NCERT Textbook - Cell Cycle and Cell Division NEET Notes | EduRev

Biology Class 11

Created by: Sushil Kumar

NEET : NCERT Textbook - Cell Cycle and Cell Division NEET Notes | EduRev

 Page 1


162 BIOLOGY
Are you aware that all organisms, even the largest, start their life from a
single cell? You may wonder how a single cell then goes on to form such
large organisms. Growth and reproduction are characteristics of cells,
indeed of all living organisms. All cells reproduce by dividing into two,
with each parental cell giving rise to two daughter cells each time they
divide. These newly formed daughter cells can themselves grow and divide,
giving rise to a new cell population that is formed by the growth and
division of a single parental cell and its progeny. In other words, such
cycles of growth and division allow a single cell to form a structure
consisting of millions of cells.
10.1 CELL CYCLE
Cell division is a very important  process in all living organisms. During
the division of a cell, DNA replication and cell growth also take place. All
these processes, i.e., cell division, DNA replication, and cell growth, hence,
have to take place in a coordinated way to ensure correct division and
formation of progeny cells containing intact genomes. The sequence of
events by which a cell duplicates its genome, synthesises the other
constituents of the cell and eventually divides into two daughter cells is
termed cell cycle. Although cell growth (in terms of cytoplasmic increase)
is a continuous process, DNA synthesis occurs only during one specific
stage in the cell cycle. The replicated chromosomes (DNA) are then
distributed to daughter nuclei by a complex series of events during cell
division. These events are themselves under genetic control.
CELL CYCLE AND CELL DIVISION
CHAPTER  10
10.1 Cell Cycle
10.2 M Phase
10.3 Significance of
Mitosis
10.4 Meiosis
10.5 Significance of
Meiosis
2015-16(19/01/2015)
Page 2


162 BIOLOGY
Are you aware that all organisms, even the largest, start their life from a
single cell? You may wonder how a single cell then goes on to form such
large organisms. Growth and reproduction are characteristics of cells,
indeed of all living organisms. All cells reproduce by dividing into two,
with each parental cell giving rise to two daughter cells each time they
divide. These newly formed daughter cells can themselves grow and divide,
giving rise to a new cell population that is formed by the growth and
division of a single parental cell and its progeny. In other words, such
cycles of growth and division allow a single cell to form a structure
consisting of millions of cells.
10.1 CELL CYCLE
Cell division is a very important  process in all living organisms. During
the division of a cell, DNA replication and cell growth also take place. All
these processes, i.e., cell division, DNA replication, and cell growth, hence,
have to take place in a coordinated way to ensure correct division and
formation of progeny cells containing intact genomes. The sequence of
events by which a cell duplicates its genome, synthesises the other
constituents of the cell and eventually divides into two daughter cells is
termed cell cycle. Although cell growth (in terms of cytoplasmic increase)
is a continuous process, DNA synthesis occurs only during one specific
stage in the cell cycle. The replicated chromosomes (DNA) are then
distributed to daughter nuclei by a complex series of events during cell
division. These events are themselves under genetic control.
CELL CYCLE AND CELL DIVISION
CHAPTER  10
10.1 Cell Cycle
10.2 M Phase
10.3 Significance of
Mitosis
10.4 Meiosis
10.5 Significance of
Meiosis
2015-16(19/01/2015)
CELL CYCLE AND CELL DIVISION 163
10.1.1 Phases of Cell Cycle
A typical eukaryotic cell cycle is illustrated by
human cells in culture. These cells divide once
in approximately every 24 hours (Figure 10.1).
However, this duration of cell cycle can vary
from organism to organism and also from cell
type to cell type. Yeast for example, can progress
through the cell cycle in only about 90 minutes.
The cell cycle is divided into two basic
phases:
l l l l l Interphase
l l l l l M Phase (Mitosis phase)
The M Phase represents the phase when the
actual cell division or mitosis occurs and the
interphase represents the phase between two
successive M phases. It is significant to note
that in the 24 hour average duration of cell
cycle of a human cell, cell division proper lasts
for only about an hour. The interphase lasts
more than 95% of the duration of cell cycle.
The M Phase starts with the nuclear division, corresponding to the
separation of daughter chromosomes (karyokinesis) and usually ends
with division of cytoplasm (cytokinesis). The interphase, though  called
the resting phase,  is the time during which the cell is preparing for division
by undergoing both cell growth and DNA replication in an orderly manner.
