Download Mitosis

General Biology lab
Mitosis and the Cell Cycle
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.
• The cell division cycle :
– 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).
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
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 :
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
1. Prophase:
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Chromatin condenses
chromosomes become visible
nuclear envelope and nucleoli disappear
spindle starts to form attach to the kinetochore ( a portion of
the centromere).
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).
Anaphase:
 Spindle fibers shorten
 the kinetochores separate
 the sister chromatids (daughter chromosomes) are pulled apart
and begin moving to the cell poles.
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.
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
 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.
Why use onion roots for viewing
mitosis?
1. The roots are easy to grow in large numbers.
2. The cells at the tip of the roots are actively dividing.
3. Because each cell divides independently of the others, a root
tip contains cells at different stages of the cell cycle.
4. The chromosomes can be stained to make them more easily.
Materials
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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
Procedure
1. Cut 2-3 mm of onion root
2. Use forceps to transfer an onion root tip into the cup of HCl
and leave for 4 minutes at 60ºC.
3. Transfer the root tip to the cup containing fixative (Carnoy)
and leave it for 4 minutes.
4. Then place the root tip on a slide.
5. Cover the root tip with a few drops of stain for 2 minutes
6. 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.
7. Observe your preparation under the microscope
Interphase
Prophase
Metaphase
Anaphase
Telophase