In today's lesson, students will create their own onion root tip slide and count the mitotic phases to determine how the nucleus divides. They will compare their slides to prepared thin section slides (both plant and animal) to better understand how scientists constructed the mitotic model of cell division. Finally, students will create a stop motion video of cell division to demonstrate what occurs in mitosis and what happens to the cell during abnormal nuclear division. Here is an overview of what students will learn today.
Before the start of class, make a circle in string on the floor of the classroom. When students arrive, dump an entire set of chromonoodles onto the floor within the circle. Have student volunteers to pair a chromonoodle with its match. Then, ask the students what would be the most efficient way to get the same number of chromosomes into two separate, identical cells. Give students time to write a response or draw a picture in their lab notebook. Have several student volunteers test their ideas using the chromonoodles.
Using the Frayer method, have students define mitosis in their lab notebooks.
Explain to the students that mitosis is a model that has been developed by scientist to explain how the cell nucleus divides. They will be using several protocol today to determine how scientists developed that model.
Based on Farrar, Jennifer and Kelsi Barnhart. 2011. "Chromonoodles: Jump Into the Gene Pool." The Science Teacher. 6:34-39.
The following supplies are needed for this activity:
For each lab pair
Using the student handout, students will make a fixed slide of a green onion root tip. First, they will need to heat the onion root tip in a test tube containing 3 mL of fixative in a warm water bath for 6 minutes at 60ºC. After six minutes, they should discard the fixative and place the fixed root tips onto a watch glass. Students should remove the bottom 2 mm of the root tip and place it onto a microscope slide. Next students should add one or two drops of aceto-orcein stain to the fixed root tip. Allow the stain to set for two minutes. Press a flat probe straight down to squash the root tip onto the slide. Put a cover slip on the specimen and press the cover slip gently with the handle of the probe.
After they have made the root tip slide, students will view their prepared slide under a light microscope at 400X. They will methodically scan the slide counting 100 cells. They will make note of the phase of mitosis each cell is in. Students will express that data as a proportion and then determine how long the cell is in each phase during a 24 hour period.
(Note: The green onions need to be placed in water for 72 hours before the lab to encourage growth of the roots. If onions are not available, this can be done with garlic cloves or any other types of Allium.)
Methodology based on Onion Root Tip Mitosis Lab as found on the Kansas Association of Biology Teachers BioBlog
At this station, students will now compare their counts to professionally prepared thin section slides. Prepared slides should be viewed under the light microscope. They will repeat the same counting protocol. The slide should be scanned and one hundred cell should be counted. The phase of cell cycle in which each cell is should be record the data table in the lab notebook. (Note: They could also use the provided student handout.)
After counting both the student prepared slide and the professional prepared slide, students should graph the average amount of time the cell nucleus stays in each part of the cell cycle. Graphs should be placed in their lab notebook.
Next, at this station, students will repeat the protocol with prepared whitefish blastula slides. Students will not be able to count one hundred cells due to the limited size of the blastula. Student will need to modify their protocol by either counting all of the viable cells in one blastula or multiple blastulas (until they count 100 cells). Students should classify the part of the cell cycle each cell is occupying and record that information in their data table.
Students should then calculate the amount of time during a 24 hour period that each phase of the cell cycle takes. Ask students to compare their findings with the findings from the onion root tip. Ask student to consider what might account for any differences they see.
Bring the class back together and discuss the importance of sample size in model development. First have students consider if it would be better to count 200, 500, or 1000 cells on one root tip when determining the time each phase takes or if counting 100 cells on 200 root tips. Encourage them to support their argument.
Next, ask students to share their data from the whitefish blastula counts. Have student that only sampled one blastula compare their findings with students that counted 100 cells. Then ask students how their findings compared with the onion root tip data.
Revisit the chromonoodle activity done at the beginning of class, ask student to look at their initial model. Based on their findings from the cell counts, have student volunteers move the chromonoodles through the process of cell division.
Allow students time to revise their initial model in their lab notebooks before turning in their lab notebooks for evaluation.
Student pairs will be given a storyboard and asked to draw a storyline for their stop motion video. Using several different colors of clay, student teams will construct model of the cell. They will take still images of the models and then change one thing about the model. Next, they will take another still. They will continue to do this to create a stop motion video. Once they have finished, pictures can be imported into a video editing program and narration can be added. Guidelines for this assignment can be found here. (Note: here is an example of completed student work.)