The interphase is divided into three further phases:
l l l l l G
1
 phase (Gap 1)
l l l l l S phase  (Synthesis)
l l l l l G
2
 phase (Gap 2)
 G
1
 phase corresponds to the interval between mitosis and initiation
of DNA replication. During G
1
 phase the cell is metabolically active and
continuously grows but does not replicate its DNA. S or synthesis  phase
marks the period during which DNA synthesis or replication takes place.
During this time the amount of DNA per cell doubles.  If the initial amount
of DNA is denoted as 2C then it increases to 4C. However, there is no
increase in the chromosome number; if the cell had diploid or 2n number
of chromosomes at G
1
, even after S phase the number of chromosomes
remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the
nucleus, and the centriole duplicates in the cytoplasm. During the G
2
phase, proteins are synthesised in preparation for mitosis while cell growth
continues.
How do plants and
animals continue to
grow all their lives?
Do all cells in a plant
divide all the time?
Do you think all cells
continue to divide in
all plants and
animals? Can you
tell the name and the
location of tissues
having cells that
divide all their life in
higher plants? Do
animals have similar
meristematic
tissues?
Figure 10.1 A diagrammatic view of cell cycle
indicating formation of two cells
from one cell
M Phase
2015-16(19/01/2015)
Page 3


162 BIOLOGY
Are you aware that all organisms, even the largest, start their life from a
single cell? You may wonder how a single cell then goes on to form such
large organisms. Growth and reproduction are characteristics of cells,
indeed of all living organisms. All cells reproduce by dividing into two,
with each parental cell giving rise to two daughter cells each time they
divide. These newly formed daughter cells can themselves grow and divide,
giving rise to a new cell population that is formed by the growth and
division of a single parental cell and its progeny. In other words, such
cycles of growth and division allow a single cell to form a structure
consisting of millions of cells.
10.1 CELL CYCLE
Cell division is a very important  process in all living organisms. During
the division of a cell, DNA replication and cell growth also take place. All
these processes, i.e., cell division, DNA replication, and cell growth, hence,
have to take place in a coordinated way to ensure correct division and
formation of progeny cells containing intact genomes. The sequence of
events by which a cell duplicates its genome, synthesises the other
constituents of the cell and eventually divides into two daughter cells is
termed cell cycle. Although cell growth (in terms of cytoplasmic increase)
is a continuous process, DNA synthesis occurs only during one specific
stage in the cell cycle. The replicated chromosomes (DNA) are then
distributed to daughter nuclei by a complex series of events during cell
division. These events are themselves under genetic control.
CELL CYCLE AND CELL DIVISION
CHAPTER  10
10.1 Cell Cycle
10.2 M Phase
10.3 Significance of
Mitosis
10.4 Meiosis
10.5 Significance of
Meiosis
2015-16(19/01/2015)
CELL CYCLE AND CELL DIVISION 163
10.1.1 Phases of Cell Cycle
A typical eukaryotic cell cycle is illustrated by
human cells in culture. These cells divide once
in approximately every 24 hours (Figure 10.1).
However, this duration of cell cycle can vary
from organism to organism and also from cell
type to cell type. Yeast for example, can progress
through the cell cycle in only about 90 minutes.
The cell cycle is divided into two basic
phases:
l l l l l Interphase
l l l l l M Phase (Mitosis phase)
The M Phase represents the phase when the
actual cell division or mitosis occurs and the
interphase represents the phase between two
successive M phases. It is significant to note
that in the 24 hour average duration of cell
cycle of a human cell, cell division proper lasts
for only about an hour. The interphase lasts
more than 95% of the duration of cell cycle.
The M Phase starts with the nuclear division, corresponding to the
separation of daughter chromosomes (karyokinesis) and usually ends
with division of cytoplasm (cytokinesis). The interphase, though  called
the resting phase,  is the time during which the cell is preparing for division
by undergoing both cell growth and DNA replication in an orderly manner.
The interphase is divided into three further phases:
l l l l l G
1
 phase (Gap 1)
l l l l l S phase  (Synthesis)
l l l l l G
2
 phase (Gap 2)
 G
1
 phase corresponds to the interval between mitosis and initiation
of DNA replication. During G
1
 phase the cell is metabolically active and
continuously grows but does not replicate its DNA. S or synthesis  phase
marks the period during which DNA synthesis or replication takes place.
