Purpose: To observe the different stages of mitosis occur in the cells of an onion root tip
Background information/research:
 In a growing plant root, the cells at the tip of the root are constantly dividing to allow the
roots to grow. Because each cell divides independently of the others, a root tip contains
cells at different stages of the cell cycle. This makes a root tip an excellent tissue to study
the stages of cell division.
 There are many different stages of the cell cycle/mitosis.
 In Interphase, the cell is preparing for Mitosis, and you see the nucleus as a dark mass in
the cell.
 In Prophase, the nuclear envelope dissolves, and the chromatin condenses into rod shaped
structures. The chromosomes are visible but not organized.
 In Metaphase, the chromosomes line up along the equator.
 In Anaphase, spindle fibers pull the chromosomes apart.
 In Telophase, the nuclear envelope reforms and two nuclei are visible.
Hypothesis: If onion cells in different stages of mitosis are looked at under a microscope, then it
will be noticeable that mitosis is not divided into clear stages, but is a continuous process.
Experiment:
Materials:
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Microscope
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Prepared slides of union (alium) root tips
Procedure:
1. Get one microscope for your lab group and cary it to your lab desk with two hands. Make
sure that the low power objective is in position and that the diaphragm is open to the
widest setting
2. Obtain a prepared slid of an onion root tip (there will be three root tips on the slide). Hold
the slide up to the light to see the pointed ends of the root sections. This is the root tip
where the cells are actively dividing. (The root tips are freshly sliced into thin sections,
then preserved when the slide was prepared.)
3. Place the slide on the microscope stage with the root tips pointing away from you. Using
the low power objective to find a root tip, and focus it with the coarse adjustment knob
until it is clearly visible. Just above the root “cap” is a region that contains many new
small cells. The larger cells in this region were in the process of dividing when the slide
was made. These are the cells that you will be observing. Center the image, then switch
to high power.
4. Observe the box like cells that are in arranged rows. The chromosomes of the cells have
been stained to make them easily visible. Select one cell whose chromosomes are clearly
visible. (if you need to change the focus when using high power, make sure to only use
the fine adjustment knob.
5. Sketch the cell that you selected in first box.
6. Look around at the cells again. Select four other cells whose internal appearances are
different from each other and the first one that you sketched. Sketch them in the
remaining boxes.
7. As you look at the cells of the root tip, you may notice that some cells seem to be empty
inside (There is no dark nucleus or visible chromosomes). This is because these cells are
three dimensional, but we are looking at just thin slices of them. (If you slice a hardboiled
egg at random, would you diffidently see the yolk in your slice? No.) We want to
continue to look at the cells, but we will ignore anywhere we cannot see the genetic
material (dark areas).
8. Looking along the rows of cells, identify what stage each cell is in. Use photos of the
different stages as a guide.
9. Use the data table to record the number of cells you see in each of the stages. The easiest
way to do this is for one person to look through the microscope, going along each row of
cells. For each cell, say out loud what stage it appears to be in. Another student can make
tally marks for each stage.
Observations/Data/Results:
Cells observed in the different stages through the high power objective lens:
Stage of Cell Cycle
Number of Cells in the Stage
% of
cells
Interphase
36%
Prophase
22%
Metaphase
10%
Anaphase
12%
Telophase
20%
Analysis of Data:
The data shows us how long each of the stages are in relation to the others. If a stage is
very short, then the cell will be finished with it quickly, and less will appear in that stage, and
vice versa. For example, the data shows that most of the cells observed at the onion root tip were
in the Interphase stage. Therefore, Interphase is most likely the longest step of the cell cycle. The
least number of cells were found in Metaphase, showing that it is most likely the shortest. The
term “most likely” is used instead of “definitely” because it is not always certain and clear what
stage a cell is in. This is because mitosis is a continuous process, not a series of clearly difined
steps.
Answering questions related to the lab:
4) The onion plant began with a single cell. That cell had X number of chromosomes.
(The exact number does not matter; we will just call that number X). How many chromosomes
are in each of the cells you observed? (Give the answer in terms of X) How do you know?
Each of the cells observed contained X chromosomes. This is because every cell (nonmutant) always contains the same number of chromosomes. During mitosis, the chromosomes
are doubled, and then split, resulting in the same number of chromosomes for the two daughter
cells as their parent had.
5) If this onion would reproduce sexually, it would need to produce sperm and/or eggs by
the process of meiosis. After meiosis, how many chromosomes would be in each sex cell?
In ach sex cell, there would be 1/2x chromosomes
6) If this onion would complete the process of sexual reproduction (fertilizing an egg
cell) How many Chromosomes would be in zygotes that are produced (In terms of X)?
In each zygote, there would be x chromosomes.
Conclusion:
The data and analysis supported my hypothesis that mitosis is a continuous
process and not a series of steps. This is because some cells appeared to in the middle of
Metaphase and Anaphase. The chromosomes where lined up, but not in a straight line, some had
already started being pulled away from the equator of the cell. (It was later deemed that
Anaphase was the best choice.) While this particular cell was one of the more obvious ones,
many of the cells were hard to distinguish into certain phases. The most common “mixed” cells
were in between interphase an prophase. The final data somewhat deviated from previous
expectations. I believed that each of the stages would be represented by approximately the same
amount of cells. For example, it seemed unlikely before the experiment that the preparation for
mitosis (Interphase) would take any longer (and therefore have any more cells representing it)
than the cell and its contents beginning to split (Telophase).The data showed, however, a
shocking 16% difference between the two stages.
I had several questions during the experiment. Two of these are: How many
Chromosomes are in each cell? At what point in the cells life does it stop going through
mitosis/the cell cycle?
Is there a point in which a cell does not undergo any stage of mitosis?
There is a sub phase of G1 called G0. At this point the cells do not develop further into
the cell cycle, or make any preparations for cell division. Instead, the cells just continue on with
life processes. This stage can last for a variety of times, sometimes for a few minutes, and
sometimes for years. The cells can leave G0 with signals from other cells and other parts of the
body.
Citations:
 "THE CELL CYCLE." SparkNotes. SparkNotes, n.d. Web. 04 May 2014.
 Olsen, Ms. Observing Mitosis Lab. Apr.-May 2014. Lab sheet.
 Mader, Sylvia S. Biology. Boston: WCB/McGraw-Hill, 1998. Print.