Basic HTML Version
Table 3
Phases of mitosis Class totals
Percent of
total count
% Estimated time
spent in phase
Estimated time spent
in mitosis
(80’)
Estimated time spent
in cell cycle
(16 h)
Mitotic index
Interphase
78
Prophase
28
Metaphase
3
Anaphase
6
Telophase
9
Total Cells Counted
124
have just determined. Use the following equation to find the
duration of each phase of mitosis in onion cells:
Duration of phase (in minutes) = percentage/100 x 80 minutes
Enter the time (in minutes) of each phase under the “Time (in
minutes)” column of the table.
4
Given that it takes on average 16 hours for onion root
tips to complete the cell cycle, calculate the average time spent
in each phase as follows and record the answers in Table 2.
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 hypothestical 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.
The mitotic index is calculated like this:
number of actively · dividing cells in field of view
Mitotic index = ––––––––––––––––––––––––––––––––––––––
total number of cells in field of view
The percentage of cells undergoing mitosis or it is defined
as the ratio of no. of cells in the dividing phase to the total
number of cells observed. This will help to identify the region
of most mitotic activities. Mitotic index help us to quantify the
cell division. Mitotic index decreases with increasing distance
from root tip. That means gradual decrease in cell division as
move from the zone of cell division to the zone of cell elon-
gation. The meristematic region in the root tip is the actively
growing region and thus the mitotic index is high.
Mitotic index is used to quantify the differences in cell division
when environmental parameters are changed. Studies are al-
ready proved that, the plants grown in space in microgravity
had a greater mitotic index than plants grown on the ground.
The gravity sensing signals in the root cap are unable to send
proper orientation signals which inhibit growth in the cells
that are distant from the root tip and root cap junction when
kept in zero gravity. This leads to mitosis in greater number of
cells and plant produce secondary roots also in high rate.
5
Use the data in Table 2 to create a pie chart or a histo-
gram with an
Excel
worksheet. The graph will show the per-
centage of time the cell spends in each phase of its life cycle.
6
Maybe you were not able to collect enough data with
the slide you had prepared. In this case, use the data in the
Table 3
to calculate the percentage of cells which are divid-
ing in relation to the total of cells. Finally, state the time,
in minutes, spent by the cells in each phase. Calculate the
mitotic index.
Questions
1
How does the mitotic index change as you move
away from the tip of the root?
2
How does the mitotic index change where there is
no gravity?
3
Calculate the mitotic index based on this micro-
photograph of onion root tip cells when they are divid-
ing. Organise the data in a table.
4
What is the meaning of a high mitotic index for a
stable animal tissue?
Osservare le fasi della mitosi
9