During this time the amount of DNA per cell doubles.  If the initial amount
of DNA is denoted as 2C then it increases to 4C. However, there is no
increase in the chromosome number; if the cell had diploid or 2n number
of chromosomes at G
1
, even after S phase the number of chromosomes
remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the
nucleus, and the centriole duplicates in the cytoplasm. During the G
2
phase, proteins are synthesised in preparation for mitosis while cell growth
continues.
How do plants and
animals continue to
grow all their lives?
Do all cells in a plant
divide all the time?
Do you think all cells
continue to divide in
all plants and
animals? Can you
tell the name and the
location of tissues
having cells that
divide all their life in
higher plants? Do
animals have similar
meristematic
tissues?
Figure 10.1 A diagrammatic view of cell cycle
indicating formation of two cells
from one cell
M Phase
2015-16(19/01/2015)
164 BIOLOGY
Some cells in the adult animals do not appear to exhibit division (e.g.,
heart cells) and many other cells divide only occasionally, as needed to
replace cells that have been lost because of injury or cell death. These
cells that do not divide further exit G
1
 phase to enter an inactive stage
called quiescent stage (G
0
) of the cell cycle. Cells in this stage remain
metabolically active but no longer proliferate unless called on to do so
depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic
cells. Against this, the plants can show mitotic divisions in both haploid
and diploid cells. From your recollection of examples of alternation of
generations in plants (Chapter 3) identify plant species and stages at which
mitosis is seen in haploid cells.
10.2 M PHASE
This is the most dramatic period of the cell cycle, involving a major
reorganisation of virtually all components of the cell. Since the number of
chromosomes in the parent and progeny cells is the same, it is also called as
equational division. Though for convenience mitosis has been divided
into four stages of nuclear division, it is very essential to understand that cell
division is a progressive process and very clear-cut lines cannot be drawn
between various stages. Mitosis is divided into the following four stages:
l l l l l Prophase
l l l l l Metaphase
l l l l l Anaphase
l l l l l Telophase
10.2.1 Prophase
Prophase which is the first stage of mitosis follows the S and G
2
 phases of
interphase. In the S and G
2
 phases the new DNA molecules formed are not
distinct but intertwined. Prophase is marked by the initiation of condensation
of chromosomal material. The chromosomal material becomes untangled
during the process of chromatin condensation (Figure 10.2 a). The centriole,
which had undergone duplication during S phase of interphase, now begins
to move towards opposite poles of the cell. The completion of prophase can
thus be marked by the following characteristic events:
l l l l l Chromosomal material condenses to form compact mitotic
chromosomes. Chromosomes are seen to be composed of two
chromatids attached together at the centromere.
l l l l l Initiation of the assembly of mitotic spindle, the microtubules, the
proteinaceous components of the cell cytoplasm help in the
process.
You have studied
mitosis in onion root
tip cells.  It has 16
chromosomes in
each cell.  Can you
tell how many
chromosomes will
the cell have at G
1
phase, after S phase,
and after M phase?
Also, what will be the
DNA content of the
cells at G
1
, after S
and at G
2
, if the
content after M
phase is 2C?
2015-16(19/01/2015)
Page 4


162 BIOLOGY
Are you aware that all organisms, even the largest, start their life from a
single cell? You may wonder how a single cell then goes on to form such
large organisms. Growth and reproduction are characteristics of cells,
indeed of all living organisms. All cells reproduce by dividing into two,
with each parental cell giving rise to two daughter cells each time they
divide. These newly formed daughter cells can themselves grow and divide,
giving rise to a new cell population that is formed by the growth and
division of a single parental cell and its progeny. In other words, such
cycles of growth and division allow a single cell to form a structure
consisting of millions of cells.
10.1 CELL CYCLE
Cell division is a very important  process in all living organisms. During
the division of a cell, DNA replication and cell growth also take place. All
these processes, i.e., cell division, DNA replication, and cell growth, hence,
have to take place in a coordinated way to ensure correct division and
formation of progeny cells containing intact genomes. The sequence of
events by which a cell duplicates its genome, synthesises the other
constituents of the cell and eventually divides into two daughter cells is
termed cell cycle. Although cell growth (in terms of cytoplasmic increase)
is a continuous process, DNA synthesis occurs only during one specific
stage in the cell cycle. The replicated chromosomes (DNA) are then
distributed to daughter nuclei by a complex series of events during cell
division. These events are themselves under genetic control.
