RP2 Preparation of stained squashes of root tips to observe the stages of mitosis under the microscope

Describe how to prepare squashes of cells from plant root tips. (5 marks)

1. Cut a thin slice from the root tip and place it on a microscope slide.

2. Soak the root tip in hydrochloric acid, then rinse.

3. Stain the sample for DNA using a suitable dye, such as toluidine blue.

4. Lower the coverslip at an angle to avoid air bubbles.

5. Press down gently to squash the tissue without pushing sideways.

Explain why root tips are chosen for this practical and state why a stain is applied when preparing root tip cells. (2 marks)

- Root tips are regions where mitosis is actively taking place.

- Stain is used to make chromosomes visible and distinguishable under the microscope.

Explain why the coverslip needs to be pressed down and why the coverslip should not be pushed sideways. (2 marks)

- To spread out the cells into a single layer so light can pass through.

- To prevent rolling cells together and breaking chromosomes.

State two reasons why root tips should be soaked in acid. (2 marks)

- To separate cells by breaking cell walls.

- To allow stain to penetrate and cells to squash more easily.

Describe how to set up and use an optical microscope. (5 marks)

1. Place the slide on the stage and turn on the light.

2. Select the lowest power objective lens.

3. Use the coarse focusing dial to bring the stage close to the lens, then focus.

4. Adjust the fine focusing dial to sharpen the image.

5. Change to a higher power lens and refocus if needed.

State three rules of scientific drawing. (3 marks)

- Draw with clear, continuous lines without shading.

- Include a magnification scale.

- Label with straight, uncrossed lines.

Explain how prophase can be identified in microscope slides. (2 marks)

- Chromosomes are visible and distinct due to condensation.

- They are randomly arranged because they are not yet attached to the spindle.

Describe how metaphase appears under the microscope. (1 mark)

Chromosomes are aligned along the cell's equator.

Explain how anaphase can be recognised. (2 marks)

- Chromatids are found at opposite poles.

- Chromatids have a V shape as they are pulled apart by spindle fibres.

Describe what is seen during telophase. (1 mark)

Two sets of chromosomes are present, one at each pole.

Define the term mitotic index. (2 marks)

- The proportion of cells undergoing mitosis.

- Calculated as the number of cells in mitosis divided by the total number of cells.

Explain how to obtain a reliable mitotic index. (3 marks)

- Count whole cells undergoing mitosis in several random fields of view.

- Use at least five fields of view for a representative sample.

- Calculate the mean value.

Suggest how to calculate the time spent in a specific mitosis stage. (2 marks)

- Find the proportion of cells in that stage.

- Multiply by the total length of the cell cycle.