15.2: Exercise

The Phases of Meiosis

Exercise 1: Modeling the Phases of Meiosis

Materials:

• Chalk (can be used directly on lab bench to draw cellular structures and then washed off)
• Chromosome modeling kits
  • Commercially available pop bead kits (e.g Carolina Biological Supply Company, Item #171100)
• 40 pop beads of one color (red)
• 40 pop beads of another color (yellow)
• 8 magnetic centromeres
• 8 laminated pictures of centrosomes (each consisting of a pair of centrioles)
• Paper towels / Kimwipes
• Fine point sharpie
• Small round stickers

Procedure:

A diploid cell with 2 homologous pairs of chromosomes (as in the previous modeling exercise) will be modeled as it moves through the meiosis. First, you will model meiosis l. Then, you will model meiosis ll as described below.

Model Meiosis l (1 diploid cell → 2 haploid cells)

• tetrads form, crossing over occurs, homologues separate
  • Interphase Before Synthesis of DNA (G1)
  • Interphase After Synthesis of DNA (G2)
  • Prophase l
  • Metaphase l
  • Anaphase l
  • Telophase l
  • Cytokinesis l

Model Meiosis ll (2 haploid cells → 4 haploid cells)

• sister chromatids separate
  • Prophase ll
  • Metaphase ll
  • Anaphase ll
  • Telophase ll
  • Cytokinesis ll

Modeling Meiosis l

1. You will build 2 pairs of homologous chromosomes (2n = 4). The first homologous pair of single chromosomes should be constructed using 12 red beads for one member of the long pair and 12 yellow beads for the other member of the long pair. You may place the magnetic centromere at any position along the chromosome, but it must be at the same position on both members of the homologous pair.

2. The second pair of single chromosomes will represent the X and Y sex chromosomes in a male. The X chromosome should be constructed using 8 red beads. To make manipulation of the chromosomes easier, place the magnetic centromere in the middle of the X chromosome so that you have 4 red beads on one side of the centromere and 4 red beads on the other side. The Y chromosome should be constructed using 2 yellow beads with a magnetic centromere between them.

3. Alternative forms of genes are called alleles. We will use the letter “B” or “b” to represent the gene for eye color. Labeled stickers will be used to represent alleles. With the Sharpie, label one small sticker with a “B” and another small sticker with a “b”. “B” will represent the dominant allele for brown eyes and “b” will represent the recessive allele for blue eyes. You may place the “B” sticker on any of the red beads of the long red chromosome. The “b” sticker must be placed at the same bead position of the “B” sticker, but on the long yellow chromosome.

4. To model the G1 phase of interphase (before synthesis of the DNA). On your lab bench, use the chalk to draw the cell membrane, nucleus, and nucleolus. Place a laminated centrosome in the cytoplasm of your cell. Place the 4 assembled chromosomes in the nucleus of your cell drawing. Remember that these represent a mass of chromatin.

5. DNA replication takes place during the S phase of interphase. Model the S phase of interphase by assembling a second strand that is identical to each of the 4 single chromosomes. The identical strands are considered sister chromatids, which are held together by their magnetic centromeres. (Note: In a living cell, the centromere is a single unit until it separates in anaphase. Therefore, consider the pair of magnets to be a single centromere.)

6. Model the G2 phase of interphase by letting your 4 assembled replicated chromosomes rest in the nucleus of your cell. Duplicate the centrosomes by placing another laminated centrosome in the cytoplasm of your cell drawing. Begin to move them towards opposite poles of the cell drawing.

7. To model the prophase I stage of meiosis, leave the chromosomes where they are in your cell drawing. In this stage, the chromatin coils and condenses into chromosomes. Using the paper towel (or Kimwipe) start erasing some of the nuclear membrane on your cell drawing. This will simulate the breakdown of the nuclear membrane. Also erase the nucleolus, as the nucleolus disappears in this stage. Continue to move your centrosomes to opposite sides of the cell. Use the chalk to draw spindle fibers beginning to form and radiating outward toward the chromosomes.

8. Prophase l of meiosis has some notable differences compared to prophase of mitosis. To further model prophase l of meiosis, arrange the condensed chromosomes so that homologous chromosomes are paired. Put homologous chromosomes next to each other in your cell drawing to simulate this process of synapsis (homologs pairing). Homologous pairs of chromosomes paired in this way are called tetrads. When tetrads form, the inner non-sister chromatids of the tetrad pair can exchange DNA by a process known as crossing over. Pieces of equivalent segments of non-sister chromatids can be exchanged from one non-sister chromatid to the other. Crossing over can occur several times along the length of the chromosomes.

• How many tetrad complexes do you have in your cell, which is 2n = 4? ____________________________________________________________

9. Exchange segments of the inner non-sister chromatids of the red and yellow beads that contain the two different alleles, “B” and “b”. Remember that the red beads represent DNA from the maternal chromatid and the yellow beads represent DNA from the paternal chromatid. You now have one crossover event for this tetrad.