CELL CYCLE AND CELL DIVISION
CHAPTER  10
10.1 Cell Cycle
10.2 M Phase
10.3 Significance of
Mitosis
10.4 Meiosis
10.5 Significance of
Meiosis
2015-16(19/01/2015)
CELL CYCLE AND CELL DIVISION 163
10.1.1 Phases of Cell Cycle
A typical eukaryotic cell cycle is illustrated by
human cells in culture. These cells divide once
in approximately every 24 hours (Figure 10.1).
However, this duration of cell cycle can vary
from organism to organism and also from cell
type to cell type. Yeast for example, can progress
through the cell cycle in only about 90 minutes.
The cell cycle is divided into two basic
phases:
l l l l l Interphase
l l l l l M Phase (Mitosis phase)
The M Phase represents the phase when the
actual cell division or mitosis occurs and the
interphase represents the phase between two
successive M phases. It is significant to note
that in the 24 hour average duration of cell
cycle of a human cell, cell division proper lasts
for only about an hour. The interphase lasts
more than 95% of the duration of cell cycle.
The M Phase starts with the nuclear division, corresponding to the
separation of daughter chromosomes (karyokinesis) and usually ends
with division of cytoplasm (cytokinesis). The interphase, though  called
the resting phase,  is the time during which the cell is preparing for division
by undergoing both cell growth and DNA replication in an orderly manner.
The interphase is divided into three further phases:
l l l l l G
1
 phase (Gap 1)
l l l l l S phase  (Synthesis)
l l l l l G
2
 phase (Gap 2)
 G
1
 phase corresponds to the interval between mitosis and initiation
of DNA replication. During G
1
 phase the cell is metabolically active and
continuously grows but does not replicate its DNA. S or synthesis  phase
marks the period during which DNA synthesis or replication takes place.
During this time the amount of DNA per cell doubles.  If the initial amount
of DNA is denoted as 2C then it increases to 4C. However, there is no
increase in the chromosome number; if the cell had diploid or 2n number
of chromosomes at G
1
, even after S phase the number of chromosomes
remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the
nucleus, and the centriole duplicates in the cytoplasm. During the G
2
phase, proteins are synthesised in preparation for mitosis while cell growth
continues.
How do plants and
animals continue to
grow all their lives?
Do all cells in a plant
divide all the time?
Do you think all cells
continue to divide in
all plants and
animals? Can you
tell the name and the
location of tissues
having cells that
divide all their life in
higher plants? Do
animals have similar
meristematic
tissues?
Figure 10.1 A diagrammatic view of cell cycle
indicating formation of two cells
from one cell
M Phase
2015-16(19/01/2015)
164 BIOLOGY
Some cells in the adult animals do not appear to exhibit division (e.g.,
heart cells) and many other cells divide only occasionally, as needed to
replace cells that have been lost because of injury or cell death. These
cells that do not divide further exit G
1
 phase to enter an inactive stage
called quiescent stage (G
0
) of the cell cycle. Cells in this stage remain
metabolically active but no longer proliferate unless called on to do so
depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic
cells. Against this, the plants can show mitotic divisions in both haploid
and diploid cells. From your recollection of examples of alternation of
generations in plants (Chapter 3) identify plant species and stages at which
mitosis is seen in haploid cells.
10.2 M PHASE
This is the most dramatic period of the cell cycle, involving a major
reorganisation of virtually all components of the cell. Since the number of
chromosomes in the parent and progeny cells is the same, it is also called as
equational division. Though for convenience mitosis has been divided
into four stages of nuclear division, it is very essential to understand that cell
division is a progressive process and very clear-cut lines cannot be drawn
between various stages. Mitosis is divided into the following four stages:
l l l l l Prophase
l l l l l Metaphase
l l l l l Anaphase
l l l l l Telophase
10.2.1 Prophase
Prophase which is the first stage of mitosis follows the S and G
2
 phases of
interphase. In the S and G
2
 phases the new DNA molecules formed are not
distinct but intertwined. Prophase is marked by the initiation of condensation
of chromosomal material. The chromosomal material becomes untangled
during the process of chromatin condensation (Figure 10.2 a). The centriole,
which had undergone duplication during S phase of interphase, now begins
to move towards opposite poles of the cell. The completion of prophase can
thus be marked by the following characteristic events:
l l l l l Chromosomal material condenses to form compact mitotic
chromosomes. Chromosomes are seen to be composed of two
chromatids attached together at the centromere.
l l l l l Initiation of the assembly of mitotic spindle, the microtubules, the
proteinaceous components of the cell cytoplasm help in the
process.
