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Mitotic Cell Division

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Objectives

Learn preparing and staining procedure to identify the stages of mitosis in onion root tip.

To differentiate between the different stages of mitosis.

Calculate the mitotic index.

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Types of Cell Division

– MitosisProduces two new daughter cells with the same

number and kind of chromosomes as the parent cell – Meiosis

Reduction division produces progeny cells with one-half the genetic content and number of chromosomes as parent cell

Produces gametes/spores

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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

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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.

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Cell Cycle

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G0

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Interphase

This phase consist of the G1,S, and G2 phases of the cell cycle.

The chromatin is diffuse. It may not look like much is going

on here, but there is a lot of activity because the cell must prepare for Mitosis:

protein synthesis, DNA synthesis, replication of other cellular

structures.

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Mitosis

There are 4 main phases: Prophase, Metaphase, Anaphase, Telophase.

Cytokinesis (division of the cytoplasm) follows and one cell becomes two.

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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

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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.

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Mitosis: Anaphase

The centromere replicates and splits

The sister chromatids separate and are pulled to opposite sides of the cell

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Mitosis: Telophase

Chromosomes now uncoil Nuclear envelope reappears and

surrounds the chromosomesCytokinesis 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

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Stages of mitosis in onion root tip cells

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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

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Experiment

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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

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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.

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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

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Mitotic index

A measure for the proliferation status of a cell population.

It is defined as the ratio between the number of cells in mitosis and the total number of cells.

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Animation

http://www.youtube.com/watch?v=s1ylUTbXyWU

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Interphase Prophase

Metaphase Anaphase

Telophase