10. Metaphase l of meiosis is also notably different than metaphase of mitosis. To model metaphase l of meiosis, move your tetrads to the equator, midway between the two centrosomes, which should now be positioned at the two opposite poles of the cell. Draw spindle fibers radiating out from the centrosomes to the homologous pairs (tetrads) which are lined up along the equatorial plane of the cell. Homologs are paired together but the maternal and paternal chromosomes are on opposite sides of the equatorial plate along the middle of the cell. Every pair of chromosomes is arranged independent of another. The side of the equatorial plate where each chromosome is arranged is completely random and independent of the side of the equatorial plate on which other chromosomes are located. Therefore, there are several different arrangements that can occur.
11. In anaphase l of meiosis, homologous chromosomes separate toward opposite poles of the cell. Centromeres do not split as they do in mitosis. For each homologous pair, move the duplicated maternal chromosome and duplicated paternal chromosome to opposite poles of the cell. Remember that in our model, the two magnets (one of each of the two sister chromatids) represent one centromere. Erase the spindle fibers and redraw them shorter and shorter as each homologous pair moves away from one another towards opposite poles of the cell.

• How does the structure of chromosomes in anaphase I of meiosis differ from that in anaphase of mitosis? ____________________________________________________________

12. Anaphase ends and telophase l begins when chromosomes reach opposite poles of the cell. Arrange the chromosomes in groups at opposite ends of the cell. You should have one long chromosome of 12 beads and either the X chromosome or the Y chromosome at each pole. Nuclear division happens in telophase. The formation of separate nuclear envelopes divide the nuclei and mark the end of telophase.

13. Model cytokinesis l by drawing the formation of a cleavage furrow to divide the cytoplasm into two and form two separate cells. Redraw the nuclear membrane around the chromosomes and draw a nucleolus inside of each nucleus. These two cells will now enter meiosis ll. Note that each daughter cell has half the number of chromosomes as the parental cell. Thus, the cells have been reduced from diploid (2n) to haploid (n). [n refers to the number of pairs of chromosomes that are characteristic for a species. Humans have a “n” of 23, so a diploid human cell has 2(23), or 46 chromosomes].

Modeling Meiosis ll

1. There is no DNA replication before the second cell division stage of meiosis. The stages of meiosis ll proceed very much like mitosis. The two cells created in meiosis l will enter into prophase ll. The chromosomes in each cell contain two sister chromatids, which are condensed and distributed throughout the nucleus. Add another laminated centrosome. There should be two centrosomes in each new cell. In your drawing be sure to note that the nuclear envelope begins to break down, the nucleolus disappears, the centrosomes move towards opposite poles, and spindle fibers begin to form and radiate toward the chromosomes. Spindle fibers attach to kinetochore proteins at the centromeres of the chromosomes.

2. To model metaphase ll, line up the individual chromosomes on the equator (middle) of each cell. Sister chromatids remain attached at the centromere during metaphase ll.

3. Model anaphase ll by pulling the two magnetic centromeres of each duplicated chromosome apart. The sister chromatids should be separated and moved toward opposite poles of each cell. After separation at the centromere, the chromatids are now called chromosomes. In anaphase II single chromosomes move towards opposite poles.

4. Anaphase ll ends and telophase ll begins when the chromosomes reach opposite poles of the cells. Nuclear division happens in telophase. The spindle fibers disassemble. Nuclear envelopes and nucleoli reappear. Condensed chromosomes begin to decondense and uncoil. The formation of separate nuclear envelopes divide the nuclei and marks the end of telophase.

5. Model cytokinesis ll by drawing the formation of a cleavage furrow to divide the cytoplasm of each cell into two separate cells.

• What is the total number of nuclei and cells now present? ___________________

• How many cells were present when meiosis began? _______________________

• How many chromosomes were present in the original parental cell? ___________

• How many chromosomes are in each new daughter cell? ___________________

• Are the chromosomes in the new daughter cells identical to the chromosomes in the original parental cell? Explain your results in terms of independent assortment and crossing over. __________________________________________________

Questions for Review

1. What is the meaning of diploid? What abbreviation do we use to represent diploid? Name 2 diploid cells in humans.

2. What is the meaning of haploid? What abbreviation do we use to represent haploid? Name 2 haploid cells in humans.

3. In what stage of the cell cycle does S phase occur? Explain why the DNA must be duplicated during the S phase of the cell cycle, prior to mitosis taking place.

4. What specific feature of cytokinesis in animal cells can you use to distinguish this process from cytokinesis in plant cells?

Practical Challenge Questions

1. In the circle below, sketch a 2n=6 haploid cell in metaphase l of meiosis.

2. A monogenic gene gives rise to a trait from a single set of alleles. A polygenic gene gives rise to a trait from several sets of alleles. Give an example of a monogenic and polygenic trait.

References

Belwood, Jacqueline; Rogers, Brandy; and Christian, Jason, Foundations of Biology Lab Manual (Georgia Highlands College). “Lab 10: Mitosis & Meiosis,” (2019). Biological Sciences Open Textbooks. 18. CC-BY

https://oer.galileo.usg.edu/biology-textbooks/18