The Cell Division
Cellular division includes
two kinds,
Mitosis:
1. It occurs in the somatic cells.
2. The resulting cell from this division receives a
complete copy of the original cell chromosomes
( 2n or diploid )
3. These cells contain the same genetic information and perform the same
functions as the original cell.
4. It plays an important role in the
reproduction in the primitive organism
and in many of the higher plant in what
is known as asexual reproduction.
5. Mitosis occurs in two main stages.
a. Nuclear division
b. Cytoplasmic division.
6. Before the cell divides, it passes
through a stage known as inter-phase
in which the chromosome material
multiplication starts, these stages are:
1. The prophase
1- The longest phase ( lasts for over half
the entire mitotic time).
2- Starts by the disappearance of the
network structure of the nucleus.
3- The chromatin condenses into distinct
chromosomes.
4- Each chromosome appears consisting of
two long identical filaments (chromatids)
5- Each 2 chromatids are attached at the centromere.
6- In animal cell; two pairs of centrioles appear close to the nucleus.
7- Each pair moves towards one of the two cell poles, where dense fibers radiate
from
them across the cytoplasm forming the spindle.
8- The nuclear membrane disappears and the nucleolus degenerates.
In plant cell, the centrioles are absents and the spindle fibers appear without them.
2-The metaphase:
1. The paired chromatids become straightly arranged and fixed by the spindle
fibers with the centromeres along the midline of the cell (equatorial plane).
2. Each centromere then divides into two one per each chromatid.
3. The Anaphase:
1- This is the shortest phase
2- Identical chromatids separate into independent chromosomes.
3- The spindle fibers contract withdrawing each of the new chromosomes
towards one of the cell poles, where two identical sets of chromosomes are
formed.
4. The Telophase
1- All changes that occur during this stage are the opposite of what happen in
the prophase.
2- Upon reaching the cell poles, chromosomes lose their identity and coil up
into a chromatin net,
3- Around which appears the nuclear membrane.
4- Then the nucleolus appears while the spindle fibers disappear and the
centrioles divide.
5- At the end of the nuclear division, the cytoplasm divides between the two
daughter cells; and a cell membrane is formed between them in the animal
cell.
6- In plant cells, a middle lamella develops in the middle of the original cell
forming the cell wall.
Meiosis (Meiotic division):
1- Occurs only in the sexual organs (the gonads).
2- The gametes resulting from this division gets half the number of
chromosomes
3- If the original cell (2n), Then after meiosis it will be 4 cells each one N or
haploid
4- This reduction in the number of the chromosomes leads to their consistency
in their number in each species after the fusion of their haploid gametes.
5- This avoids the duplication and misbalance of the chromosomal material
duplicates once in the two nuclear division.
6- Meiosis passes continuously through two consecutive stages each is formed
as four phases as follows
The first meiotic division:
1.
123-
Prophase1:
It is the longest stage (in time) and the most important and complex.
The chromatin reticulum uncoils and the chromosomes become distinct.
Homologous chromosomes then arrange in pairs, each pair appears formed of
four even chromatids called a tetrad.
4- The process of chiasmata formation and crossing over occurs between each two
inner chromatids of the tetrads, where some parts are exchanged between the
chromatids.
5- Centrioles in animal cells divide near the end of this phase
6- The spindle appears
7- The nucleolus and the nuclear membrane disappear.
Crossing over is a renewable source of genetic variation due to the new genetic
arrangements.
2. Metaphase 1:
1- Homologous pairs of chromosomes arrange on the midline or the equator of the
cell.
2- They become pulled by the spindle fibers that connect with the centromere in
each pair of them (compare this with what happens in mitosis).
3. Anaphase1:
1- The chromosomes separate form one another
2- Each one moves as a pair of chromatids (without division of centromeres) by the
pull of the spindle fibers to be directed to one of the cell poles so the whole
chromosomes are separated here and not the chromatids as in mitosis.
3- These chromosomes become distributed at random over the resulting cells,
having no relation with one another.
4. Telophase1:
1- Haploid chromosomes (half their original number) arrive to each cell pole.
2- They aggregate into a sphere to be enveloped by a nuclear membrane,
3- The a nucleus appears.
