All cells arise from other cells

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Question 8
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A student prepared a stained squash of cells from the root tips of an onion bulb (Allium cepa) to calculate a mitotic index. She:

  1. Cut the terminal 4 mm4\text{ mm}4 mm from 8 onion8\text{ onion}8 onion roots.
  2. Placed the root tips into a watch glass containing 5 cm35\text{ cm}^35 cm3 of 1 M1\text{ M}1 M hydrochloric acid for 10 minutes10\text{ minutes}10 minutes.
  3. Rinsed the root tips thoroughly in distilled water.
  4. Placed one of the root tips on a microscope slide and added a few drops of aceto-orcein stain.
  5. Placed a coverslip onto the slide, covered it with a paper towel, and pressed gently but firmly downwards on the root tip.
  6. Observed the prepared slide using an optical microscope.

Figure 1

A diagram showing a microscope field of view of onion root tip cells, labeled Figure 1. The view contains 40 distinct cells in total. 34 of these cells are in interphase, showing a circular nucleus with a granular chromatin appearance. 6 cells are in active mitotic stages: 2 in prophase (chromosomes condensed and visible as threads), 1 in metaphase (chromosomes aligned at the equator), 1 cell labeled Cell H in anaphase (chromatids separated and moving towards opposite poles), and 2 in telophase (chromosomes clustered at opposite poles with a cell plate forming).

1.

Suggest why the student soaked the root tips in hydrochloric acid in step 2.

[2]
2.

Pressing the coverslip downwards enabled the student to observe the stages of mitosis clearly. Explain why.

[2]
3.

Name the stage of mitosis shown in Cell H in Figure 1. Explain the appearance of this cell.

[2]
4.

Use Figure 1 to calculate a mitotic index for the cells in this field of view.

[1]
5.

Other students in the class followed the same protocol, but calculated different mitotic indices. Apart from student errors, suggest two explanations why.

[2]

All cells arise from other cells Questions

  1. A Level
  2. /Biology
  3. /All cells arise from other cells