You have studied
mitosis in onion root
tip cells.  It has 16
chromosomes in
each cell.  Can you
tell how many
chromosomes will
the cell have at G
1
phase, after S phase,
and after M phase?
Also, what will be the
DNA content of the
cells at G
1
, after S
and at G
2
, if the
content after M
phase is 2C?
2015-16(19/01/2015)
CELL CYCLE AND CELL DIVISION 165
Cells at the end of prophase, when viewed under the
microscope, do not show golgi complexes, endoplasmic
reticulum, nucleolus and the nuclear envelope.
10.2.2 Metaphase
The complete disintegration of the nuclear envelope marks
the start of the second phase of mitosis, hence the
chromosomes are spread through the cytoplasm of the cell.
By this stage, condensation of chromosomes is completed
and they can be observed clearly under the microscope. This
then, is the stage at which morphology of chromosomes is
most easily studied. At this stage, metaphase chromosome
is made up of two sister chromatids, which are held together
by the centromere (Figure 10.2 b). Small disc-shaped
structures at the surface of the centromeres are called
kinetochores. These structures serve as the sites of attachment
of spindle fibres (formed by the spindle fibres) to the
chromosomes that are moved into position at the centre of
the cell. Hence, the metaphase is characterised by all the
chromosomes coming to lie at the equator with one chromatid
of each chromosome connected by its kinetochore to spindle
fibres from one pole and its sister chromatid connected by
its kinetochore to spindle fibres from the opposite pole (Figure
10.2 b). The plane of alignment of the chromosomes at
metaphase is referred to as the metaphase plate.  The key
features of metaphase are:
l l l l l Spindle fibres attach to kinetochores of
chromosomes.
l l l l l Chromosomes are moved to spindle equator and get
aligned along metaphase plate through spindle fibres
to both poles.
10.2.3 Anaphase
At the onset of anaphase, each chromosome arranged at
the metaphase plate is split simultaneously and the two
daughter chromatids, now referred to as chromosomes of
the future daughter nuclei, begin their migration towards
the two opposite poles. As each chromosome moves away
from the equatorial plate, the centromere of each
chromosome is towards the pole and hence at the leading
edge, with the arms of the chromosome trailing behind
(Figure 10.2 c). Thus, anaphase stage is characterised by
Figure 10.2 a and b : A diagrammatic
view of stages in mitosis
2015-16(19/01/2015)
Page 5


162 BIOLOGY
Are you aware that all organisms, even the largest, start their life from a
single cell? You may wonder how a single cell then goes on to form such
large organisms. Growth and reproduction are characteristics of cells,
indeed of all living organisms. All cells reproduce by dividing into two,
with each parental cell giving rise to two daughter cells each time they
divide. These newly formed daughter cells can themselves grow and divide,
giving rise to a new cell population that is formed by the growth and
division of a single parental cell and its progeny. In other words, such
cycles of growth and division allow a single cell to form a structure
consisting of millions of cells.
10.1 CELL CYCLE
Cell division is a very important  process in all living organisms. During
the division of a cell, DNA replication and cell growth also take place. All
these processes, i.e., cell division, DNA replication, and cell growth, hence,
have to take place in a coordinated way to ensure correct division and
formation of progeny cells containing intact genomes. The sequence of
events by which a cell duplicates its genome, synthesises the other
constituents of the cell and eventually divides into two daughter cells is
termed cell cycle. Although cell growth (in terms of cytoplasmic increase)
is a continuous process, DNA synthesis occurs only during one specific
stage in the cell cycle. The replicated chromosomes (DNA) are then
distributed to daughter nuclei by a complex series of events during cell
division. These events are themselves under genetic control.
CELL CYCLE AND CELL DIVISION
CHAPTER  10
10.1 Cell Cycle
10.2 M Phase
10.3 Significance of
Mitosis
10.4 Meiosis
10.5 Significance of
Meiosis
2015-16(19/01/2015)
CELL CYCLE AND CELL DIVISION 163
10.1.1 Phases of Cell Cycle
A typical eukaryotic cell cycle is illustrated by
human cells in culture. These cells divide once
in approximately every 24 hours (Figure 10.1).
