RP 2

Question 1

describe how to prepare squashes of cells from plant root tips

Answer 1

cut a thin slice of root tip using scalpel and mount onto a slide
soak tip in HCl acid then rinse
stain for DNA
lower coverslip using a mounted needle at an angle
squash firmly pressing down on glass slip

Question 2

why are root tips used

Answer 2

where mitosis occurs

Question 3

why is a stain used

Answer 3

to distinguish chromosomes

Question 4

why squash/press down on cover slip

Answer 4

create a single layer of cells so light passes through to make chromosomes visible

Question 5

why not push cover slip sideways

Answer 5

avoid rolling cells together/breaking chromosomes

Question 6

why soak roots in acid

Answer 6

separate cells/cell walls
to allow stain to diffuse into cells
to stop mitosis
to allow cells to be more easily squashes

Question 7

what is a mitotic index

Answer 7

proportion of cells undergoing mitosis

Question 8

what is the formula for mitotic index

Answer 8

number of cells undergoing mitosis/total number of cells in sample

Question 9

explain how to determine a reliable mitotic index from observed squashes

Answer 9

count cells in mitosis in field of view (count only whole cells to standardise counting)
divide this by total number of cells in field of view
repeat many times (representative sample)
calculate a (reliable) mean

Question 10

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

Answer 10

identify proportion of cells in named phase at any one time
- number of cells in that phase / total number of cells observed
multiply by length of cell cycle