The cell cycle and mitosisEdexcel A-Level Biology B: Subtopic test

1. 1
   A student prepared a temporary squash of onion root tip, stained it with acetic orcein and examined it with a light microscope. In one field of view she counted 200 cells. Of these, 24 were in the stages of mitosis and the rest were in interphase.

   (a)

   Why is the root tip, rather than the mature part of the root, used for this investigation?

   [1 mark]

   • AThe cells in the root tip are all in interphase and are easy to count
   • BThe meristem contains many cells that divide frequently, so many are visible in mitosis
   • CThe root tip cells have no cell walls, so they separate easily
   • DThe root cap contains the cells with the most chromosomes

   (b)

   What is the mitotic index of this sample?

   [1 mark]

   • A0.12
   • B0.14
   • C7.3
   • D0.88

   (c)

   Explain why the root tips are warmed in hydrochloric acid and then squashed under a coverslip.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A human skin cell in the G1 stage of interphase contains 46 chromosomes. Skin cells divide by mitosis to replace those lost from the surface of the body.

   (a)

   How many chromatids are present in this cell at metaphase?

   [1 mark]

   • A23
   • B46
   • C92
   • D184

   (b)

   Compared with its DNA content in G1, what is the DNA content of this cell at the start of mitosis?

   [1 mark]

   • AHalf as much, because chromosome number is halved in G2
   • BThe same, because DNA is replicated only during mitosis
   • CFour times as much, because each chromatid has replicated twice
   • DTwice as much, because DNA was replicated in S phase

   (c)

   Describe what happens to the chromosomes during anaphase.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Colchicine is a chemical extracted from the autumn crocus. It prevents spindle fibres from forming. When added to a culture of dividing cells, the cells stop dividing and their chromosomes remain condensed. Colchicine has been investigated as a treatment for some cancers.

   (a)

   Explain why cells treated with colchicine accumulate in metaphase and cannot complete mitosis.

   [3 marks]

   (b)

   Suggest why colchicine might slow the growth of a tumour, and why it may also cause side effects in a patient.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Mitosis occurs in many different organisms and tissues. A human embryo grows from a single zygote to billions of cells, a cut on a finger heals within days, and a spider plant produces small plantlets on long stems.

   (a)

   Describe the stages of mitosis and explain how they produce two genetically identical nuclei.

   [6 marks]

   (b)

   Discuss the importance of mitosis in these examples, and evaluate the advantages and disadvantages of asexual reproduction for the spider plant.

   [6 marks]

   Total for question 4: 12 marks

End of questions