WJEC Biology for AS: Study and Rev Guide

Isla’s answer a) During prophase the nuclear membrane disappears ✓ and the chromosomes condense appearing as two distinct chromatids ✓ joined at centromere. ✓ b) Interphase occurs in both experiments as DNA quantity doubles. ✓ When placed in the new drug, there is no halving of DNA at 20 hours as seen in the garlic grown in water. ✓ This shows that there is no cytokinesis because the lack of spindle fibres prevents anaphase. ✓ Examiner commentary Isla has learnt the key events in prophase and was able to relate the graph to her knowledge of cytokinesis. Isla achieves 6/6 marks Exam tip Learn the key events in mitosis (and meiosis) and be able to recognise drawings of each stage. You should be able to use data given to reach or support a conclusion. Ceri’s answer a) During prophase the chromosomes condense ✓ and centrioles appear. ✗ q b) When placed in the drug, DNA does not decrease, ✗ so cell division has not happened. ✗ w Examiner commentary q Reference to centrioles appearing is incorrect as they are absent in higher plants. w Reference to DNA decreasing is too vague. The question asks how the results show that the new drug inhibits mitosis: references to cell division not occurring is also too vague, when information in the question details that the new drug prevents spindle formation. Ceri should link this to anaphase not occurring, and that subsequently cytokinesis will not occur. Ceri achieves 1/6 marks Q & A 5 133 Questions and answers The diagram on the right shows the relative lengths of the cell cycle in actively dividing cells taken from the root tip of a garlic plant. (a) Describe the changes that occur to the nucleus of a plant cell during prophase. [3] (b) A new drug has been developed that inhibits mitosis by preventing the formation of the spindle fibres. Garlic bulbs were grown in a solution of the new drug and the quantity of DNA present in a cell from the root tip was measured over the 24-hour length of the cell cycle. The results are shown below together with the results from garlic bulbs grown in water. Using your knowledge of the cell cycle, explain how the results of this experiment show that the new drug inhibits mitosis. [3] Prophase Metaphase Anaphase Telophase Cytokinesis 0 0 2 4 6 4 8 Time / hrs Time / hrs Garlic grown in new drug solution Garlic grown in water Quantity of DNA per cell / arbitrary units Quantity of DNA per cell / arbitrary units 12 16 20 24 0 0 2 4 6 4 8 12 16 20 24