In this lab, we each looked at 3 onion root tip samples. In each sample ,we were asked to examine the many cells in the root tip and look at the cell life cycles in each sample. We counted how many cells were in each cycle in each cross section to get an idea of how long cells spend in each cell life cycle.
Purpose: To determine how long cells spend in each phase of the cell life cycle
Background: During the cell cycle, a cell is first in interphase, where the DNA is jumbled while being read. The cell later moves to the stage, prophase, where DNA is made into chromosomes and in an unorganized fashion. When looking at this in a cell, it can look dotted or spotty. Later, the chromosomes move to the center of the cell, attached by spindles in metaphase. During anaphase, the chromatids separate to opposing sides of the cell until it finally splits into two daughter cells in telophase. During telophase, a new cell wall is made between the split chromosomes. Cells spend the longest time in interphase when the cell is functioning and spends the least amount of time during the mitotic phases of metaphase, anaphase, and telophase.
- place onion root tip slide under microscope and magnify it 400 times, making sure it’s in focus.
- count the average number of cells seen in the microscope by counting the cells on the outer length and width then multiplying it.
- count the amount of cells in interphase, prophase, metaphase, anaphase, and telophase, then calculate the percentages per phase, making sure the percentages add up to 100%.
- repeat for the other two cross sections
Analysis and Conclusion:
- What stage were the majority of the cells in? interphase
- What percentage of the cells were in each stage?
- interphase: 81%
- prophase: 12.46%
- metaphase: 1.65%
- anaphase: 1.37%
- telophase: 2.03%
- What evidence shows that mitosis is a continuous process, not a series of separate events? It is a cycle so it shows that the cell goes from telophase then repeats the process. Each phase is dependent on the preceding stage.
- The onion plant began as a single cell. That cell had X number of chromosomes. How many chromosomes are in each of the cells that you observed? How do you know? There were 4X chromosomes because at the end of interphase there are 4X chromosomes. Cells double themselves and then split, so even though we start with X, after the cell cycle finishes it ends with 4X.
- If this onion would reproduce sexually, it would need to produce sperm and/or eggs by the process of meiosis. After meiosis, how many chromosomes would be in each sex cell? 1 4X –> 2 2X –> 4 1X. 2X are the daughter cells and 1X are the sex cells.
- If this onion would complete the process of sexual reproduction how many chromosomes would be in the zygotes that are produced? The sperm and egg cells combine during reproduction and since each sex cell is 1X, then combining the two would make 2X.
When looking at all the onion root tips it was obvious to see that cells spend a majority of their time in interphase. Some data was removed to create a more accurate class average. The highest percentage was interphase at 81%, following was prophase at 12.46%, then metaphase at 1.65%, then anaphase at 1.37%, then telophase at 2.03%. This was expected but not exactly accurate. There was expected trial error when counting the cells. The microscope could only zoom 400 times and cells could be better counted at a higher magnification. Also, we were not looking at a square cross section so the amount of cells present was an estimated guess. Another possibility for error was that there was confusion between prophase and interphase because sometimes they look similar. Also counting through a microscope was hard because it was difficult to remember if we had counted cells before so percentages were off and not accurate.