However, this duration of cell cycle can vary
from organism to organism and also from cell
type to cell type. Yeast for example, can progress
through the cell cycle in only about 90 minutes.
The cell cycle is divided into two basic
phases:
l l l l l Interphase
l l l l l M Phase (Mitosis phase)
The M Phase represents the phase when the
actual cell division or mitosis occurs and the
interphase represents the phase between two
successive M phases. It is significant to note
that in the 24 hour average duration of cell
cycle of a human cell, cell division proper lasts
for only about an hour. The interphase lasts
more than 95% of the duration of cell cycle.
The M Phase starts with the nuclear division, corresponding to the
separation of daughter chromosomes (karyokinesis) and usually ends
with division of cytoplasm (cytokinesis). The interphase, though  called
the resting phase,  is the time during which the cell is preparing for division
by undergoing both cell growth and DNA replication in an orderly manner.
The interphase is divided into three further phases:
l l l l l G
1
 phase (Gap 1)
l l l l l S phase  (Synthesis)
l l l l l G
2
 phase (Gap 2)
 G
1
 phase corresponds to the interval between mitosis and initiation
of DNA replication. During G
1
 phase the cell is metabolically active and
continuously grows but does not replicate its DNA. S or synthesis  phase
marks the period during which DNA synthesis or replication takes place.
During this time the amount of DNA per cell doubles.  If the initial amount
of DNA is denoted as 2C then it increases to 4C. However, there is no
increase in the chromosome number; if the cell had diploid or 2n number
of chromosomes at G
1
, even after S phase the number of chromosomes
remains the same, i.e., 2n.
In animal cells, during the S phase, DNA replication begins in the
nucleus, and the centriole duplicates in the cytoplasm. During the G
2
phase, proteins are synthesised in preparation for mitosis while cell growth
continues.
How do plants and
animals continue to
grow all their lives?
Do all cells in a plant
divide all the time?
Do you think all cells
continue to divide in
all plants and
animals? Can you
tell the name and the
location of tissues
having cells that
divide all their life in
higher plants? Do
animals have similar
meristematic
tissues?
Figure 10.1 A diagrammatic view of cell cycle
indicating formation of two cells
from one cell
M Phase
2015-16(19/01/2015)
164 BIOLOGY
Some cells in the adult animals do not appear to exhibit division (e.g.,
heart cells) and many other cells divide only occasionally, as needed to
replace cells that have been lost because of injury or cell death. These
cells that do not divide further exit G
1
 phase to enter an inactive stage
called quiescent stage (G
0
) of the cell cycle. Cells in this stage remain
metabolically active but no longer proliferate unless called on to do so
depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic
cells. Against this, the plants can show mitotic divisions in both haploid
and diploid cells. From your recollection of examples of alternation of
generations in plants (Chapter 3) identify plant species and stages at which
mitosis is seen in haploid cells.
10.2 M PHASE
This is the most dramatic period of the cell cycle, involving a major
reorganisation of virtually all components of the cell. Since the number of
chromosomes in the parent and progeny cells is the same, it is also called as
equational division. Though for convenience mitosis has been divided
into four stages of nuclear division, it is very essential to understand that cell
division is a progressive process and very clear-cut lines cannot be drawn
between various stages. Mitosis is divided into the following four stages:
l l l l l Prophase
l l l l l Metaphase
l l l l l Anaphase
l l l l l Telophase
10.2.1 Prophase
Prophase which is the first stage of mitosis follows the S and G
2
 phases of
interphase. In the S and G
2
 phases the new DNA molecules formed are not
distinct but intertwined. Prophase is marked by the initiation of condensation
of chromosomal material. The chromosomal material becomes untangled
during the process of chromatin condensation (Figure 10.2 a). The centriole,
which had undergone duplication during S phase of interphase, now begins
to move towards opposite poles of the cell. The completion of prophase can
thus be marked by the following characteristic events:
l l l l l Chromosomal material condenses to form compact mitotic
chromosomes. Chromosomes are seen to be composed of two
chromatids attached together at the centromere.
l l l l l Initiation of the assembly of mitotic spindle, the microtubules, the
proteinaceous components of the cell cytoplasm help in the
process.
