An Investigation into the Mitotic Nuclear Division of Allium Sativum Root Tip Cells, and the Relative Duration of Each Phase of this Cellular Cycle

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An Investigation into the Mitotic Nuclear Division of Allium Sativum Root Tip Cells, and the Relative Duration of Each Phase of this Cellular Cycle.

Aim

To conduct an investigation into the relative durations of the phases that occur in the mitotic nuclear division of meristematic Allium Sativum root tip cells, evaluating the validity of a hypothesis proposed that states that these mitotic phase durations will be of different.

Hypothesis

The Expected Results: The Relative Length of Each Stage of Mitotic Nuclear Division

Through the process of conducting background research, it has been possible to suggest a theoretical ratio concerning the timing for each stage of the mitotic cycle;

“Although the stages of mitosis are necessarily shown as static events, it must be emphasized that the process is a continuous one and the names “anaphase”, “metaphase”, etc., do not imply that the process of mitosis comes to a halt at this juncture. Moreover, the stages shown are not selected at regular intervals of time, e.g. in the embryonic cells of a particular grasshopper the timing at 38°C is as follows: prophase 100 min, metaphase 15 min, anaphase 10 min, telophase 60 min.”  

These specified times essentially form the ratio that will be used throughout the investigation (dividing each stage in the ratio by a factor of 5 provides the ratio (20:3:2:12), and allow a range of hypotheses to be formed. Although it is therefore possible to make a quantitative hypothesis concerning the physical number of cells undergoing mitosis in a sample of 50 meristematic allium cells, it is considered better scientific practice in this investigation to compare the existing, researched ratio to the experimental data that will be gained. In this manner, a greater amount of comparative biological analysis can be conducted; using the information and environment concerning both the researched ratio and the experimental ratio to offer some insight and scientific reason into why the length of stages varies, or remains constant. In this investigation, it is believed that the durations of each phase in the process of mitotic nuclear division vary, for the following biological and biochemical reasons.

Stage B: Prophase: 20 relative time units

Prophase is the cellular cycle stage of mitosis in which chromatin condense into a highly ordered structure called a chromosome (7, 8). Since the genetic material has been duplicated, there are two identical copies of each chromosome in the cell. The other important cellular organelles in the prophase stage are the centrioles (1). During prophase, the two centrioles (which replicate independently of mitosis) begin synthesizing microtubules, and forming a mitotic spindle between them. By increasing the length of the spindle, (growing the microtubules) the centrioles push apart to the poles of the cellular nucleus. The nuclear membrane also disappears (12), breaking down into smaller, microscopic vesicles that are distributed throughout the cellular body. The vast number of biological processes occurring in this mitotic phase mean that it should have the longest duration of the cycle. Similarly, prophase should also have the greatest energy requirement, revealing that a possible reason for the relatively long duration of the phase is the amount of energy that must be supplied to the cell for the process of nuclear division to occur.

Stage C: Metaphase: 3 relative time units

During metaphase, each of the centrioles reaches a polar region of the cell. The centromeres of the chromosomes convene along the equator of the cell, a line that is equidistant from the two centrosome poles. This occurs due to the interactions of the centrioles and the microtubule spindle fibres and the forces exerted on the centromere of the chromosomes. Because it is the locomotive forces provided by the microtubule spindle fibres that act upon the chromosome centromeres in this stage, this phase requires a relatively small amount of energy compared to that of prophase, and hence has a relatively brief duration.

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Stage D: Anaphase: 2 relative time units

Within anaphase two distinct, comparable processes occur. During early anaphase the chromatids abruptly separate and move towards the relative poles of the cell. This is achieved by the shortening of the microtubule spindle fibres, and the forces that are mainly exerted at the centromeres. When the chromatids are fully separated, the later stage of anaphase begins. This involves the microtubules elongating and sliding relative to each other to drive the spindle poles (and hence centrioles) further apart, encouraging elongation and constriction of the cell membrane and cytoplasm. Due to the nature of the forces exerted in this stage of this process, the cell requires minimal amounts of energy in comparison to the other stages, and hence has a far shorter duration that the other stages involved in this form of nuclear division.