4- The cytoplasm starts to divide giving two daughter cells.
5- Each cell enters through a short inter-phase where the chromosomes lose their
identity within the nucleus but their nuclear material never duplicates.
6- After a short period, the cell enters into the second meiotic division.
Exercise 3 Mitosis and meiosis
1.
2.
3.
Mitosis occurs in the
a. Somatic cells.
b. Gametes
c. All the above
d. Non of the above
In mitosis the resulting cell from this division receives
a. A complete copy of the original cell chromosomes
b. Half of the original
c. 3 Times the original
d. Double the original
………………… plays an important role in the reproduction in the primitive
organism and in many of the higher plant in what is known as asexual
reproduction.
a. Meiosis
b. Mitosis
c. Both of them
d. Non of them
4.
Mitosis occurs in two main stages which are Nuclear division and ……
a. Cytoplasm division.
b. Mitochondria division
c. ER division
d. All the above
5.
Before the cell divides, it passes through a stage known as
a. Anaphase
b. Prophase
c. Metaphase
d. Inter-phase
6.
During ………….. the chromosome material multiplication starts,
a. Anaphase
b. Prophase
c. Metaphase
d. inter-phase
7.
The longest phase ( lasts for over half the entire mitotic time) is
a. Anaphase
b. Prophase
c. Metaphase
d. inter-phase
8.
It starts by the disappearance of the network structure of the
nucleus.
a. Anaphase
b. Prophase
c. Metaphase
d. inter-phase
9.
The chromatin condenses into distinct
a. Chromosomes.
b. DNA
c. RNA
d. Non of the above
10. Each chromosome appears consisting of two long identical filaments
called:
a. Chromatids
b. Centromeres
c. Centerioles
d. Spindle
11.
Each 2 chromatids are attached at the ………
a. Chromatids
b. Centromeres
c. Centerioles
d. Spindle
12.
In animal cell two pairs of ……………. appear close to the nucleus.
a. Chromatids
b. Centromeres
c. Centerioles
d. Spindle
13. Each pair of centerioles moves towards one of the two cell poles,
where dense fibers radiate from them across the cytoplasm forming the
…………..
a. Chromatids
b. Centromeres
c. Centerioles
d. Spindle
14. In plant cell, the ………….. are absents and the spindle fibers appear
without them.
a. Chromatids
b. Centromeres
c. Centerioles
d. Spindle
15.
The paired chromatids become straightly arranged and fixed by the
spindle fibers with the centromeres along the midline of the cell (equatorial
plane) and each centromere then divides into two one per each chromatid
during:
a. Anaphase
b. Prophase
c. Metaphase
d. inter-phase
16. the shortest phase is
a. Anaphase
b. Prophase
c. Metaphase
d. inter-phase
17.
Identical chromatids separate into independent chromosomes and the
spindle fibers contract withdrawing each of the new chromosomes towards
one of the cell poles, where two identical sets of chromosomes are formed
during:
a. Anaphase
b. Prophase
c. Metaphase
d. inter-phase
18.
All changes that occur during this stage are the opposite of what
happen in the prophase.
a. Anaphase
b. Prophase
c. telophase
d. inter-phase
19.
Upon reaching the cell poles, chromosomes lose their identity and
coil up into:
a. Chromatids
b. Centromeres
c. Centerioles
d. Chromatin net
20.
At the end of the nuclear division, the cytoplasm divides between
the two daughter cells; and a cell membrane is formed between them in the :
a. Animal cell.
b. Plant cell
c. All cells
d. Non of the above
21. In plant cells, a middle ………………. develops in the middle of the original
cell forming the cell wall.
a. Lamella
b. Chromosome
c. Net
d. Pole
22.
Occurs only in the sexual organs (the gonads).
a. Meiosis
b. Mitosis
c. All the above
d. Non of the above
23.
The gametes resulting from meiosis gets ………….. the number of
chromosomes
a. ½
b. ¼
c. twice
d. equal
24.
If the original cell (2n), Then after meiosis it will be 4 cells each
onecontains:
a. N
b. 2N
c. ½ N
d. 4 N
25.
The term haploid refers to
a. N
b. 2N
c. ½ N
d. 4 N