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.
MethodsEach 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.ResultsBoth 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).