Stage E: Telophase: 12 relative time units

At this stage, the microtubules continue to lengthen, further elongating the cell. Corresponding sister chromosomes attach at opposite ends of the cell. A new nuclear envelope, using fragments of the parent cell's nuclear membrane, forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. Cytokinesis may occur at the same time the nuclear envelope is reforming, although both mitosis and cytokinesis are distinct processes. In plant cells, vesicles derived from the Golgi apparatus move to the middle of the cell along microtubule scaffold. This structure directs packets of cell wall materials which coalesce into a disk-shaped structure called a cell plate. The cell plate eventually develops into a proper cell wall, separating the two nuclei. As Telophase is essentially the reverse of Prophase occurring in a dividing cellular body, it can be assumed that the energetic requirements of the Telophase stage are almost as great of that of prophase, and hence that the duration of the phase is almost as long. Telophase must also be assumed to be a relatively long phase in the cycle due to the large number of biological reactions and methods of transport occurring, not only in the formation of one cellular nucleus, but two distinct nuclei.

The Null Hypothesis

In order to demonstrate that the expected results and the experimental, observed results are concurrent to an appropriate degree, it is necessary to conduct some form of statistical analysis. This is where the concept of a null hypothesis is involved; in statistics, a null hypothesis is a hypothesis set up to be nullified or refuted in order to support an alternative hypothesis. When used, the null hypothesis is presumed true until statistical evidence in the form of a hypothesis test indicates otherwise. It is in this manner that the concordance of both the expected and observed results will be analysed; comparing the difference (and hence, degrees of freedom) between the cell counts. An appropriate biological and genetic model to use when comparing these cellular quantities is the Chi-Squared test; an assessment that can be directly used to evaluate the level of difference between the predicted results (which will be calculated from the suggested time ratio in the background research section), and the observed results (which will be gained through the experimental process). It is hypothesized that the null hypothesis will be accepted; demonstrating the relationship stated in the initial hypothesis; the statement of the expected phase durations.

Correlation between Inter-Species Organisms, i.e. between eukaryotic plant and animal cells.

In the process of background research, the approximate timing for each of the stages of mitotic nuclear division in a sample of embryonic grasshopper cells has been offered. In addition to analysing the concept of a null hypothesis; a detailed analysis can also be completed pertaining to the correlation between the relative timings for each phase between both the embryonic grasshopper, and root tip allium cells. Although it would seem unlikely that such a comparison could be made effectively, the cells in question share a wealth of common features, and the significance of mitosis in each is essentially the same. As both cells are eukaryotic in nature, the process of mitotic nuclear division should be related in each, even when accounting for the cytological differences between the cells (for example, the allium cells are plant-like in nature and will lack the centrioles present in the embryonic animal cells). A major similarity between the cells in question is the rôle and biological significance of the cells in their respective organism; the root tip cells of the allium are meristematic in nature; a characteristic that is analogous to stem cells in animals, as both are undifferentiated (or totiopotent/pluripotent, to use the correct biological vocabulary). It is for these suggested reasons that there should be a minimal amount of difference between the cellular mitotic phase lengths of each of the samples of data in question, a theory that may be further evaluated using statistical means.


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Results

The Raw Results

A Table of Results to Demonstrate the Relative Quantities of Cells Undergoing each Relevant Stage in the Process of Mitotic Nuclear Division

Calculating an Average Number of Cells that were Observed at Each Phase of Mitosis

Please Note: Unless explicitly stated, the analysis of results and the evaluation of the investigation will refer to the mean average of data calculated. It should additionally be noted that the values contained in the above table (especially the average results) have been rounded to whole numbers of cells to ...

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This is a very detailed and well written report. 1. Researched information should be referenced. 2. Several sections, including the safety and limitations sections, are extremely detailed and show a professional approach. 3. The evaluation should suggest further research opportunities. 4. Some of the sentences are a bit long winded. *****