Required Practical 2

Question 1

Method

Prepare the root tip; Use the microscope; Calculate mitotic index

Answer 1

1. Heat 1 mol dm^-3 HCl at 60°C in a water bath.
2. Cut a small sample of the root tip using a scalpel.
3. Transfer root tip to HCl and incubate for 5 minutes.
4. Remove from HCl and wash sample in cold distilled water and remove the very tip using a scalpel.
5. Place tip on microscope slide and add a few drops of stain (e.g. toluidine blue O) - makes the chromosomes visible and will therefore show which cells are undergoing mitosis.
6. Lower the cover slip down carefully onto the slide - make sure no air bubbles are in the slide as it may distort the image, and that the coverslip doesn’t slide sideways as it could damage the chromosomes.
7. Place under a microscope and set objective lens on lowest
   magnification.
8. Use the coarse adjustment knob to move the lens down to just above the slide.
9. Use the fine adjustment knob to carefully re-adjust the focus until the image is clear (you can use a higher magnification if needed).
10. Calculate mitotic index by dividing cells undergoing mitosis (cells with chromosomes visible) by the total number of cells (in decimals, unless asked otherwise).

Question 2

Why is the root tip soaked in HCl?

Answer 2

To break down cell walls and allow tissues to spread more easily.

Question 3

Why is the root tip stained?

Answer 3

To make the chromosomes visible by binding to the DNA.

Question 4

Why do you press the cover slip downwards and not sideways?

Answer 4

It could damage the chromosomes.

Question 5

Why do you squish the root tip with the cover slip until it is flat?

Answer 5

To create a single layer of cells so that light can pass through allowing internal structure to be seen.

Question 6

Mitotic index

Answer 6

Mitotic index = number of cells undergoing mitosis (chromosomes visible) ÷ number of cells present in sample

Question 7

Magnification

Answer 7

Magnification = image size ÷ actual size

Question 8

Conversions

cm to mm

Answer 8

× 10

Question 9

Conversions

cm to µm

Answer 9

× 10,000

× 10^-4

Question 10

Conversions

mm to µm

Answer 10

× 1000

× 10³