Observing Mitosis Lab

Onion Root Tip Mitosis
Getty/Ed Reschke

We have all seen illustrations in textbooks of how mitosis works. While these types of diagrams are definitely beneficial for visualizing and understanding the stages of mitosis in eukaryotes and connecting them all together to describe the process of mitosis, it is still a good idea to show students how the stages actually look under a microscope in an actively dividing group of cells.

Necessary Equipment for This Lab

In this lab, there are some necessary equipment and supplies that would need to be purchased that go beyond what would be found in all classrooms or homes. However, most science classrooms should already have some of the necessary components of this lab and it is worth the time and investment to secure the others for this lab, as they can be used for other things beyond this lab.

Onion (or Allum) root tip mitosis slides are fairly inexpensive and easily ordered from various scientific supplies companies. They can also be prepared by the teacher or students on blank slides with coverslips. However, the staining process for homemade slides are not as clean and exact as those that are ordered from a professional scientific supply company, so the visual may be somewhat lost.

Microscope Tips

Microscopes used in this lab do not have to be expensive or high powered. Any light microscope that can magnify at least 40x is sufficient and can be used to complete this lab. It is recommended that students are familiar with microscopes and how to use them correctly before beginning this experiment, as well as the stages of mitosis and what happens in them. This lab can also be completed in pairs or as individuals as your amount of equipment and skill level of the class allows.

Alternatively, photos of onion root tip mitosis can be found and either printed onto paper or put into a slideshow presentation in which the students can do the procedure without the need for microscopes or the actual slides. However, learning to use a microscope properly is an important skill for science students to have.

Background and Purpose

Mitosis is constantly happening the meristems (or growth regions) of roots in plants. Mitosis occurs in four phases: prophase, metaphase, anaphase, and telophase. In this lab, you will determine the relative length of time each phase of mitosis takes in the meristem of an onion root tip on a prepared slide. This will be determined by observing the onion root tip under the microscope and counting the number of cells in each phase. You will then use mathematical equations to figure out time spent in each phase for any given cell in an onion root tip meristem.

Materials

Light microscope

Prepared Onion Root Tip Mitosis Slide

Paper

Writing utensil

Calculator

Procedure

1.  Create a data table with the following headings across the top: Number of Cells, Percentage of all Cells, Time (min.); and the stages of mitosis down the side: Prophase, Metaphase, Anaphase, Telophase.

2.  Carefully put the slide on the microscope and focus it under low power (40x is preferred).

3.  Choose a section of the slide where you can clearly see 50-100 cells in the different stages of mitosis (each “box” you see is a different cell and the darker stained objects are chromosomes).

4.  For each cell in your sample field of view, determine whether it is in prophase, metaphase, anaphase, or telophase based on the appearance of the chromosomes and what they should be doing in that phase.

5.  Make a tally mark under the “Number of Cells” column for the correct stage of mitosis in your data table as you count your cells.

6.  Once you have finished counting and classifying all of the cells in your field of view (at least 50), calculate your numbers for “Percentage of All Cells” column by taking your counted number (from Number of Cells column) divided by the total number of cells you counted. Do this for all stages of mitosis. (Note: you will need to take your decimal you get from this calculation times 100 to make it into a percentage)

7.  Mitosis in an onion cell takes approximately 80 minutes. Use the following equation to calculate data for your “Time (min.)” column of your data table for each stage of mitosis:  (Percentage/100) x 80

8.  Clean up your lab materials as directed by your teacher and answer the analysis questions.

Analysis Questions

1.  Describe how you determined which phase each cell was in.

2.  In which phase of mitosis was the number of cells the greatest?

3.  In which phase of mitosis was the number of cells the fewest?

4.  According to your data table, which phase takes the least amount of time? Why do you think that is the case?

5.  According to your data table, which phase of mitosis lasts the longest? Give reasons as to why this is true.

6.  If you were to give your slide to another lab group to have them repeat your experiment, would you end up with the same cell counts? Why or why not?

7.  What could you do to tweak this experiment in order to get more accurate data?

Expansion Activities

Have the class compile all of their counts into a class data set and recalculate the times. Lead a class discussion on the accuracy of data and why it is important to use large amounts of data when calculating in science experiments.