1. General Biology lab
Mitosis and the Cell Cycle

2. Objectives
To prepare slides of onion root tips demonstrating the stages of mitosis (somatic cell division) and identification of cells in the various stages of mitosis.

3. The cell division cycle : It includes :
is a sequence of events in a eukaryotic cell between one mitotic division and another.
It includes :
1. Interphase (G1 phase, S phase, and G2 phase)
Chromatin appears dispersed, DNA replication occurs. (Chromatin is a mass of uncoiled DNA and associated proteins called histones).
2. M phase. In the M-phase both the nucleus and the cytoplasm divide (mitosis and cytokinesis).

4. Cell cycle:
G1 - initial growth cycle
S - DNA synthesis (replication)
G2 - second growth stage
M - Mitosis and cytokinesis.
G1+S+G2= Interphase

5. Duration of the cell cycle in Human cells (in hours)
It takes about 16 hours
Interphase
Mitosis G1 S G2 M 5 7 3 1
Duration of phases of Mitosis:
Prophase: minutes
Metaphase: 3 minutes
Anaphase: minutes
Telophase: 18 minutes
Vary among cell types
Consistent among cell types

7. Mitosis
Mitosis :
Produces two new daughter cells with the same number and kind of chromosomes as the parent cell.
Mitosis, or nucleus division, is the first part of M-phase and in consists of four stages :
Prophase
Metaphase
Anaphase
Telophase

8. 1. Prophase: Chromatin condenses chromosomes become visible
nuclear envelope and nucleoli disappear
spindle starts to form attach to the kinetochore ( a portion of the centromere).

9. 2. Metaphase
All chromosomes align in one plane at the center of the cell called the equatorial plane (also referred to as the metaphase plate).

10. Anaphase: Spindle fibers shorten the kinetochores separate
the sister chromatids (daughter chromosomes) are pulled apart and begin moving to the cell poles.

11. Telophase: the last stage of division
The chromosomes gradually de-condense to form the chromatin seen in interphase.
Formation of a new nuclear envelope around each group of chromosomes.
The nucleoli reappear.

12. Cytokinesis ( cytoplasmic division)
Usually occurs at the end of telophase.
In plant cells cytokinesis is accomplished by the formation of a cell plate.
Animal cells separate by forming a cleavage furrow.

13. Preparing An Onion Root Tip Squash
Onion bulbs have been rooted in water.
Growth of new roots is due to the production and elongation of new cells.
Mitotic divisions are usually confined to the cells near the tip of the root.

14. Why use onion roots for viewing mitosis?
The roots are easy to grow in large numbers.
The cells at the tip of the roots are actively dividing.
Because each cell divides independently of the others, a root tip contains cells at different stages of the cell cycle.
The chromosomes can be stained to make them more easily.

15. Materials Onion bulb with roots Crystal violet and other stains.
Fixative solution (1 part glacial acetic acid to 3 parts ethanol).
1M HCl
Clean slide & cover
Forceps

16. Procedure Cut 2-3 mm of onion root
Use forceps to transfer an onion root tip into the cup of HCl and leave for 4 minutes at 60ºC.
Transfer the root tip to the cup containing fixative (Carnoy) and leave it for 4 minutes.
Then place the root tip on a slide.
Cover the root tip with a few drops of stain for 2 minutes
Put a cover slip over the root, put a paper towel or other absorbent paper and with your thumb firmly press on the cover slip.
Observe your preparation under the microscope

19. Interphase
Prophase
Metaphase
Anaphase
Telophase