14.2: Exercise

The Phases of Mitosis

Exercise 1: Modeling the Phases of Mitosis with Pop Beads

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
• 4 laminated pictures of centrosomes (each consisting of a pair of centrioles)
• Paper towels / Kimwipes

Procedure:

Using models is a great way to represent natural structures and processes that are too small, or too large, or too complex to observe directly. By building chromosomes from the pop beads and manipulating them to model cell division (mitosis and meiosis) you will enhance your understanding of the nature of chromosomes and the cellular structures needed to perform cell division. In this simulation, your cell will be a diploid (2n) cell with 4 chromosomes. It will contain 2 homologous pairs of chromosomes. In order to differentiate between the pairs of homologs, one pair of homologous chromosomes will be longer than the other. One chromosome of each pair is red and represents maternal DNA (genetic material contributed by a female’s egg). The other chromosome of each pair is yellow and represents paternal DNA (genetic material contributed by a father’s sperm). Thus, for each pair of homologous chromosomes, one should be red and one should be yellow. The strands of pop beads represent the DNA in the form of chromatin during the G1, S, and G2 phases. They then represent chromosomes as they enter the phases of mitosis.

This diploid cell with 2 homologous pairs of chromosomes will be modeled as it moves through the following phases of mitosis:

• Interphase (uncondensed DNA) Before Synthesis of DNA (G1)
• Interphase (uncondensed DNA) After Synthesis of DNA (G2)
• Prophase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis

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 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 homologous pair of single chromosomes should be constructed using 6 red beads for one member of the short pair and 6 yellow beads for the other member of the 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.

3. Use the pop beads 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.

4. 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.)

5. 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.

6. To model the prophase stage of mitosis, 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.

7. At prometaphase the centrosomes are at opposite poles of the cell. During this phase the chromosomes continue to condense, the nuclear membrane completely breaks down, and the spindle fibers start to attach to the kinetochores of the centromeres. These are called kinetochore microtubules. Other spindle fibers radiate outward from the centrosomes, but these do not attach to the kinetochores. These spindle fibers are called nonkinetochore microtubules.

8. To model metaphase, move the centromeres of your chromosomes to lie on an imaginary plane midway between the centrosomes that are now positioned at opposite poles of the cell. Line up the individual chromosomes in the middle (equator) of the cell. Sister chromatids remain attached at the centromere during metaphase.

9. To model anaphase, pull the magnetic centromeres apart and slide them towards opposite poles of the cell. Keep them attached to your drawn kinetochore microtubules, but use the paper towel to erase them, making them shorter. With the chalk, lengthen the nonkinetochore microtubules and have them overlap in the middle of your cell. After separation at the centromere, each sister chromatid is now referred to as an individual chromosome. Anaphase ends when the chromosomes reach the opposite ends of the cell.

10. Model telophase by piling up your chromosomes at each pole. Erase your spindle fibers, as they disappear during this stage. Even though we can’t simulate it, the chromosomes will start to decondense and uncoil back into chromatin. Redraw a nuclear envelope around each pile of chromosomes. Also add nucleoli to your drawing. The formation of separate nuclear envelopes divide the nuclei and marks the end of telophase.

11. The division of the cytoplasm, or cytokinesis, results in the formation of two separate cells. To model cytokinesis in animal cells, fungi, and slime molds, leave the piles of chromosomes at their separate poles. Draw indentations of the cell membrane inwards, towards the cytoplasm, on the sides of the cell where there are no centrosomes. This represents a cleavage furrow,which eventually “pinches” the cell’s cytoplasm into two separate cells. The proteins actin and myosin contribute to the formation of the cleavage furrow. In plant cells, membrane-bound vesicles migrate to the center of the cell (the equatorial plane) and fuse together to form a cell plate. The cell plate eventually divides the cytoplasm into two separate cells. Materials needed to build the cell wall are released from the vesicles at the cell plate, forming a new cell wall.

• How do the daughter cells you formed compare to the original parent cell? _________________________________________________________________

Observe the phases of Mitosis in Plant Cells

Exercise 2: Observing the Phases of Mitosis in the Onion Root Tip

Materials:

• Prepared slide of the onion root tip
• Compound light microscope

Procedure:

1. Examine a slide of a longitudinal section of an onion root tip. Adjust the slide to view the region just above the root cap, where there are likely to be dividing cells.

2. Focus on the dividing cells using the 4x scanning objective lens, then switch to the 10x objective and then the 40x objective.

3. Survey the slide to find a cell in each phase of mitosis. Draw a cell for each phase in the boxes provided.

Interphase

DNA is uncondensed and in the form of chromatin. Individual chromosomes are not visible. The nuclear membrane is intact. The nucleolus is visible.

