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 using a scalpel and mount onto slide
2. soak root tip in hydrochloric acid then rinse
3. stain for DNA
4. lower coverslip using mounted needle at 45 degrees without trapping air bubbles
5. squash by firmly pressing down on glass slip but do not push sideways

Question 2

why are root tips used

Answer 2

they are where mitosis occurs

Question 3

why is a stain used

Answer 3

to distinguish the chromosomes

Question 4

why do you squash the coverslip

Answer 4

spreads out the cells to create a single layer so light passes through to make chromosomes visible

Question 5

why do you not push the coverslip sideways

Answer 5

to avoid rolling the cells together and breaking the chromosomes

Question 6

why do you soak the roots in acid

Answer 6

• separate cells and 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 lense
3. use coarse focusing dial to move stage close to lens, 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 objective lense then refocus

Question 8

what are the rules of scientific drawing

Answer 8

• look similar to specimen/image
• no sketching/shading only clear continuous lines
• include a magnification scale
• label with straight uncrossed lines

Question 9

explain how prophase can be identified

Answer 9

• chromosomes are visible and distinct because of condensing
• randomly arranged because no spindle activity because not attached to spindle fibres

Question 10

explain how metaphase can be identified

Answer 10

chromosomes lined up on equator because attaching to spindle fibres

Question 11

explain how anaphase can be identified

Answer 11

• chromatids at poles of spindle
• chromatids v shaped because pulled apart at their centromeres by spindle fibres

Question 12

explain how telophase can be identified

Answer 12

chromosomes in two sets on at each pole

Question 13

what is mitotic index

Answer 13

proportion of cells undergoing mitosis:
number of cells undergoing mitosis/total number if cells in sample

Question 14

explain how to determine a reliable mitotic index from observed squashes

Answer 14

• count cells in mitosis in field of view
• count only whole cells
• divide this by total number of cells in field of view
• repeat with at least 5 fields of view selected randomly for a representative sample
• calculate reliable mean

Question 15

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

Answer 15

1. identify proportion of cells in named phase = number of cells in phase/total number of cells observed
2. multiply by length of cell cycle