Mitosis and the cell cycleEdexcel A-Level Biology A: Subtopic test

1. 1
   A student prepares a root tip squash from garlic roots to observe the stages of mitosis. She cuts the last few millimetres from the tip of a root, warms it in hydrochloric acid, adds a stain that binds to DNA, and then squashes it under a coverslip before viewing it with a light microscope.

   (a)

   Why does the student cut her sample from the tip of the root rather than from further up the root?

   [1 mark]

   • AThe root tip has the thickest cell walls, which hold the stain best
   • BThe root tip contains meristem cells that are dividing by mitosis
   • CThe root tip is the only part of the root that contains chromosomes
   • DThe root tip contains xylem, which takes up the stain

   (b)

   Why does the student squash the root tip under the coverslip?

   [1 mark]

   • ATo break open the nuclei so the chromosomes are released
   • BTo separate DNA from protein so the stain can bind
   • CTo kill the cells at the same stage of mitosis
   • DTo spread the cells into a thin layer so that light can pass through and cells can be seen separately

   (c)

   State the purpose of (i) warming the root tip in hydrochloric acid and (ii) adding the stain.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Using a light microscope, the student examines a root tip squash and counts 200 cells. She finds 168 cells in interphase, 14 in prophase, 6 in metaphase, 4 in anaphase and 8 in telophase. The whole cell cycle in this species takes 18 hours.

   (a)

   Which description matches the cells that the student records as being in metaphase?

   [1 mark]

   • AChromosomes are lined up across the equator of the cell, attached to spindle fibres at their centromeres
   • BChromatids are being pulled towards opposite poles of the cell
   • CNuclear envelopes are re-forming around two groups of chromosomes
   • DChromosomes are condensing and the nuclear envelope is breaking down

   (b)

   What is the mitotic index of the 200 cells?

   [1 mark]

   • A0.84
   • B0.32
   • C0.16
   • D0.08

   (c)

   Calculate the mean time that a cell in this root tip spends in mitosis. Give your answer in hours to two significant figures.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A chemotherapy drug prevents spindle fibres from forming. Tumour cells exposed to the drug are found with condensed chromosomes, each made of two chromatids, scattered through the cell. The cells do not divide and many later die.

   (a)

   Explain why chromosomes cannot be separated into two daughter nuclei when spindle formation is prevented.

   [3 marks]

   (b)

   Suggest why this drug causes side effects such as hair loss and damage to the lining of the gut but has little effect on most other body cells.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A strawberry grower propagates a high-yielding variety using runners, which are stems that grow along the ground and form roots and a new plant at their tips. All the new plants are genetically identical to the parent plant.

   (a)

   Explain how a cell prepares for mitosis and how mitosis ensures that both daughter cells are genetically identical to the parent cell.

   [6 marks]

   (b)

   Evaluate the use of runners rather than seeds to produce new strawberry plants. Include a justified conclusion.

   [6 marks]

   Total for question 4: 12 marks

End of questions