Introduction


The growth and development of every
organism depends on the precise
replication of the genetic material during
each cell division.
Cell division is the process by which cells
reproduce themselves.
2
Objectives
•
•
Learn preparing and staining procedure
to identify the stages of mitosis in onion
root tip.
To differentiate between the different
stages of mitosis.
Types of Cell Division

Mitosis
• Mitosis is the process by which a
eukaryotic
cell
separates
the
chromosomes in its cell nucleus into two
identical sets, in two separate nuclei.
• It is generally followed immediately by
cytokinesis,
• The outcome of this process: two new
daughter cells with the same number and
kind of chromosomes as the parent cell.




New body (somatic) cells are formed by
mitosis.
Meiosis ( Reduction division )
produces progeny cells with one-half the
genetic
content
and
number
of
chromosomes as parent cell
The formation of male and female gametes
in animal cells or spores in plant cells is by
meiosis.
5
Mitosis

Purpose
• Mitosis occurs in order for organisms to grow
and develop.
• In order to replenish dead or dying cells such
as skin cells, and cells in the digestive tract.

Karyokinesis
• process of nuclear division (division of genetic
material).

Cytokinesis
• Process of dividing cytoplasm/cell.

Mitosis occurs only in eukaryotic cells and the
process varies in different species. For example:
 animals undergo an "open" mitosis, where the
nuclear envelope breaks down before the
chromosomes separate.
 Fungi and yeast undergo a "closed" mitosis,
where chromosomes divide within an intact
cell nucleus.

Prokaryotic cells, which lack a nucleus, divide by
a process called binary fission.
7
The Cell Cycle
•
The life of a cell is divided into three stages
known as the cell cycle:
1. Interphase: cell carries out normal functions and
prepares to divide.
2. Mitosis: nucleus divides splits into two.
3. Cytokinesis: cell and contents divide into two
daughter cells.
Interphase
•
•
•
•
•
•
The cell prepares itself for cell division.
This phase consist of the G1 (first gap), S
(synthesis), and G2 (second gap) phases.
The chromatin is diffuse.
protein synthesis,
DNA synthesis,
Replication of other cellular structures.

S phase: where each chromosome is
duplicated and consists of two sister
chromatids
joined
together
by
a
centromere.
Now, the nucleus and cell increase in
 size,
and chromosomes are fully
extended.

11
Mitosis
•
There are 4 main phases:
• Prophase,
• Metaphase,
• Anaphase,
• Telophase.
•
Cytokinesis (division of the cytoplasm) follows and
one cell becomes two.
the nucleus has to migrate into the center of the cell
before mitosis can begin.
•
Mitosis: Prophase
Major processes during this phase:
• Chromosomes condense and form visible bodies.
• Chromosomes become thicker, shorter, and easily visible
when stained under the light microscope.
• Two “sister chromatids” join near their middle at a
structure called the centromere.
• The nucleolus and the nuclear membrane disappear.
• The mitotic apparatus the spindle, begins to organize
within the cell.
Mitosis: Metaphase
•
•
•
Chromosomes become aligned at midpoint or
equator between poles of the cell.
Are at their thickest and shortest structure.
They are easily identified as two longitudinally
double sister chromatids.
Mitosis: Anaphase
•
•
The centromere replicates and splits
The sister chromatids begin to separate and
migrate to the opposite poles separate of the cell.
Mitosis: Telophase
•
•

•
•
Chromosomes now uncoil
Nuclear envelope reappears and surrounds the
chromosomes
Cytokinesis
The cytoplasm and all its contents are divided between the
2 daughter cells (cytoplasmic division), membrane creates
between the 2 new daughter cells
In plants, such as the onion root tip cells, this is seen as
the formation of a cell plate
Cell Cycle
Stages of mitosis in onion root tip cells
Mitosis in Root Tip
•
•
•
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.
Materials
•
•
•
•
•
•
•
•
•
Slides & cover slips
Microscope
Fresh onion root tips
Fixative ( methanol-acetic acid 3:1 v/v)
Forceps
1 M HCl
Razor blade
Stain
Paper towel, or absorbent paper
Method
•
•
•
•
•
•
•
•
Cut 2-3 mm of onion root
Use forceps to transfer an onion root tip into the cup
of HCl.
Leave for 4 minutes
Transfer the root tip to the cup containing fixative
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
Cover the root tip with one to two drops of 45%
acetic acid
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 low power
(X10) of a microscope
Search the slide to find cells in various stages
of cell division, once you have located cells in
division, change to high power (X40) & try to
observe several stages of division.
Record the number of cells in each stage.
Count at least three full fields of view. You
should have counted over 200 cells.
Record your data in the table
Calculate the percentage of cells in each
phase and record in the table
Interphase
Prophase
Anaphase
Metaphase
Telophase
Animation
http://www.youtube.com/watch?v=s1ylUTb
XyWU

http://www.johnkyrk.com/mitosis.html