required practical 2

Question 1

Describe how to prepare squashes of cells from plant root tips

Answer 1

1. Cut a thin slice of root tip (5mm from end) using scalpel and mount onto a slide
2. Soak root tip in hydrochloric acid then rinse
3. Stain for DNA (e.g.. with toluidine blue)
4. Lower coverslip using a mounted needle at 45 degree Celsius without trapping air bubbles
5. Squash by firmly pressing down on glass slip but do not push sideways

Question 2

Explain why root tips are used

Answer 2

● Where dividing cells are found / mitosis occurs

Question 3

Explain why a stain is used

Answer 3

● To distinguish chromosomes
● Chromosomes not visible without stain

Question 4

Explain why the cover slip needs to be squashed / pressed down

Answer 4

● (Spreads out cells) to create a single layer of cells
● So light passes through to make chromosomes visible

Question 5

Explain why the cover slip should not be
pushed sideways

Answer 5

● Avoid rolling cells together / breaking chromosomes

Question 6

Give two reasons why the roots should be soaked in acid

Answer 6

● Separate cells / cell walls
● To allow stain to diffuse into cells
● To allow cells to be more easily squashed
● To stop mitosis

Question 7

Describe how to set-up and use an optical microscope

Answer 7

1 Clip slide onto stage and turn on light
2 Select lowest power objective lens (usually x 4)
3 a. Use coarse focusing dial to move stage close to lens
b. Turn coarse focusing dial to move stage away from
lens until image comes into focus
4 Adjust fine focusing dial to get clear image
5 Swap to higher power objective lens, then refocus

Question 8

What are the rules of scientific drawing?

Answer 8

✓ Look similar to specimen / image - draw parts
to the same scale / relative size
✓ No sketching - only clear, continuous lines
✓ No shading / hatching
✓ Include a magnification scale (eg. x 400)
✓ Label with straight, uncrossed lines

Question 9

Explain how the stages of mitosis can be identified

Answer 9

Question 10

What is a mitotic index (MI)?

Answer 10

● Proportion of cells undergoing mitosis (with visible chromosomes)
● Mitotic index = number of cells undergoing mitosis / total number of cells in sample

Question 11

Explain how to determine a reliable MI from observed squashes

Answer 11

● Count cells in mitosis in field of view
● Count only whole cells / only cells on top and right edges → standardise counting
● Divide this by total number of cells in field of view
● Repeat with many / at least 5 fields of view selected randomly → representative sample
● Calculate a reliable mean

Question 12

Suggest how to calculate the time cells are in a certain phase of mitosis

Answer 12

1. Identify proportion of cells in named phase at any one time
   ○ Number of cells in that phase / total number of cells observed
2. Multiply by length of cell cycle