The growth and the development of every organism
depend exclusively on the multiplication and enlargement
of its cells. The development of a multicellular organism
from the unicellular zygote is achieved by the cell division,
growth and differentiation.
The division of the nucleated cells is achieved by two
integral activities such as division of the nucleus (called
Karyokinesis) and the division of the cytoplasm (that is called
cytokinesis). Usually the Karyokinesis is followed by the
cytokinesis, but sometimes it does not follow and results in
multinucleated cells.
In animals and plants following three types of cell
division have been observed:
1.
Amitosis,
2.
Mitosis
3.
Meiosis
1.
Amitosis: It is the means of asexual reproduction in
unicellular organisms like bacteria and protozoans. It
also occurs in embryonic membranes of vertebrates. In
Amitosis, the splitting of nucleus is followed by
cytoplasmic contriction. In Amitosis, two daughter cells
are formed without the occurrence of any nuclear event.
It is also known as direct nuclear division.
2.
Mitosis: It takes place as a result of mitotic division
One cell divides into 2 daughter cells which are
quantitatively and qualitatively identical to the mother
cell. No change in the Chromosome number. In between
two successive mitotic divisions there is a rest period
called the interphase. Mitosis has 4 phases- Prophase,
Metaphase, Anaphase & Telophase.
?
Prophase : Disappearance of nuclear membrane and
nucleolus along with the doubling of chromosomes.
?
Metaphase : Formation of spindle, chromosomes join
themselves to the equatorial plane of the spindle.
?
Anaphase : Centromeres divide longitudinally. Sister
chromatids move towards opposite poles.
?
Telophase : Grouping of chromatids at each pole along
with the formation of new nuclear membrane and
nucleolus.
3.
Meiosis: Also called Reproductive cell division because
it is associated with all sexual reproduction. As a result
of division. in the daughter cells chromosomes number
become half to that of the mother cell. Each division
results into 4 daughter cells in contrast to 2 daughter
cells in each mitotic divisions.
Division includes 2 phases i.e. Reduction division and
Equational division (Meiosis) consist of same 4 phases, i.e.,
Prophase, Metaphase, Anaphase & Telophase. But Prohase
I (Division I) has 5 substages. Leptotene, Zygotene,
Pachytene, Diplotene & Diakinesis. The characteristics of
each of the substage are:
?
Leptotene : Close association of homologous
chromosomes.
?
Zygotene: Pairing of homologous chromosomes, the
phenomena is called Synapsis. As a result, Bivalent
Chromosomes are formed.
?
Pachytene : Chiasma formation and crossing over.
?
Diplotene : Terminations of chiasma starts.
?
Diakinesis : Disappearance of nuclear membrane and
nucleolus.
In Metaphase I spindle formation and rearrangement of
chromosomes. In Anaphase I separation of homologous
chromosomes. In Telophase grouping of chromosomes and
formation of nuclear membrane and nucleolus. This division
is followed by Division I
depend exclusively on the multiplication and enlargement
of its cells. The development of a multicellular organism
from the unicellular zygote is achieved by the cell division,
growth and differentiation.
The division of the nucleated cells is achieved by two
integral activities such as division of the nucleus (called
Karyokinesis) and the division of the cytoplasm (that is called
cytokinesis). Usually the Karyokinesis is followed by the
cytokinesis, but sometimes it does not follow and results in
multinucleated cells.
In animals and plants following three types of cell
division have been observed:
1.
Amitosis,
2.
Mitosis
3.
Meiosis
1.
Amitosis: It is the means of asexual reproduction in
unicellular organisms like bacteria and protozoans. It
also occurs in embryonic membranes of vertebrates. In
Amitosis, the splitting of nucleus is followed by
cytoplasmic contriction. In Amitosis, two daughter cells
are formed without the occurrence of any nuclear event.
It is also known as direct nuclear division.
2.
Mitosis: It takes place as a result of mitotic division
One cell divides into 2 daughter cells which are
quantitatively and qualitatively identical to the mother
cell. No change in the Chromosome number. In between
two successive mitotic divisions there is a rest period
called the interphase. Mitosis has 4 phases- Prophase,
Metaphase, Anaphase & Telophase.
?
Prophase : Disappearance of nuclear membrane and
nucleolus along with the doubling of chromosomes.
?
Metaphase : Formation of spindle, chromosomes join
themselves to the equatorial plane of the spindle.
?
Anaphase : Centromeres divide longitudinally. Sister
chromatids move towards opposite poles.
?
Telophase : Grouping of chromatids at each pole along
with the formation of new nuclear membrane and
nucleolus.
3.
Meiosis: Also called Reproductive cell division because
it is associated with all sexual reproduction. As a result
of division. in the daughter cells chromosomes number
become half to that of the mother cell. Each division
results into 4 daughter cells in contrast to 2 daughter
cells in each mitotic divisions.
Division includes 2 phases i.e. Reduction division and
Equational division (Meiosis) consist of same 4 phases, i.e.,
Prophase, Metaphase, Anaphase & Telophase. But Prohase
I (Division I) has 5 substages. Leptotene, Zygotene,
Pachytene, Diplotene & Diakinesis. The characteristics of
each of the substage are:
?
Leptotene : Close association of homologous
chromosomes.
?
Zygotene: Pairing of homologous chromosomes, the
phenomena is called Synapsis. As a result, Bivalent
Chromosomes are formed.
?
Pachytene : Chiasma formation and crossing over.
?
Diplotene : Terminations of chiasma starts.
?
Diakinesis : Disappearance of nuclear membrane and
nucleolus.
In Metaphase I spindle formation and rearrangement of
chromosomes. In Anaphase I separation of homologous
chromosomes. In Telophase grouping of chromosomes and
formation of nuclear membrane and nucleolus. This division
is followed by Division I
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