Topic 2 : 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 (eg. with toluidine blue)
4. Lower coverslip using a mounted needle at 45

o 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. (1)

Answer 2

● Where dividing cells are found / mitosis occurs

Question 3

Explain why a stain is used. (1) ● To distinguish chromosomes
● Chromosomes not visible without stain

Answer 3

● To distinguish chromosomes

Question 4

Explain why the cover slip needs to be
squashed / pressed down. (1)

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

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. (1)

Answer 5

● Avoid rolling cells together / breaking chromosomes

Question 6

Give two reasons why the roots should
be soaked in acid. (2)

Answer 6

• to stop mitosis
• seperate cells

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

✓ No shading / hatching
✓ Include a magnification scale (eg. x 400)
✓ No sketching

Question 9

Explain how the stages of mitosis can be identified (look at email)

Answer 9

Prophase :
● Chromosomes visible / distinct → because condensing
● But randomly arranged → because no spindle activity / not
attached to spindle fibre

Metaphase ● Chromosomes lined up on equator → because attaching to spindle

Anaphase
● Chromatids (in two groups) at poles of spindle

● Chromatids V shaped → because being pulled apart at their
centromeres by spindle fibres

Telophase ● Chromosomes in two sets, one at each pole

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