LAB 2
THE CELL CYCLE AND THE STAGES OF MITOSIS
LEARNING OBJECTIVES
AFTER COMPLETING THIS LABORATORY, YOU SHOULD BE ABLE TO:
1.
2.
3.
Describe the cell cycle.
Identify stages of mitosis from prepared slides.
By examining the mitotic region of an onion root tip (Allium cepa), calculate the percentage of time a cell
spends in the various stages if the cell cycle.
THE CELL CYCLE
Mitosis is the process of cellular division that is responsible for cell and tissue growth and development. For this
reason it only occurs in areas known as ‘zones of growth’. In plants this happens in the cambium layers and one of
these areas of cell division is located at the root tip.
The root tip of and onion is a location of growth and consists of a terminal quiescent zone, surrounded by actively
dividing cells and covered with a protective root cap. Behind the root cap, three regions are apparent as the cells
begin differentiating: the region of cell division at the tip of the root; behind this lies the region of elongation; and
behind this lies the region of differentiation.
Within the region of cell division, you should be able to locate many stages of mitosis taking place.
Cells come into existence through the division of their parent cells and most of the cells divide in turn producing
daughter cells. Usually, this occurs during mitosis, when genetic material is duplicated, and one copy is passed on
to each daughter cell. Mitosis is generally followed by cytokinesis, or cytoplasmic division, in which the rest of the
cell divides in half forming two new cells. Sometimes, however, cytokinesis does not occur and a cell with many
nuclei is formed.
Mitosis is just one part of the cell cycle (see Figure 1.3)
FIGURE 1.1: THE CELL CYCLE
The cell cycle illustrated in the figure above can be summarized as follows:
TABLE 1.1 – A HYPOTHETICAL CELL CYCLE
Phase
Percentage of Time at Phase
Interphase
(G1, S-phase, G2)
87.5
Mitosis
Prophase
6.9
Metaphase
2.2
Anaphase
1.7
Telophase
1.7
Total
100
ACTIVITY 1: OBSERVING THE STAGES OF MITOSIS
(5 MARKS PER DRAWING- TOTAL 20 MARKS)
MATERIALS



Unlined paper
Pencil
Computer access (for microscope)
PROCEDURE
1. Log into the RWSL Microscope. You will be examining a slide of the mitotic region of an onion root tip
(Allium cepa).
2. Make proper biological drawings of cells in each of the four stages of mitosis (see Appendix A). Drawings
should be large (one per page) on blank paper. They must be correctly headed and labeled and include
drawing magnification. Please include a calculation of drawing magnification on the back of the page.
3. Figure 1.2 illustrates the kind of drawing you should submit.
4. The drawing will be submitted to your instructor via mail.
FIGURE 1.2: EXAMPLE DRAWING OF A STAGE OF MITOSIS IN AN ONION ROOT CELL
Onion Root Cell
Allium cepa (* see below)
Metaphase
X1100
*
As mentioned in Appendix A, underline genus and species names in your drawings.
ACTIVITY 2: CALCULATING THE PERCENT TIME SPENT IN EACH STAGE OF MITOSIS
At the time when a slide of an onion root tip was prepared, the cells in the region of cell division were arrested at their
current phase within the cell cycle. Some were fixed at the time of interphase and others were fixed at some stage of
mitosis. The duration of each stage in the cell cycle of the onion root tip can be estimated by determining the proportion
of cells arrested at each stage of mitosis and interphase.
Let’s assume that you examined a slide and determined the stage at which 100 cells were arrested at time of
fixation. Table 1.2 is a summary of your results.
It is known that onion root tip cells take about 16 hours to complete the cell cycle. By determining the percentage
of cells in each stage of mitosis and in interphase, you can calculate the amount of time spent in each stage. For
example, if ten cells out of 100 were found to be in prophase, the percentage of cells is 10/100 x 100 = 10%. This
shows that any one of the hypothetical cells spends 10% of the time in prophase, so they spend 0.10 x 16 hours or
1.6 hr (1 hr and 36 min) in that stage.
PROCEDURE
Examine the slide provided of an onion root tip. Count and record the stages of the cycle of each of the cells in
your field of view, then calculate the percentage of cells in each stage, and enter the results in Table 1 (there is no
calculation if you have the patience to count exactly 100 cells; just enter the number of cells). Keep in mind that
you must count enough cells to make a representative sample. If you count too few your data will likely reflect the
hypothetical estimates provided earlier. If you are using the highest power ocular, you may have to record more
than one field of view.
Calculate the hours and minutes spent in each stage, assuming the entire cell cycle takes 16 hours (0.1 hr = 6 min).
TABLE 1.2 DATA ON STAGES IN PLANT CELLS
Cell Cycle Stage
Number of Cells
% of Total Cells
Hours and Minutes
in the Stage
in the Stage
in Stage
100
16:00
Interphase
Prophase
Metaphase
Anaphase
Telophase
Total
(Calculations 5 marks)
DISCUSSION QUESTIONS
1.
2.
3.
4.
How do your data compare to that of the hypothetical times given in Table 1.1? If it is not exactly the
same, discuss some of the reasons this might be. (3 marks)
Why is the tip of the onion root chosen for this activity? (1mark)
How do the sizes of cells in Cytokinesis compare with those in Prophase? Why? (2 marks)
Compare and contrast mitosis and meiosis. You may use drawings to illustrate and add to your discussion.
(10 marks)