Observing Mitosis Lab
Name: ___________________________________________________ Date: _________________ Period : _____________
Background:
A quick overview of cell division
The genetic information of plants, animals and other eukaryotic organisms resides in several (or many) individual DNA
molecules, or chromosomes. For example, each human body cell possesses 46 chromosomes, while each cell of an onion
possesses 8 chromosomes. All cells must replicate their DNA when dividing. DNA replication yields an identical pair of DNA
molecules (called sister chromatids) attached at a region called the centromere. DNA replication in eukaryotes is followed by
the process called mitosis which assures that each daughter cell receives one copy of each of the replicated chromosomes.
During the process of mitosis, the chromosomes pass through several stages known as prophase, metaphase, anaphase and
telophase. The actual division of the cytoplasm is called cytokinesis. During each of the preceding stages, particular events
occur that contribute to the orderly distribution of the replicated chromosomes prior to cytokinesis.
In a growing plant root, the cells at the tip of the root are constantly dividing to allow the root 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. First, you will first observe and draw cells in the different phases of the cell
cycle. Second, you will then estimate the proportion of time that cells in actively dividing tissues, such as the root tip, actually
spend in Interphase, M-phase (Mitosis) and cytokinesis.
Materials:
microscope
prepared slides of onion (allium) root tips
photomicrographs of onion root tips
PROCEDURE PART I:
1. Obtain a prepared slide of an onion root tip (there will be two to three root tips on a 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 were actively dividing. (The root tips were
freshly sliced into thin sections that were preserved and stained when the slide was prepared.)
2. Place the slide on the microscope stage with the root tips pointing away from you. Use the low-power objective to find a
root tip, and focus it with the coarse adjust until it is clearly visible. Just above the root “cap” is a region that contains many
new small cells. The larger cells of 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 higher power.
3. Observe the box-like cells that are arranged in rows. The chromosomes of the cells have been stained to make them easily
visible. Select one cell that represents each stage below and whose chromosomes are clearly visible.
4. Draw a cell in each stage listed below. Use pictures on board for reference.
Interphase
Prophase
Telophase
Cytokinesis
Metaphase
Anaphase
NOTE: 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 hard boiled egg at random, would you definitely see the yolk in your slice?) We want to continue to look
at the cells, but we will ignore any where we cannot see the genetic material (dark areas).
PROCEDURE: Part II
1.
2.
3.
In the photomicrographs of the root tip sections, identify the stage of the cell cycle that each cell is in.
Place your data in the table below.
To increase your number of data points, pool your data together with your lab partners. Then, estimate the
percentage of time the onion root tip cells spend in each different stage of the cell cycle.
Stage of cell
cycle
Interphase
M-Phase
Number of
cells:
A=your root tip
Number of cells:
B=your partners
Total Number
of cells in
each phase
(A+B)
Percentage
of cells in
each stage of
the cell cycle
Proportion of
time in each
stage of the
cell cycle (%)
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Total number of all cells
4.
Calculate the percentage of time cells spend in each phase: 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 cells in that stage.
EXAMPLE CALCULATION: Percent of cells in stage X 1,440 minutes = _______minutes of cell cycle spent in stage.
ANALYSIS:
1. Using your data table draw a pie chart to represent your quantitative data (label the chart properly). Show the number of
minutes that onion root tip cells spend in each phase of cell division. Color each phase on your graph with colored pencils.
Produce a key that corresponds with the color on your graph. Draw/color your pie chart on a separate sheet of paper and
attach to this handout.
2. Why are onion root tips selected for the study of mitosis?
3. Why did you pool your data with your lab partners’ data?
4. The onion plant began as 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 that you observed? (Give the answer in terms
of X.) How do you know?
5. What can you conclude about the proportion of time cells spend in M-phase of the cell cycle versus the whole cell cycle?
6. BONUS (3 points): Obtain a prepared “White Fish Blastula” slide from the teacher. A blastula is an early stage of
development, occurring after the sperm has fertilized the egg. Underneath your pie chart, create the heading “White Fish
Blastula”. Draw and title a cell in each stage of the cell cycle (Interphase, Prophase, Metaphase, Anaphase, Telophase,
Cytokinesis). Drawings must be clear and neat to receive points.