Experiment 1: Observation of Mitosis in a Plant Cell
In this experiment, you will look at the different stage of mitosis in an onion cell. Remember that mitosis only occupies one to two hours of the cell cycle, whereas interphase can take anywhere from 18 – 24 hours. Using this information and the data from your experiment, you can estimate the percentage of cells in each stage of the cell cycle.
Materials
Onion (Allium) Root Tip Digital Slide Images
Procedure:
Part 1: Calculating Time Spent in Each Cell Cycle Phase
- The length of the cell cycle in the onion root tip is about 24 hours. Predict how many hours of the 24 hour cell cycle you think each step takes. Record your predictions, along with supporting evidence, in Table 1.
- Examine the onion root tip slide images on the following pages. There are four images, each displaying a different field of view. Pick one of the images, and count the number of cells in each stage. Then count the total number of cells in the image. Record the image you selected and your counts in Table 2.
- Calculate the time spent by a cell in each stage based on the 24 hour cycle using the equation below. Record your data in Table 2:
Hours of Stage =
24 x Number of Cells in Stage
Total Number of Cells Counted
Part 2: Identifying Stages of the Cell Cycle
- Observe the images of the onion root tip.
- Locate a good example of a cell in each of the following stages: interphase, prophase, metaphase, anaphase, and telophase.
- Draw the dividing cell in the appropriate area for each stage of the cell cycle, exactly as it appears. Include your drawings in Table 3.
- Label the cell cycle stage on the onion root tip image (located after the tables).
The Correct Answer and Explanation is :
To analyze mitosis in onion root tip cells, follow the given procedure and calculations.
Part 1: Calculating Time Spent in Each Cell Cycle Phase
Using the total cell cycle time of 24 hours, we determine the time spent in each phase based on the number of cells observed in each stage. The formula is:
[
\text{Hours of Stage} = \left(\frac{24 \times \text{Number of Cells in Stage}}{\text{Total Number of Cells Counted}}\right)
]
Using the images provided (which I currently cannot view), typical results for an onion root tip slide may show:
| Stage | Predicted % of Cycle | Observed Number of Cells | Calculated Hours in Stage |
|---|---|---|---|
| Interphase | 80-90% | ~80 | ( \frac{24 \times 80}{100} = 19.2 ) hrs |
| Prophase | ~5% | ~10 | ( \frac{24 \times 10}{100} = 2.4 ) hrs |
| Metaphase | ~3% | ~6 | ( \frac{24 \times 6}{100} = 1.44 ) hrs |
| Anaphase | ~1-2% | ~2 | ( \frac{24 \times 2}{100} = 0.48 ) hrs |
| Telophase | ~1-2% | ~2 | ( \frac{24 \times 2}{100} = 0.48 ) hrs |
These numbers may vary depending on your specific observations.
Part 2: Identifying Stages of the Cell Cycle
- Interphase: The longest stage where the cell grows, performs normal functions, and prepares for division. The nucleus is visible, and DNA is duplicated but not yet condensed.
- Prophase: Chromatin condenses into visible chromosomes, the nuclear envelope starts to break down, and spindle fibers begin forming.
- Metaphase: Chromosomes align in the middle of the cell, attached to spindle fibers.
- Anaphase: Sister chromatids separate and move toward opposite poles.
- Telophase: Chromosomes de-condense, nuclear envelopes reform, and cytokinesis (cell division) begins.
Explanation (300 words)
Mitosis is essential for growth, repair, and asexual reproduction in plants. The onion root tip is an ideal specimen for studying mitosis because it has rapidly dividing cells. Interphase is the longest phase, taking up 80-90% of the cycle, as the cell prepares for division by growing and replicating DNA.
Mitosis itself consists of four stages. Prophase is characterized by the condensation of chromatin into chromosomes, spindle fiber formation, and nuclear envelope breakdown. Metaphase follows, where chromosomes align at the cell’s equator due to the action of spindle fibers. Anaphase occurs next, with sister chromatids pulled apart toward opposite poles, ensuring each daughter cell will receive an identical set of chromosomes. Finally, in telophase, nuclear membranes reform around the separated chromosomes, which then decondense, marking the end of mitosis.
Cytokinesis follows, dividing the cytoplasm and forming two identical daughter cells. Since mitosis only lasts for 1-2 hours, interphase is predominant when observing onion root tip cells. The distribution of cells in different stages can be used to estimate the relative duration of each phase, reinforcing the understanding that mitosis is a quick but crucial process in the cell cycle.
If you need calculations based on specific images, upload the cell count data, and I can process it accurately.