job. Meiosis 2 separates sister chromatids.

[!!!!!!] question 10 +++++++++[/!!!!!!]

[q] Which process results in four haploid cells with single chromosomes?

[c] meiosis 1

[c*] meiosis 2

[f] No. Meiosis 1 separates homologous pairs. The result is two haploid cells with doubled chromosomes.

[f] That’s correct. Meiosis 2 separates sister chromatids. The result is four haploid daughter cells, each of which has single chromosomes.

[!!!!!!] question 11 +++++++++[/!!!!!!]

[q] Which process reduces chromosomes from two sets to one set?

[c*] meiosis 1

[c] meiosis 2

[f] Exactly. Meiosis 1 separates homologous pairs. The result is two daughter cells, each with one set of chromosomes. In each set, the chromosomes are still doubled.

[f] No. Meiosis 2 separates sister chromatids. Meiosis 1 has already cut the number of chromosomes down to one set.

[!!!!!!] question 12 +++++++++[/!!!!!!]

[q] Which process changes doubled chromosomes into single chromosomes.

[c] meiosis 1

[c*] meiosis 2

[f] No. Meiosis 1 separates homologous pairs. Each resulting chromosome is still doubled.

[f] Yes. Meiosis 2 separates sister chromatids. This changes doubled chromosomes (consisting of two sister chromatids)  into single chromosomes.

[x]
[restart]

[/qwiz]

6. Comparing Mitosis and Meiosis

At this point, it makes sense to review the differences between mitosis and meiosis. To do so, try the quiz below.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Meiosis: meiosis v. mitosis”]

[h]Meiosis Quiz 2: Meiosis v. Mitosis

[i]
[!!!!!!] question 1+++++++++[/!!!!!!]
[q] The kind of cell division associated with sexual reproduction:
[c*] meiosis
[c] mitosis
[f] Correct. Sexual reproduction requires production of haploid gametes, and these are produced through meiosis.
[f] No. Mitosis is associated with asexual reproduction (in unicellular eukaryotes). In multicellular organisms like humans, mitosis is how we grow and develop.
[!!!!!!] question 2+++++++++[/!!!!!!]
[q] Somatic cells reproduce themselves through
[c] meiosis
[c*] mitosis
[f] No. Meiosis is how sex cells (egg and sperm cells) are produced.
[f] Yes. Mitosis is how the somatic cells that make up our body tissues reproduce themselves. You grew from a single celled zygote into the trillions of cells that make you up today through mitosis.
[!!!!!!] question 3+++++++++[/!!!!!!]
[q] Gametes are created through
[c*] meiosis
[c] mitosis
[f] Yes. Meiosis is how gametes (egg and sperm cells) are produced.
[f] No. Mitosis is how the somatic cells that make up our body tissues are produced.
[!!!!!!] question 4+++++++++[/!!!!!!]
[q] Genetic variation arises through
[c*] meiosis
[c] mitosis
[f] Yes. Meiosis creates variation. Meiosis involves two processes, crossing over and independent assortment, that mix up genes in a way that creates brand new combinations of genes.
[f] No. Mitosis creates an exact duplicate of the parent cell. The daughter cells which result from mitosis are genetic clones of the parent cell. No new genetic variation is produced.
[!!!!!!] question 5+++++++++[/!!!!!!]
[q] Which type of cell division cuts chromosome number in half?
[c*] meiosis
[c] mitosis
[f] Yes. Meiosis reduces chromosome number from two sets to one set (from diploid to haploid).
[f] No. Mitosis keeps the chromosome number exactly the same.
[!!!!!!] question 6+++++++++[/!!!!!!]
[q] Which type of cell division creates two daughter cells that are genetic clones of the parent cell?
[c] meiosis
[c*] mitosis
[f] No. Meiosis creates four daughter cells that are genetically distinct from the parent cell.
[f] Yes. Mitosis results in two daughter cells that are exact genetic clones of the parent cell.
[!!!!!!] question 7+++++++++[/!!!!!!]
[q] Animals grow from single celled zygotes into multicellular organisms through
[c] meiosis
[c*] mitosis
[f] No. Meiosis is how multicellular organisms make gametes for reproduction.
[f] Yes. Mitosis is how animals grow from a single-celled zygotes into multicellular adults.
[!!!!!!] question 8+++++++++[/!!!!!!]
[q] Animals create sperm and egg cells created through
[c*] meiosis
[c] mitosis
[f] Yes. Meiosis is how gametes (egg and sperm cells) are produced.
[f] No. Mitosis is how the somatic cells that make up our body tissues are created.
[!!!!!!] question 9+++++++++[/!!!!!!]
[q] Which type of cell division involves only one round of cell division?
[c] meiosis
[c*] mitosis
[f] No. During meiosis, cells divide twice as they reduce chromosome number from diploid to haploid, and then divide doubled chromosomes into single chromosomes.
[f] Yes. Mitosis involves only one round of cell division as doubled chromosomes are split into single chromosomes, and the cell splits in half.
[!!!!!!] question 10+++++++++[/!!!!!!]
[q] Which type of cell division involves two rounds of cell division?
[c*] meiosis
[c] mitosis
[f] Yes. During meiosis, cells divide twice as they reduce chromosome number from diploid to haploid, and then divide doubled chromosomes into single chromosomes.
[f] No. Mitosis involves only one round of cell division as doubled chromosomes are split into single chromosomes, and the cell splits in half.
[!!!!!!] question 11+++++++++[/!!!!!!]
[q] Which of the numbered diagrams below represents meiosis?

[c] 1
[c*] 2
[f] No. In diagram 1, you see only one round of cell division. During meiosis, cells divide twice as they reduce chromosome number from diploid to haploid, and then divide doubled chromosomes into single chromsomes.
[f] Excellent. Diagram 2 represents meiosis, which you can see is happening because of the two rounds of cell division that are occurring, and because the chromosome number is being reduced in half (four in the parent cell, two in the daughter cells).
[!!!!!!] question 12+++++++++[/!!!!!!]
[q] Which of the numbered diagrams below represents mitosis?

[c*] 1
[c] 2
[f] Yes. In diagram 1, you see only one round of cell division. In addition, you can see that the chromosome number stays the same in both parent cells and daughter cells. Both of these indicate that what you’re seeing is mitosis.
[f] No. Diagram 2 represents meiosis, which you can see is happening because of the two rounds of cell division that are occurring, and because the chromosome number is being reduced in half (four in the parent cell, two in the daughter cells). In mitosis, you see only one round of cell division, and the chromosome number stays the same.
[!!!!!!] question 13+++++++++[/!!!!!!]
[q] Which of the following statements about mitosis and meiosis is true?
[c] Mitosis and meiosis are universal processes that occur in both prokaryotes and eukaryotes.
[c] Mitosis occurs in prokaryotes. Meiosis occurs in eukaryotes.
[c] Mitosis and meiosis occur in prokaryotes only.
[c*] Mitosis and meiosis occur in eukaryotes only.
[f] No. Mitosis and meiosis never occur in prokaryotes. Next time, choose another answer.
[f] No. Mitosis and meiosis never occur in prokaryotes. Next time, choose another answer.
[f] No. Mitosis and meiosis never occur in prokaryotes. Next time, choose another answer.
[f] Exactly. Both mitosis and meiosis are cell division processes that are unique to eukaryotes.
[x]
[restart]

[/qwiz]

What Now?

Now that we’ve got the basics of meiosis covered, we can go onto the next (and most important) topic: How meiosis creates variation.

  1. Click to go on to tutorial 3 (how meiosis creates variation)
  2. Click here to go to the meiosis main menu