RP2- preparing & observing a root tip squash

Question 1

give the equation to calculate mitotic index

Answer 1

number of cells in mitosis/ total number of cells

Question 1

what is meristem tissue

Answer 1

regions of unspecialised cells in plants which can undergo cell division

Question 2

why do we use the root tip

Answer 2

• this is where the cells are dividing
• more meristem tissue at the tip
• cells further from tip are no longer dividing

Question 3

why does mitotic index decrease as distance from root tip increases

Answer 3

less meristem tissue further from the tip, so less cells are dividing
at the tip there is more damage so there is more cell division to repair tissue

Question 4

first step is fixing- describe and explain this step

Answer 4

This kills the cells while causing minimum changes to them. Place the tissue in concentrated hydrochloric acid
- this stops cell division and also hydrolyses the middle lamella so that the cells can be separated easily

Question 5

second step is staining- describe and explain

Answer 5

Place the root tip on a slide and add a stain that will bind to the chromosomes- makes chromosomes visible

stain used is acetic orcein- stains chromosomes dark red

Question 6

third step is preparing a root tip squash- describe and explain

Answer 6

Squash the stained root tip by placing a coverslip, this produces a single layer of cells so you can see individual cells
allow light to pass through specimen

Question 7

what is maceration

Answer 7

breaking up the root tip
with mounted needle to separate the cells

Question 8

Explain why the student:
used only the first 5 mm from the tip of an onion root
(1)

Answer 8

it is where dividing cells are found

Question 9

explain why the student:
pressed down firmly on the cover slip (1)

Answer 9

to spread out the cells so light passes through
thin layer

Question 10

Describe and explain what the student should’ve done when counting cells to make sure that the mitotic index he obtained for this root tip was accurate. (2)

Answer 10

• examine a large number of cells to ensure a representative sample
• repeat count to ensure the figures are correct

Question 11

give 2 precautions taken during this experiment

Answer 11

1. Safety specs: stop chemicals going into eyes.
2. Lab coat: protect skin, and wash hands if make contact with acetic orcein.

Question 12

Why must you not push the coverslip sideways when preparing the slide?

Answer 12

cells may get rolled up, damaging the chromosomes

Question 13

Describe how you would determine a reliable mitotic index from tissue observed
with an optical microscope. (3)

Answer 13

1. Count cells in mitosis in field of view;
2. Divide this by total number of cells in field of
   view;
3. Repeat many/at least 5 times

Question 14

another student didn’t undergo the exact same procedure and as a result saw no cells undergoing mitosis
suggest 3 reasons why she didn’t see any cells in mitosis (3)

Answer 14

• no stain used
• not root tip
• more than 1 layer of cells as cells not squashed firmly enough

Question 15

suggest why the student soaked the root tips in hydrochloric acid (2)

Answer 15

• stop mitosis
• hydrolyse cell wall
• allows stain to pass into cells

Question 16

other students in the class followed the same method but calculated different mitotic indexes.

Apart from student errors, suggest 2 explanations why (2)

Answer 16

• root from different species
• roots are different ages
• roots undergo mitosis at different rates
• student may have calculated a mean