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THE CELL DIVISION
Mitosis and Meiosis
Cell division is a process by which the cellular material is divided between 2
new daughter cells.
Types of cell division:
There are 2 major types of cell division, these are mitosis and meiosis.
1- Mitosis (indirect division):

This type occurs in somatic cells of higher organisms, and is the means of
population growth in unicellular organisms.

This cell division results in two daughter cells, each having the same number
of chromosomes of the mother cell.
2- Meiosis (reduction division):

This type occurs in the diploid germ cells of animals and results in 4
daughter cells each having a haploid number of chromosomes (i.e. half the
number of chromosomes of the original cell).

It involves exchange of the chromosomal segments between homologous
(identical) chromosomes.
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The Phases of Mitosis:
(A) Prophase:
1. The chromosomal threads become tightly coiled, thus become shorter and
denser and could be seen as chromosomes, each is formed of 2 sister chromatids
(replicated chromosome) attached together at the centromere.
2. The nuclear envelope dissolves.
3. The nucleolus disappears.
4. In the cytoplasm of animal cells, the centrioles, with a surrounding fan of
astral rays, start to migrate toward opposite poles of the cell.
5. The spindle fibers begin to form.
(B) Metaphase:
1. The spindle fibers are fully formed and astral rays appear around the
centrioles at the cell poles (in animal cells).
2. The chromosome are arranged in the equatorial plane and their centromeres
appear attached to the spindle fibers at their kinetochores. These kinetochores
are 2 disk-shaped structure in each centromere.
(C) Anaphase:
The centromeres split and the 2 chromatids of each chromosome become
separated and pulled toward the corresponding cell pole (centriole in animal
cells). Now each chromatid is a chromosome.
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(D) Telophase:
1. The chromosome have reached the opposite poles.
2. The spindle disperses.
3. The chromosome become diffuse.
4. The nucleolus reappears.
5. A nuclear envelope forms around each chromosome set, thus a nucleus is
formed.
Cytokinesis:

This involves the division of the cytoplasm into 2 nearly equal cells.

The process usually begins during telophase and sometimes during
anaphase.

The cytoplasm gradually constrict at the equatorial plane (along the midline
of the spindle).

This process is not part of mitosis.

In plant cells, cytokinesis involves formation of a cell plate between the 2
daughter cells. This plate is formed as a result of fusion of vesicles arranged at
the equatorial plane. A membrane bound space, i.e. the cell plate, is formed.
This space is then filled with pectin thus forming the middle lamella. Each
daughter cell secretes its own cell wall on the sides of this lamella.
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The stages of mitosis in animal cells
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The stages of mitosis in animal cells
Prophase
Metaphase
Anaphase
Telophase
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145 ZOO
Draw the phases of the mitotic cell division as see them under the microscope
and label the different structures:
Prophase. (mag. ………… x).
Metaphase. (mag. ………… x).
Anaphase. (mag. ………… x).
Telophase. (mag. ………… x).
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Early prophase
Late prophase
Metaphase
Anaphase
Telophase
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The stages of mitosis in plant cells
Prophase
Metaphase
Anaphase
Telophase
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Draw the phases of the mitotic cell division as see them under the microscope
and label the different structures:
Prophase. (mag. ………… x).
Metaphase. (mag. ………… x).
Anaphase. (mag. ………… x).
Telophase. (mag. ………… x).
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The Phases of Meiosis:
Meiosis includes 2 successive nuclear divisions:
1. Meiosis I: during which homologus chromosomes separate.
2. Meiosis II: during which sister chromatids of each chromosome separate.
Interphase:
Meiosis I:
(A) Prophase:
1. The homologus chromosomes pair together so that they align along their
length precisely and thus form a tetrad (4 chromatids).
2. The aligned chromatids of the 2 chromosomes interchange segments in a
process called crossing over.
3. The chromatin threads condense.
4. The nuclear envelope dissolves.
5. The nucleolus disappears.
6. The spindle begins to form.
(B) Metaphase:
1. The spindle is fully formed.
2. Homologus pair of chromosomes line up in the equatorial plane (as tetrads).
3. The spindle fibers attach to each homologus at its kinetochore (centromere).
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4. In animal cells, centriols and astral rays are also present at the opposite poles
of the cell.
(C) Anaphase:
The homologus chromosomes separate; each chromosome (consisting of 2 sister
chromatids) of the homologus pair move toward one pole of the cell.
(D) Telophase:
1. Homologus chromosomes have reached the opposite poles where only half
(haploid = 1 n) the number of chromosomes can be seem.
2. A nuclear envelope may form around the chromosomes.
3. Cytokinesis may take place (depending on the species).
Meiosis II:
It resembles mitosis except that it is not preced by DNA replication, since this
has already taken place before meiosis I.
(A) Prophase:
1. The chromosomes condense fully again.
2. The nuclear envelope breaks down.
3. The spindle fibers begin to appear.
(B) Metaphase:
1. The spindle is fully formed.
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2. The chromosomes (1 n), each of 2 sister chromatids, line up in the equatorial
plane.
3. The spindle appear attached to the kinetochore of each of the chromosome.
(C) Anaphase:
1. Sister chromatids separate as the centromers split up.
2. Each chromatid, now is a chromosome, moves towards one of the opposite
poles,
(D) Telophase:
1. The chromosomes have reached the poles.
2. The spindle disappears.
3. A nuclear envelope forms around each set of chromosomes.
4. The chromosomes disperse into chromatin threads.
Cytokinesis:
This process proceeds during telophase to separate the 2 daughter cells, and to
separate the 4 daughter cells (if it has not occurred after telophase I).
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Prophase I
Metaphase I
Anaphase I
Telophase I
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Prophase II
Metaphase II
Anaphase II
Telophase II
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Draw the phases of the meiotic cell division as see them under the microscope
and label the different structures:
Prophase I. (mag. ………… x).
Metaphase I. (mag. ………… x).
Anaphase I. (mag. ………… x).
Telophase I. (mag. ………… x).
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Draw the phases of the meiotic cell division as see them under the microscope
and label the different structures:
Prophase II. (mag. ………… x).
Metaphase II. (mag. ………… x).
Anaphase II. (mag. ………… x).
Telophase II. (mag. ………… x).
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