Cell Division

The story of the cell cycle is often told only through the perspective of the chromosomes as they replicate and then divide. This resource beautifully illustrates the role of the cytoskeleton in that process. Understanding the cell cycle from this perspective enables students to appreciate the nuanced and significant role of the cytoskeleton at each phase of the cell cycle, also giving meaning and context to a topic filled with vocabulary. Explore each step individually to build understanding as you progress through the cell cycle.

Here are some suggestions for implementation:

Interphase: Encourage students to observe this video multiple times and record their observations. They may notice how the cytoskeleton is continually regenerated to maintain the overall shape of the cell, an important concept for later. They may also notice the movement of different cytoskeleton filaments within the cell, representing vesicle transport within the cell. This is especially true near the center of the cell, the location of the centrioles near the nucleus. This is a great time to introduce the three categories of cytoskeletal components: microfilaments, intermediate filaments, and microtubules. Each of these helps maintain cell structure and function throughout interphase and is essential as the cell progresses through the cell cycle.

To support students’ understanding of the cytoskeleton, a simple model using pop beads, stacking bricks, or small squares of paper could be used (each bead or brick representing a monomer of the filament or microtubule). Students can build longer filaments by adding pieces to one end or the other or make them shorter by removing pieces. The addition or removal of pieces changes the relative position of each piece. Adding pieces to one end only will push the other end away and, conversely, removing pieces from one end will seem to pull the other end closer. This principle can be used to explain and support different cell shapes and the movement of chromosomes during the subsequent phases.

Examples of questions to ask students:

• How might the cytoskeleton be used in cell division?
• What other function(s) might the cytoskeleton perform in the cell?  Can they create a simple model to illustrate their thinking?

Prophase: Again, encourage students to identify and ask questions about the different structures, paying close attention to the white cytoskeletal filaments. As the centrioles move toward the poles of the cell, they create a region of cytoskeletal production called the centrosome. Be mindful of the vocabulary you introduce to your students. The long filaments “spiderweb” away from the centrosomes. Encourage your students to explain how this might occur, using the simple model created in the previous section. (There are many terms associated with cell division, with similar roots and sounds; choose what is appropriate for the level of your course and the needs of your students.)

Prometaphase: During this phase, the spindle fibers formed between the centrosomes and the newly made microtubules attach to the kinetochore of each chromosome. Encourage students to modify their models to explain how a spindle fiber could be produced and how it might affect cell structure. Also, introduce the idea of the kinetochore and point out that each replicated chromosome will be attached to microtubules from each of the centrosomes.

Examples of questions to ask students:

• Why must the nuclear envelope degrade to proceed? How can you illustrate this?
• How might spindle fibers spanning the length of the cell be produced?
• How would this process differ if the centrosomes were not on opposite poles of the cell?

Metaphase: The hallmark of this phase is the metaphase plate, where all of the chromosomes are lined up in the middle of the cell. The deeper understanding is how the cell is using the cytoskeleton to transition from prometaphase to metaphase. Encourage students to propose an explanation for how the chromosomes moved toward the center of the cell. Have them compare the images from prometaphase and metaphase and identify differences that might explain the process at work. Note the location of the centrosomes during both phases. Each centrosome pushes the chromosome from each side until they are lined up in the middle of the cell.

Examples of questions to ask students:

• What might happen if chromosomes were attached to only one kinetochore? How would this affect what metaphase might look like?
• Looking at the cytoskeleton spindle fiber (microtubules spanning the length of the cell not attached to chromosomes), how might these structures affect the cell structure – as a preview of anaphase?

Anaphase: Students will see that the chromosomes have divided, and each new set is moving toward opposite ends of the cell. Encourage students to create a model to explain how this division is occurring with regard to the cytoskeleton. There will likely be multiple explanations, including the following:  

• The cytoskeleton is broken down at the centrosome, making the microtubules shorter, thus pulling the chromosomes apart.
• The microtubules are broken down at the kinetochore, and the chromosomes “eat” their way up each microtubule (this is what the cell is actually doing!).
• The microtubules are broken down from the middle, making them shorter and separating the chromosomes.

Encourage students to justify their thinking with models and evidence from the images. Students may draw on the other images from the series to further support their thinking. Have students evaluate and critique different ideas to reach a consensus explanation.

Telophase: Have students compare the progression through the cell cycle up to this point. Have them identify patterns in how the cytoskeleton is enabling each step. Now is a good time to revisit the images and information about the basic functions of the cytoskeleton during interphase (cell shape and transport).

Examples of questions to ask students:

• Describe the shape of the cell as it progresses from metaphase to telophase.  How can you use the cytoskeleton to make sense of this claim?
• What might be happening to the other cell structures (organelles and such) during telophase?

Cytokinesis: Using the amazing video of cytokinesis, encourage students to propose a mechanism for how this might occur by applying all of their learning and thinking about the cytoskeleton.

Examples of questions to ask students:

• How does successful cell division depend on the coordination of each step of M phase?
• What would happen if a cell divided without each step of M phase occurring correctly?

Conclusion: Now is the chance for students to celebrate their learning. This could be used as a summative or another formative assessment.

Here are some ideas for how students could share their learning:

• Create an annotated model of the video presented in this section, using their models from the previous steps to support and explain.
• Create a narration of the video, describing the structures and processes occurring at each phase.
• Identify key points throughout the process and describe why each point is essential for successful cell division.

Finally, remember that consistency in the use of words and terms is huge when teaching the cell cycle. There are a lot of terms and many sound very similar or share roots. Here are some examples:  

• Replicated chromosomes versus unreplicated chromosomes
• Chromatids, chromosomes, and chromatin
• Centromere, centrosome, and centriole
• Intermediate filiments, microtubules, and actin filiments