1. 1
   A student made a stained squash of a garlic (Allium sativum) root and counted 400 cells in each of two regions. In the region within 1 mm of the root tip, 312 cells were in interphase, 38 in prophase, 18 in metaphase, 12 in anaphase and 20 in telophase. In a region 5 mm behind the tip, 392 cells were in interphase, 5 in prophase, 1 in metaphase, 1 in anaphase and 1 in telophase. Cells in the region 5 mm behind the tip were noticeably longer than those near the tip.

   (a)

   What is the mitotic index of the cells within 1 mm of the tip?

   [1 mark]

   • A0.22
   • B0.28
   • C0.78
   • D0.095

   (b)

   Which is the best explanation for the lower mitotic index 5 mm behind the tip?

   [1 mark]

   • ACells 5 mm behind the tip divide so quickly that they are rarely seen in mitosis
   • BCells near the tip are dividing by meiosis rather than mitosis
   • CCell proliferation for growth is concentrated in the apical meristem, while cells further back are enlarging and differentiating
   • DCells 5 mm behind the tip are all dead, so they cannot divide

   (c)

   Justify the conclusion that interphase is the longest stage of the cell cycle in the cells near the root tip.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Researchers grew human cells in culture in which the histone proteins carried a red fluorescent label and the tubulin of microtubules carried a green fluorescent label, and filmed a cell dividing. First, the red-labelled material in the nucleus became gathered into short, compact rods. Next, green fibres from both ends of the cell attached to every rod and the rods lined up across the middle. Then each rod split into two and the halves moved apart towards opposite ends as the green fibres shortened. Finally, a groove appeared around the middle of the cell and deepened until two cells were formed.

   (a)

   Why did the red-labelled material become gathered into compact rods?

   [1 mark]

   • AThe nuclear membrane was folding around each chromosome
   • BHistones were forming the spindle microtubules
   • CDNA was being replicated to form the chromatids
   • DDNA was supercoiled around histones, condensing the chromosomes

   (b)

   What caused the groove around the middle of the cell?

   [1 mark]

   • AVesicles fusing to form a cell plate
   • BA ring of actin and myosin contracting and pulling the plasma membrane inwards
   • CSpindle microtubules pushing the membrane outwards at the poles
   • DHistones binding to the plasma membrane

   (c)

   Explain why chromosomes must be condensed before they are moved to the poles.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In early sea urchin embryos, cells divide rapidly and in step with each other. Researchers measured the concentration of a protein called cyclin B through several cell cycles. Its concentration rose steadily through interphase, reached a peak just before the start of mitosis, then fell sharply to almost zero during mitosis before rising again in the next cycle. When the embryos were treated with a drug that blocks protein synthesis, cyclin B did not accumulate and the cells remained in G2 without entering mitosis. When cells were engineered to make a form of cyclin B that cannot be broken down, they entered mitosis but did not complete it and did not divide.

   (a)

   Outline the events of the cell cycle from the start of G1 to the end of cytokinesis, including the growth that occurs during interphase.

   [3 marks]

   (b)

   Using the data, explain how cyclins control progression through the cell cycle.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A 58-year-old patient has a tumour in the wall of the colon. A biopsy shows that its cells divide rapidly and have spread into the muscle layer beneath the lining of the bowel. Sequencing of tumour cells reveals a mutation in the KRAS gene, a proto-oncogene, which makes the KRAS protein permanently active, and loss of both functional copies of the APC gene, a tumour suppressor gene. Scans reveal two small tumours in the liver, made of cells that resemble colon lining cells. A second patient has a lipoma: a soft, slow-growing lump of fat cells under the skin, enclosed by a capsule, which has been the same size for five years and has not spread.

   (a)

   Explain how mutations in proto-oncogenes and tumour suppressor genes can lead to uncontrolled cell division, with reference to this patient.

   [6 marks]

   (b)

   Using the information, compare the colon tumour and the lipoma, and discuss which of them should be described as cancer.

   [6 marks]

   Total for question 4: 12 marks

End of questions