‫الجامعة االسالمية – غزة‬
The Islamic University
Faculty of Science
‫كلية العلوم‬
Biology and Biotechnology
Department
‫قسم االحياء والتكنولوجيا الحيوية‬
General Genetics
Lab. 2
Mitosis and the Cell Cycle
2010/2011
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.
Background:

Prokaryotic cells (bacteria) reproduce asexually
by binary fission. Bacterial cells have a single
circular chromosome, which is not enclosed by a
nuclear envelope. During binary fission the
bacterial chromosome is duplicated, the cell
elongates, and the two chromosomes migrate to
opposite ends of the cell. Each daughter cell
receives one chromosome and is identical to the
parent cell. Binary fission is a relatively fast and
simple process.


Eukaryotes are diploid, which means they have
two sets of chromosomes; one set of
chromosomes is inherited from each parent.
For example in a growing plant root, the cells at
the tip of the root are constantly dividing to allow
the root to grow. Because each cell divides
independently of the others, a root tip contains
cells at different stages of the cell cycle. This
makes a root tip an excellent tissue to study the
stages of cell division.

The cell division cycle is a sequence of events in a
eukaryotic cell between one mitotic division and another.
It includes interphase (G1 phase, S phase, and G2
phase) and M phase. In the M-phase both the nucleus
and the cytoplasm divide (mitosis and cytokinesis).

Chromosomes are visible only during mitosis when they
separate between the daughter cells. The process of
mitotic division ensures that both daughter cells will gain
identical set of chromosomes containing identical genetic
information.
Cell cycle:
G1 - initial growth cycle
S - DNA synthesis
(replication)
G2 - second growth stage
M - Mitosis and
cytokinesis.
G1+S+G2= Interphase
As shown in the figure, the part of the cell cycle during which the nucleus of the cell
is dividing (which is called mitosis) occupies approx. 10% of the time taken for the
whole cycle. The Cytokinesis phase takes approx. half of this time.
Most of the cell cycle is the period in which the cell is not dividing (called interphase).
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
Vary among cell types
Consistent among cell types
Duration of phases of Mitosis:
Prophase: 36 minutes
Metaphase: 3 minutes
Anaphase: 3 minutes
Telophase: 18 minutes
Interphase

Chromatin appears dispersed, DNA
replication occurs.
(Chromatin is a mass of uncoiled DNA and
associated proteins called histones).
Mitosis

Mitosis, or nucleus division, is the first
part of M-phase and in consists of four
stages (prophase, metaphase, anaphase
and telophase).
Mitosis is the first part of M-phase and
divided into four distinct phases:
Prophase: Chromatin condenses,
chromosomes become visible, nuclear
envelope and nucleoli disappear, spindle
starts to form attach to the kinetochore ( a
portion of the centromere).
 Each replicated chromosome comprises two
chromatids, both with the same genetic
information (called sister chromatids).


Metaphase: All chromosomes align in
one plane at the center of the cell called
the equatorial plane (also referred to as
the metaphase plate).

Anaphase: Spindle fibers shorten, the
kinetochores separate, and the sister
chromatids (daughter chromosomes) are
pulled apart and begin moving to the cell
poles.
Telophase:
Telophase, the last stage of division, is
marked by a noticeable condensation of
the chromosomes, followed by formation
of a new nuclear envelope around each
group of chromosomes. The
chromosomes gradually de-condense to
form the chromatin seen in interphase.
The nucleoli reappear.

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.

Preparing An Onion Root Tip
Squash:

It is possible for you to make your own stained
preparations of onion root tips and observe
mitotic figures (2n=16). 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. Follow the procedure
outlined below to make your own root tip
preparation.
Materials:








Onion bulb with roots
Crystal violet
fixative solution (1 part glacial acetic acid to 3 parts ethanol).
1M HCl
clean slide & cover
Forceps
Razor blade
Fingernail polish
Procedure:


1. Obtain two small cups, label one “ HCL” and
the other “Carnoy.” Pour in enough of each to
just cover the bottom of its cup.
2. Obtain an onion bulb that is just beginning to
show the emergence of roots. Cut a small piece
of root off and use forceps to place it in the cup
of 1M HCL for 4 min at 60ºC.



3. Transfer the root into the cup of carnoy
fixative. The root should remain in the carnoy
fixative for 4 minutes.
4. After the 4 minutes, place the root on a slide.
Then, cut off the bottom 1 or 2mm of the root tip
& discard the rest.
5. Cover the root with a 1drop of Crystal Violet
for 2 minutes. After the 2 minutes, blot away the
stain; be careful not to touch the root tip.
6. When the stain has been blotted away, cover
the root tips with drops of water to remove the
stain. Apply water drops until water runs mostly
clear. Blot slide without touching root tip.
 7. Gently lower a cover slip over the root tip.
Cover the slide with a paper towel and with your
thumb (or a pencil eraser), firmly press on the
cover slip; Do not twist the cover slip. The
pressure will spread the cells into a single layer.




8. Then using the fingernail polish, carefully
paint the edges of the cover slip. Make sure not
to move the cover slip. Finally allow the slide to
dry.
9. Now you are ready to view you slide under
low power. Place your prepared slide on the
microscope.
10. Use the low power objective on your
microscope to look for thin layers of cells and
then use the 40X power objective to observe
mitotic stages in individual cells.
11. Use the table below to record your data :
Interphase
Prophase
Metaphase
Anaphase
Telophase
Total
number of
cells
percent of
cells
100%
12. Draw a cell in each of the four main stages
of mitosis and interphase stage:
Interphase
Prophase
Metaphase
Anaphase
Telophase
pictures
from the
last
semester