PAG 1.1

Question 1

Equipment

Answer 1

• Light microscope with low power and high power objective lenses e.g. x 4, x10 and x40
• Eyepiece graticule
• Stage micrometer
• Permanent slides showing the stages of mitosis from Allium spp.

Question 2

aim

Answer 2

To use a light microscope to observe and identify the stages of mitosis including calibrating and using an eyepiece graticule and making annotated scientific drawings showing cells in different stages of mitosis.

Question 3

why the need to be sure to move the microscope stage up to almost touching the microscope slide while not looking down the microscope and then to focus DOWN.

Answer 3

This will avoid broken cover pieces and slides.

Question 4

Calibration method

Answer 4

• In order for correct symbol (µm) to be used and to take into consideration small differences between microscopes, the graticule must be calibrated prior to the activity being carried out. The technique for the calibration of the graticule is as follows-
  1. Use a stage micrometer. This is a microscope slide with an accurate scale etched on it. In the example below, 1 mm is divided into 10 parts. So on this stage micrometer, each small division is equal to 0.1 mm, which is 100 micrometres (µm).2. Insert the graticule into the eyepiece lens and line up the two scales similar to the diagram below.3. It is then possible to count the number of divisions on the eyepiece graticule equivalent to each division on the stage micrometer and hence calculate the length that one eyepiece division is equivalent to.4. For the example below, three divisions (shown in red) are equal to 100 µm, so each division is equal to 33.3 µm. This might be, for example, at low power.5. The process should be repeated with each objective lens and you will then have a calibration factor for each lens. You are now ready to examine the prepared slide-

Question 5

why must the graticule be calibrated prior to the activity being carried out

Answer 5

In order for correct symbol (µm) to be used and to take into consideration small differences between microscopes

Question 6

Observing the specimen

Answer 6

1. Place the slide showing the stages of mitosis on the microscope stage.
2. Without putting your eye to microscope but looking at the slide itself, twist the focusing dial until the microscope stage has moved up so that the slide is just about to touch the lens.
3. Then use the coarse focusing dial to focus DOWN until you can clearly see the cells in the field of view. Make sure the cells are in the centre of the field of view (the circle of light you can see when looking down the microscope).
4. Rotate to the medium power lens and again focus until you can see the cells and the chromosomes in them clearly.
5. Then rotate to the high power lens and use the fine focusing dial only to bring the chromosomes into distinct view.
6. Take time to look carefully and identify each stage of mitosis that can be seen.
7. The eyepiece graticule can be used to measure the length of the chromosomes. Aim to do this carefully for three separate chromosomes depending on the actual slide being used.
8. Make scientific annotated drawings of the stages of mitosis that you have seen and identified. It should be possible from your drawings to know which stage has been drawn. An example of a labelled scientific drawing is shown below.

Question 7

1. What is the purpose of mitosis for a living organism?

Answer 7

Growth and repair of tissues, asexual reproduction.

Question 8

2. What is a key distinguishing visible feature of each stage of mitosis?

Answer 8

Prophase – chromosomes visible,
metaphase – chromosomes line up,
anaphase – chromatids separate,
telophase – two nuclei visible, cell splits.

Question 9

3. Once active cell division ends, the cells will enter interphase. Explain why it is incorrect to say that these cells are “resting”.

Answer 9

Interphase is not resting as e.g. checking the DNA for errors, protein synthesis, respiration, mRNA synthesis all occur during this time.

Question 10

4. Why is a sample from the roots of Allium spp. a good specimen for studying mitosis?

Answer 10

Cells are actively growing so it is easy to find cells in various stages of mitosis.

Question 11

5. State which stage of the cell cycle the majority of the cells in your specimen were in and suggest why.

Answer 11

Interphase, cells spend most time of the cell cycle in this stage so is inevitable that many cells in a root tip will be in interphase.