Figure \(\PageIndex{1}\): Interphase in plant cells.

Prophase

Chromatin begins to condense into distinguishable chromosomes. These “puffy” structures are seen throughout the nucleus. Nucleoli begin to disappear. In late prophase (often called prometaphase) the nuclear membrane is no longer visible.

Figure \(\PageIndex{2}\): Prophase in plant cells.

Metaphase

The chromosomes line up in the middle of the cell. Spindle fibers attach to kinetochores at the centromere and extend to the poles of the cell.

Figure \(\PageIndex{3}\): Metaphase in plant cells.

Anaphase

Centromeres split, separating each former chromatid into two individual chromosomes. The chromosomes move toward opposite poles.

Figure \(\PageIndex{4}\): Anaphase in plant cells.

Telophase and Cytokinesis

Chromosomes reach the poles. The nuclear envelopes begin to reform. The formation of a cell plate forms between the two cells to carry out cytokinesis.

Figure \(\PageIndex{5}\): Telophase in plant cells.

Figure \(\PageIndex{6}\): Cytokinesis in plant cells.

Observe the phases of Mitosis in Animal Cells

Exercise 3: Observing the Phases of Mitosis in the Whitefish Blastula

Materials:

• Prepared slide of whitefish blastula
• Compound light microscope

Procedure:

The blastula is an early embryonic stage where many of the cells are dividing at any one time.

1. Focus on the dividing cells using the 4x scanning objective lens, then switch to the 10x objective and then the 40x objective.

2. Survey the slide to find a cell in each phase of mitosis. Draw a cell for each phase in the boxes provided.

Interphase

The DNA is uncondensed and in the form of chromatin. Individual chromosomes are not visible. The nuclear membrane is intact. The nucleolus is visible.

Figure \(\PageIndex{7}\): Interphase in animal cells.

Prophase

Chromatin begins to condense and chromosomes are distinguishable. These “puffy” structures are seen throughout the nucleus. The nucleoli begin to disappear. In late prophase (often called prometaphase) the nuclear membrane is no longer visible.

Figure \(\PageIndex{8}\): Prophase in animal cells.

Metaphase

The chromosomes line up in the middle of the cell. Spindle fibers attach to kinetochores at the centromere and extend to the poles of the cell.

Figure \(\PageIndex{9}\): Metaphase in animal cells.

Anaphase

Centromeres split, separating each former chromatid into two individual chromosomes. The chromosomes move toward opposite poles.

Figure \(\PageIndex{10}\): Anaphase in animal cells.

Telophase and Cytokinesis

Chromosomes reach the poles. The nuclear envelopes begin to reform. A cleavage furrow forms between the two cells to carry out cytokinesis.

Figure \(\PageIndex{11}\): Telophase in animal cells.

Figure \(\PageIndex{12}\): Cytokinesis in animal cells.

• Why would the method of cytokinesis in animal cells not work in plant cells? Explain. __________________________________________________________________

How Long Does a Cell Spend in Each Phase of the Cell Cycle?

Exercise 4: Determining Time Spent in Different Phases of the Cell Cycle (Optional)

Materials:

• Laptop
• Calculator

Procedure:

1. Obtain a laptop.
2. Open a web browser and go to the following site: http://www.biology.arizona.edu(opens in new window)

3. This site will provide an interactive test of your ability to identify the stages of mitosis. It will also allow you to calculate the duration of the stages identified in the laboratory exercise you just completed, but the website will give standard results for the entire class.
4. Go to the Cell Biology section, and find the activity “Online Onion Root Tips”.
5. Keep clicking on “Next” at the bottom of the page until you get to the screen: Determining Time Spent in Different Phases of the Cell Cycle
6. Click on “Next” at the bottom of the page.
7. Identify each stage shown to you by the program. When a picture of a cell pops up in a stage of mitosis, simply click on the phase in which the cell belongs. If you make a mistake, read the explanation for why you were mistaken before making a new selection.
8. When you are finished, use the formula given below and record your results in the table.
9. The duration of each stage of mitosis can be determined by using the following formula. Compute the length of time for each stage and place your calculations in the table below.

(Number of cells in a stage ÷ Total number of cells) x 1440 (min in a day) = minutes a cell spends

in each stage in one day

Data Table for Mitosis Stages

Number of cells in each stage	Interphase ______	Prophase ______	Metaphase ______	Anaphase _______	Telophase _______	Total 36
Percent of cells (# of cells / Total)						100%
Time (in minutes) spent in Stage – use calculation above

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(opens in new window)