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A student prepared a stained squash of cells from the root tips of garlic to calculate a mitotic index - AQA - A-Level Biology - Question 3 - 2022 - Paper 3
Question 3
A student prepared a stained squash of cells from the root tips of garlic to calculate a mitotic index. He: 1. cut the end 5 mm from 10 garlic roots 2. placed the ro... show full transcript
Worked Solution & Example Answer:A student prepared a stained squash of cells from the root tips of garlic to calculate a mitotic index - AQA - A-Level Biology - Question 3 - 2022 - Paper 3
Step 1
Suggest why the student soaked the root tips in hydrochloric acid in step 2.
Answer
Soaking the root tips in hydrochloric acid serves multiple purposes:
- Breaking Down Cell Walls: The acid helps to break down the links between the cell walls, which facilitates the separation of the cells, allowing them to be observed more easily under a microscope.
- Allowing Stain Entry: It permits the stain used for observation to diffuse into the cells effectively, improving visibility under microscopic examination.
Step 2
Explain why pressing the coverslip downwards enabled the student to observe the stages of mitosis.
Answer
Pressing the coverslip downwards creates a single-layer of cells, which allows for a clearer view of the individual cells. Additionally, it spreads out the cells, enabling better light penetration, which is essential for observing the various stages of mitosis under the microscope.
Step 3
Name the stage of mitosis shown in cell G. Explain the appearance of this cell.
Answer
Stage of mitosis: Anaphase.
Explanation: During anaphase, the chromatids are pulled apart towards opposite poles of the cell. This is visible as the cell exhibits a V-shape appearance, with the chromosomes moving away from the center. The separation indicates that the genetic material is being equally distributed in preparation for cell division.
Step 4
Use Figure 6 to calculate a mitotic index for the cells in this field of view.
Answer
To calculate the mitotic index, count the number of cells in mitosis (e.g., the stages observed) and divide by the total number of cells observed. For example, if there are 3 cells in mitosis out of a total of 30 cells, then:
ext{Mitotic Index} = rac{ ext{Number of Cells in Mitosis}}{ ext{Total Number of Cells}} = rac{3}{30} = 0.1
Thus, the mitotic index is 0.1.
Step 5
Apart from student errors, suggest two explanations why students calculated different mitotic indices.
Answer
- Differences in Sample Preparation: Garlic roots can vary in age or growth conditions. Different root tips may exhibit varying stages of mitosis due to developmental differences.
- Field of View Variation: Students may have looked at different areas of the slide or examined varying numbers of root tips, leading to discrepancies in the total cell count and observed mitotic figures.