USE OF ONION ROOT TIPS FOR MITOTIC DIVISION OBSERVATIONS

Mark Meredith

15247 N. State Hwy 28, Dardanelle, AR72834

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 / Class
Number of Cells/Stage / Duration (min.) / Number of Cells/Stage / Duration (min.)
  1. Total number of nuclei
  2. Number in prophase
  3. Number in metaphase
  4. Number in anaphase
  5. Number in telophase
  6. Total mitotic figures
  7. Number in interphase
/ 357
11
3
3
3
20
337 / ―
44.37
12.10
12.10
12.10
―
1359.33 / 3437
193
39
33
91
347
3090 / ―
80.86
16.34
13.83
38.13
―
1294.62

Table 2. Mitotic index at various levels in the root tip

Level / Student / Class
Number of Nuclei / Number of Mitotic Figures / Mitotic Index / Number of Nuclei / Number of Mitotic Figures / Mitotic Index / %
IA
IB
IC
II
III
IV / 54
49
51
123
93
72 / 7
7
6
22
14
7 / 12.96
14.29
11.76
17.89
15.05
9.72 / 1627
1560
1350
2728
2082
1287 / 97
142
119
169
70
17 / 6.0
9.1
8.8
6.2
3.4
1.3 / 65.9
100
96.7
68.1
37.4
14.3

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.