4 2 A student investigated the number of dividing cells in an onion root tip. The student made a root tip squash and counted the number of cells at each stage of the cell cycle. The table shows the results. Number of cells at each stage of the cell cycle Prophase Metaphase Anaphase Telophase Interphase 4 2 1 1 90 (a) (i) Calculate the mitotic index for these results using the formula Mitotic index = number of actively dividing cells total number of cells × 100 (2) Answer .............................................................. (ii) The mitotic index depends on the distance from the root tip. The graph shows the relationship between the mitotic index and the distance from the root tip. 2.0 1.5 1.0 0.5 0 14 12 10 8 6 4 2 0 Distance from root tip / mm Mitotic index Turn over 5 Explain the relationship shown in the graph. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (b) The student used the following method to prepare the root tip squash. 1. Cut a small piece from the tip of a growing onion root and place it in weak acid. 2. Transfer the root tip to a microscope slide. 3. Add a drop of stain. 4. Use a mounted needle to flatten the piece of root and place a coverslip on top. 5. View using low power objective lens of a microscope. 6. Count the cells at each stage of the cell cycle. Justify two improvements to this method. (2) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (Total for Question 2 = 6 marks)