Observing Mitosis in Onion Cells


Title: Observing Mitosis in Onion Cells

Purpose: The purpose of this lab is to observe the process of mitosis in onion root tip cells.


The parts of plants that actively grow, and therefore undergo mitosis, are the roots and edges of leaves. Cells are constantly growing, so the many cells can be seen in different stages of growth and division.

Interphase can be identified by a solid colored nucleus in the middle of the cell, and sometimes there is a dark spot in the nucleus, which is the nucleolus. Prophase can be identified as a cell similar to a cell in interphase, but the nucleus has white gaps that separate the dark mass. This is the condensing of chromatin to form chromosomes for mitosis. Metaphase can be identified by the dark colored chromosomes lined up in the middle of the cell. Anaphase can be identified by the separation of the chromosomes from the middle of the cell. Since the chromosomes are pulled by the spindle fibers at the centromere, the chromosomes make a U/V shape when being pulled. Telophase can be observed when two dark masses of chromosomes are on opposite ends of one cell.


  1. Microscope
  2. Allium onion root tip slide


  1. Set up the slide of the onion root tip under the microscope, with the tips pointing away. Start observing one root tip at a time, since there are three tips on one slide.
  2. Focus the slide with the smallest magnification. Once the slide is in focus, repeat with the next largest magnification lens until the slide is focused in the lens with the largest magnification.
  3. Observe the tip of the root and count how many cells are in each stage of the cell cycle (interphase, prophase, metaphase, anaphase, and telophase.) Record the data.)
  4. Repeat steps 1-3 for the other two root tips.

Data and Observations:

This is the data that I collected.

Screen Shot 2016-12-04 at 11.16.09 AM.png

These are the class averages.

Screen Shot 2016-12-04 at 11.05.51 AM.png


  1. The majority of cells observed were in the interphase stage.
  2. The percentage of cells in each stage according to the class averages was
    • interphase: 81%
    • prophase: 12.43%
    • metaphase: 1.65%
    • anaphase: 1.37%
    • telophase: 2.03%
  3. The observations of this lab demonstrate that mitosis is a continuous process, not a series separate events. There were no definitive signs for each stage, as differentiating and identifying the stages of the cell cycle were difficult. Many cells looked like they could be either in one stage or the next.
  4. In the cells observed in the lab, each cell had 4x chromosomes. Normal cells have 2x (2 copies of a set of chromosomes), but cells that undergo mitosis have had their DNA replicated, therefore making 4x amount of chromosomes.
  5. In a sex cell, there are x amount of chromosomes. Each sex cell has half as many chromosomes as a regular cell does.
  6. A zygote would have 2x amount of chromosomes. A sperm and egg cell that each has x amount of chromosomes combine to make a fertilized egg that hs 2x amount of chromosomes.


Overall, the class averages were fairly close to the true percentages of time cells spend in each cell cycle. The class average for interphase was 81%, while the real percentage is 80%. The class average for prophase was 14.59%, and the real percentage is 12%. The class average for metaphase was 1.65%, but the real percentage is 1%. The class average for anaphase was 1.37%, but the real percentage is 2%. The class average for telophase was 2.03%, but the real percentage is 5%. The personal data is more inaccurate than the averages, but this can be attributed to several errors.

One error comes from the nature of the slide and the process of making the slides. As the onion root tips are made into microscope slides, some of the cells were killed. Also, since the slide is just a thin slice of the root tip, the sample does not include every nucleus of every cell. Not all cells observed were seen with nuclei, which made the percentages calculated more inaccurate.

Another error comes from the microscopes. The magnification of the even the strongest lens of the microscopes used was not strong enough for the cells to be accurately counted. Since around 250-300 cells were in each image of the lens, there was much room for error and inaccuracy. If the lens could magnify the image more, the number of cells counted in each stage would be more accurate.

Human error was also a problem that led to an inaccurate calculation of percentages. Keeping track of which cells were and were not counted already was difficult because the counter had to look in and out of the lens to write the data down. Differentiating between stages, specifically interphase and prophase, was also difficult. If there was a way to take a picture of the image in the lens, the counter could count the cells more accurately and make marks on a physical or digital copy of the picture.


2 thoughts on “Observing Mitosis in Onion Cells

  1. Edith Leung

    I didn’t know what mitosis was before reading this, so it’s a little hard for me to comment on this. I can see that it is the way that cells divide and grow.

    I would say that one thing you could improve is the organization of the information. The data as a list is a little hard to read. Maybe put in a chart instead?

    Liked by 1 person

  2. Michelle, A very well organized post. You clearly describe the challenges of the lab. I love seeing cells under the microscope even though it is challenging. Just wait…next semester we get to look at living cells! Stay tuned 🙂


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