You have studied
mitosis in onion root
tip cells.  It has 16
chromosomes in
each cell.  Can you
tell how many
chromosomes will
the cell have at G
1
phase, after S phase,
and after M phase?
Also, what will be the
DNA content of the
cells at G
1
, after S
and at G
2
, if the
content after M
phase is 2C?
2015-16(19/01/2015)
CELL CYCLE AND CELL DIVISION 165
Cells at the end of prophase, when viewed under the
microscope, do not show golgi complexes, endoplasmic
reticulum, nucleolus and the nuclear envelope.
10.2.2 Metaphase
The complete disintegration of the nuclear envelope marks
the start of the second phase of mitosis, hence the
chromosomes are spread through the cytoplasm of the cell.
By this stage, condensation of chromosomes is completed
and they can be observed clearly under the microscope. This
then, is the stage at which morphology of chromosomes is
most easily studied. At this stage, metaphase chromosome
is made up of two sister chromatids, which are held together
by the centromere (Figure 10.2 b). Small disc-shaped
structures at the surface of the centromeres are called
kinetochores. These structures serve as the sites of attachment
of spindle fibres (formed by the spindle fibres) to the
chromosomes that are moved into position at the centre of
the cell. Hence, the metaphase is characterised by all the
chromosomes coming to lie at the equator with one chromatid
of each chromosome connected by its kinetochore to spindle
fibres from one pole and its sister chromatid connected by
its kinetochore to spindle fibres from the opposite pole (Figure
10.2 b). The plane of alignment of the chromosomes at
metaphase is referred to as the metaphase plate.  The key
features of metaphase are:
l l l l l Spindle fibres attach to kinetochores of
chromosomes.
l l l l l Chromosomes are moved to spindle equator and get
aligned along metaphase plate through spindle fibres
to both poles.
10.2.3 Anaphase
At the onset of anaphase, each chromosome arranged at
the metaphase plate is split simultaneously and the two
daughter chromatids, now referred to as chromosomes of
the future daughter nuclei, begin their migration towards
the two opposite poles. As each chromosome moves away
from the equatorial plate, the centromere of each
chromosome is towards the pole and hence at the leading
edge, with the arms of the chromosome trailing behind
(Figure 10.2 c). Thus, anaphase stage is characterised by
Figure 10.2 a and b : A diagrammatic
view of stages in mitosis
2015-16(19/01/2015)
166 BIOLOGY
the following key events:
l l l l l Centromeres split and chromatids separate.
l l l l l Chromatids move to opposite poles.
10.2.4 Telophase
At the beginning of the final stage of mitosis, i.e., telophase,
the chromosomes that have reached their respective poles
decondense and lose their individuality. The individual
chromosomes  can no longer be seen and chromatin material
tends to collect in a mass in the two poles (Figure 10.2 d).
This is the stage which shows the following key events:
l l l l l Chromosomes cluster at opposite spindle poles and their
identity is lost as discrete elements.
l l l l l Nuclear envelope assembles around the chromosome
clusters.
l l l l l Nucleolus, golgi complex and ER reform.
10.2.5 Cytokinesis
Mitosis accomplishes not only the segregation of duplicated
chromosomes into daughter nuclei (karyokinesis), but the
cell itself is divided into two daughter cells by a separate
process called cytokinesis at the end of which cell division is
complete (Figure 10.2 e). In an animal cell, this is achieved
by the appearance of a furrow in the plasma membrane.
The furrow gradually deepens and ultimately joins in the
centre dividing the cell cytoplasm into two. Plant cells
however, are enclosed by a relatively inextensible cell wall,
thererfore they undergo cytokinesis by a different
mechanism. In plant cells, wall formation starts in the centre
of the cell and grows outward to meet the existing lateral
walls. The formation of the new cell wall begins with the
formation of a simple precursor, called the cell-plate that
represents the middle lamella between the walls of two
adjacent cells. At the time of cytoplasmic division, organelles
like mitochondria and plastids get distributed between the
two daughter cells. In some organisms karyokinesis is not
followed by cytokinesis as a result of which multinucleate
condition arises leading to the formation of syncytium (e.g.,
liquid endosperm in coconut). Figure 10.2 c to e : A diagrammatic
view of stages in Mitosis
2015-16(19/01/2015)
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