USE OF ONION ROOT TIPS FOR MITOTIC DIVISION
OBSERVATIONS
Mark Meredith
15247 N. State Hwy 28, Dardanelle, AR 72834
Introduction
The zone of cell division on the onion root tip is a great place to
observe mitotic cell divisions. Through the observation and comparison of
different regions of the root tip, the duration of mitotic stages and mitotic
indexes are determined for the various divisions of the root tip.
Methods
Each group used a compound microscope to analyze the different
sections of the onion root tip. The root tip was visually divided into four
sections sized by the field of view through the microscope. Section I, the
section nearest the tip, was subdivided into three divisions, A, B, and C. The
first section, Section I, was used for determining the duration of mitotic
divisions. The cells we then counted and classified on the basis of their
mitotic stage. Cells not in a mitotic stage but in interphase were also
counted. The total number of cells within the first section was determined by
combining the numbers from the different classifications. The duration of
mitotic stages was then determined using a formula based on the assumption
that from the beginning of interphase to the end of telophase lasts about 24
hours.
The mitotic index (MI), which is the percentage of cells undergoing
mitosis, was determined for each division of the root tip. In order to
determine the mitotic index for each region, the total number of nuclei must
first be determined, and from that number, the number of cells undergoing
some stage of mitosis must be determined. Knowing both numbers from a
section allows for the calculation of the mitotic index. Knowing the mitotic
index for each division will allow for identification of regions undergoing the
most mitotic activities.
Results
Both the results from our group and the combined class results
indicated similar mitotic stage durations (Table 1). Both data sets show that
the longest mitotic duration occurs during prophase. Because prophase has
the longest duration, it is understood that more cells will be undergoing
prophase rather than any other mitotic stage. By determining the mitotic
index, the area experiencing the most mitotic division is determined (Table
2). A comparison of the mitotic index percentage of each stage shows the
area of highest mitotic action and the gradual decline in division as the
samples move from the zone of cell division to the zone of cell elongation
(Figure 1).
Table 1. Duration of mitotic stages
Student
1.
2.
3.
4.
5.
6.
7.
Total number of nuclei
Number in prophase
Number in metaphase
Number in anaphase
Number in telophase
Total mitotic figures
Number in interphase
Number of
Cells/Stage
357
11
3
3
3
20
337
Class
Duration Number of
(min.)
Cells/Stage
―
3437
44.37
193
12.10
39
12.10
33
12.10
91
―
347
1359.33
3090
Duration
(min.)
―
80.86
16.34
13.83
38.13
―
1294.62
Table 2. Mitotic index at various levels in the root tip
Student
Class
Level
Number
Number
Number
of
of
of
Mitotic
Mitotic Number of Mitotic
Mitotic
Nuclei
Figures
Index
Nuclei
Figures
Index
IA
54
7
12.96
1627
97
6.0
IB
49
7
14.29
1560
142
9.1
IC
51
6
11.76
1350
119
8.8
II
123
22
17.89
2728
169
6.2
III
93
14
15.05
2082
70
3.4
IV
72
7
9.72
1287
17
1.3
%
65.9
100
96.7
68.1
37.4
14.3
120
Mitotic Index %
100
80
60
40
20
0
Series1
IA
IB
IC
II
III
IV
65.9
100
96.7
68.1
37.4
14.3
Level
Figure 1. Comparison of mitotic index and average cell length at various
levels in the root tip. Mitotic index and average cell length expressed as
percentage of maximum observed.
Discussion
The first section of the experiment looked at the zone of cell division
on the root tip. By determining the number of cells experiencing the different
stages of mitosis, the duration of each stage and most abundant stage was
determined. Prophase was by far the most abundantly represented stage in
this portion of the onion root. The large amount of prophase cells is due to
the large duration of prophase in comparison to the durations of the other
mitotic stages. A cell in prophase is undergoing the condensing of sister
chromatids and the development of mitotic spindle from microtubules that
are produced by centrosomes. It is understandable that these processes take
more time when compared to the durations of the subsequent mitotic stages.
Mitotic stages aside, cells in interphase are far more abundant. Put simply,
interphase is the stage in which a cell exists while not undergoing mitosis.
The remainder of the experiment sought to determine the amount of
mitotic figures within each section of the root tip. With this knowledge the
mitotic index can be calculated and converted into a percentage that then can
be used to determine the sections most experiencing mitotic divisions. As
was expected, the results shown that the region shortly behind the root cap,
also known as the zone of cell division, had the highest number of mitotic
figures present. The results of our group held fairly consistent with the data
of the combined class (Table 2). The percentage of the mitotic indexes were
then graphed showing the increase in mitotic figures shortly after the root cap
and the gradual decrease as the sections move from the zone of cell division
to the zone of cell elongation.
Appendix
Duration of mitotic stages:
Time / mitotic stage = (number of cell / stage) / (total number of cell) x (24hr
/ 1 mitotic) x (60 min. / 1hr)
Mitotic Index:
M.I. = (number of mitotic figures / number of nuclei) x 100
*To calculate %, use the highest mitotic index as 100% and divide other
mitotic index values by it.
References
Brooker, R. J. 2005. Genetics: Analysis and Principles, Second Edition.
McGraw Hill, New York, New York, USA.
Cohen, NS 1969. Elements of Cytology, 2nd ed. New York: Brace and
World
Jensen, WA and Kavaljian LG 1958. An analysis of cell morphology and the
periodicity of division in the root tip of Allium cepa. Am. J. of
Botany 45:365-372.
Mazia, D 1961. Mitosis and the physiology of cell division. In J Brachet &
AE Mirsky (eds.) The Cell. Vol 3 NY: Academic Press.