AP Biology Mitosis Lab
Introduction
The purpose of this lab is to study the process of mitosis in onion root tip cells. This way
you can see what each stage of mitosis looks like in a plant cell and then calculate the
relative duration of the phases of mitosis in plant meristematic tissue.
Roots consist of different regions. The root cap functions in protection. The apical
meristem is the region that contains the highest percentage of cells undergoing mitosis.
The region of elongation is the area in which growth occurs. The region of maturation
is where root hairs develop and where cells differentiate to become xylem, phloem, and
other tissues (Figure 3.1)
Figure 3.1a: Median Longitudinal Section
Figure 3.1b: Apical Meristem Tip Close Up
Exercise 3A.1
Examine prepared slides of onion root tips. Locate the meristematic region of the onion
with the 10X objective, and then use the 40X objective to study individual cells. For
convenience in discussion, biologists have described certain stages, or phases, of the
continuous mitotic cell cycle. Identify one cell that clearly represents each phase. Sketch
and label the cell in the boxes below.
Table 3.1
Interphase
Prophase
Metaphase
Anaphase
Telophase
Exercise 3A.2: Time for Cell Replication
(To be completed at home)
Overview
To estimate the relative length of time that a cell spends in the various stages of cell
replication, you will examine the meristematic region of onion root tips. The length
of the cell cycle is approximately 24 hours for cells in actively dividing onion root
tips.
Procedure
It is hard to image that you can estimate how much time a cell spends in each phase
of cell replication from a slide of dead cells. Yet this is precisely what you will do
in this part of the lab. Since you are viewing prepared slides, you cannot get any
information about how long it takes for a cell to divide. What you can determine is
how many cells are in each phase. From this, you can infer the percent to of time
each cell spends in each phase.
1. Go to the following website and read the introduction for the activity. Click on
“Next” at the bottom of the page when finished.
http://www.biology.arizona.edu/cell_bio/activities/cell_cycle/cell_cycle.html
2. Read through the descriptions of each phase of mitosis. Pay attention to the
images provided to help guide your understanding. Click “Next” on the bottom
of the page when finished.
3. Read through the assignment description.
4. Go through the activity, determining what phase is represented by each of the
pictures of the online root tip given. At the end, record how many cells were
found in each phase of cell replication in the data table provided on the next
page (Table 3.2).
5. Calculate the percentage of cells in each phase.
6. Consider it takes, on average, 24 hours (or 1,440 minutes) for onion root tip
cells to complete the cell cycle. You can calculate the amount of time spent in
each phase of the cell cycle from the percent of ells in that stage using the
formula below.
Percent of cells in stage x 1,440 minutes = ____ min. of cell cycle spent in stage
Table 3.2
Interphase
Prophase
Metaphase
Anaphase
Telophase
Number
of cells
Percent of
cells
Time in
each stage
(min)
Total
36
100%
1,440 min
Questions:
1. What form does DNA take in each phase of the cell cycle? Check the appropriate
column in the table below.
Phase
G1
S
G2
Prophase
Metaphase
Anaphase
Telophase
Chromatin
Chromosome
2. Why is each form of DNA (chromatin and chromosomes) necessary? Be
thorough.
3. If DNA could be formed into chromosomes in the normal human somatic G1 cell
(it doesn’t but what if it did), how many chromosomes would there be and would
they be single or double?
4. In metaphase of the same cell, how many chromosomes would there be and would
they be single or double?
5. After telophase in the same cell, how many chromosomes would there be and
would they be single or double?
6. Explain why onion root tip is selected for the study of mitosis. Name another
tissue that would be good for this type of study and state why it would be a good
tissue to use.
7. In interphase and early prophase, the nucleus is still present. How can you tell
one phase from the other under the light microscope?
8. In mid to late prophase, the nucleus is almost gone, chromosomes are condensing,
and spindles are reforming. At the end of telophase, the nucleus beings to reform,
chromosomes are de-condensing, and spindles are disappearing. These would
look extremely similar (i.e. it would be hard to tell if things are coming or going).
How could you distinguish between the two?
9. What is the purpose of the kinetochores?
10. How does cytokinesis differ in plant and animal cells?
11. Look at your results in Table 3.2. Summarize your data for this part of the lab.
(Make inferences about the relative amount of time the root tip cells spend in each
phase of the cell cycle.
12. Why does the number of cells in each phase equate to the relative time spent in
each phase?
13. In the online lab activity, how would your results have been different if your
observations had not been restricted to the area of the root tip in the meristematic
region?
14. Explain why you think the longest phase lasts the longest.
15. What is the difference between early and late prophase? Early and late telophase?