CP 3

Question 1

Outline the procedure to prepare a root tip slide

Answer 1

•remove 2mm(accept 1mm>3mm)of the tip and place on a slide (1)

• use WARM acid and submerge roots in it (1)
• remove acid using water (1)
• add named stain i.e. acetic orcein (1)
• use of named fixative, e.g. fixation in ethanoic (acetic) ethanol (1)
• macerate or tease with a needle (1)
• use coverslip and squash press firmly vertically / without moving the coverslip (1)

Question 2

Where in plants can cells undergoing mitosis be found?

Answer 2

Meristem tissue at shoot and root tips

Question 3

What is the mitotic index?

Answer 3

The ratio of cells undergoing mitosis to the total number of cells in a sample.

Question 4

State the formula for the mitotic index.

Answer 4

Mitotic index = Number of cells dividing / Total number of cells

Question 5

State the hazards and precautions for reagents used in this procedure.

Answer 5

HCl - corrosive, avoid contact with skin, wear eye protection

Toluidine Blue O stain / acetic orcein - irritant, avoid contact with skin, wear eye protection

Scalpel - cut away from fingers

Question 6

Explain why the root tip is heated with acid.

Answer 6

The root tip is heated with acid to break up the tissues into individual cells.

The cellulose walls of plant cells are held together by pectins such as calcium pectate.

Treatment with hydrochloric acid dissolves the middle lamellae in order to break up the cellulose cell wall.

This allows the stain to permeate and the tip to be squashed more easily

Question 7

What effect will maceration and pressing the slide preparation have on the dividing cells?

Answer 7

Pressing the preparation will separate the cells in the meristem tissue into individual cells in a single layer.

Question 8

What information do the cell counts give you about each stage of mitosis?

Answer 8

The cell counts show the relative duration of each stage in the cell cycle.

The longer a phase, the more cells are likely to be going through that phase at any point in time

Question 9

What is the role of mitosis in the life of an organism?

Answer 9

Mitosis produces identical daughter cells for growth, replacement and repair

Question 10

Why is the sample squashed?

Answer 10

To flatten the sample and reduce the number of layers, reducing overlap so the cells can be observed more clearly.

Question 11

Why is acetic orcein used ?

Answer 11

Acetic orcein stains the dna and chromosomes to allow us to observe stages of cell cycle and mitosis

It kills the cells without disturbing the structure of the cell

Question 12

How do you caculate how much time is spentr in one of the cell cycle stages?

Answer 12

Number of cells in the stage/ total number of cells X 100

Question 13

Describe how to prepare a microscope slide of root tissue to show stages of mitosis.
(4)

Answer 13

 use warm acid (1)
 remove acid using water (1)
 add named stain eg acetic orcein(1)
 macerate or tease with needle (1)
 use coverslip and squash

Question 14

Describe some errors that could have occured in this practical

Answer 14

Error in process of preparing slide, which gives distorted images
- contaminating sample/ damage to sample

Error in focussing / using appropriate magnifications

Errors in calculation of mitotic index

Question 15

Describe one safety precaution that should be taken when carrying out this procedure.

Answer 15

1. safety goggles when handling acid / stain
2. care (with scalpel) when cutting root tip
3. care with slide when squashing root tip ;

Question 16

Explain the role of mitosis in the development of roots. (2)

Answer 16

1. idea of increasing cell number
2. idea of replacing {damaged / dead } cells OR idea of repairing (damaged) tissue
3. to produce genetically identical cells

Question 17

Describe some limitations of this practical

Answer 17

• Limitations to resolution or magnifcation - could be overcome by going from ight microscope to electron microscope
• Some specimins become distorted/damaged in preparation and so lose their natural structure
• Depending on which plane a tissue is cut, cells organelles can look lmany different shapes. One plane may give a false impression in 2d, of the 3d structure