CHAPTER 10
CELL CYCLE AND CELL DIVISION
10.1 Cell Cycle Are you aware that all organisms, even the largest, start their life from a single cell?
10.2 M Phase 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.
10.3 Significance of
All cells reproduce by dividing into two, with each parental cell giving rise to two
Mitosis
daughter cells each time they divide. These newly formed daughter cells can
10.4 Meiosis 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
10.5 Significance of
cycles of growth and division allow a single cell to form a structure consisting of
Meiosis
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 163
ase
Figure 10.1 A diagrammatic view of cell cycle indicating
formation of two cells from one cell
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:
z Interphase
z M Phase (Mitosis phase)
(cytokinesis). The
The M Phase represents the phase when the actual
interphase, though called
cell division or mitosis occurs and the interphase
the resting phase, is the
represents the phase between two successive M
time during which the
phases. It is significant to note that in the 24 hour
cell is preparing for
average duration of cell cycle of a human cell, cell
division by undergoing
division proper lasts for only about an hour. The
both cell growth and
interphase lasts more than 95% of the duration of cell
DNA replication in an
cycle.
orderly manner. The
The M Phase starts with the nuclear division, corresponding to the separation of
interphase is divided into
daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm
three further phases:
, z G1 phase (Gap 1)
z S phase (Synthesis)
z G2 phase (Gap 2)
G1 phase corresponds to the interval between mitosis and initiation of DNA
replication. During G1 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
How do plants and
the centriole duplicates in the cytoplasm. During the G 2 phase, proteins are
animals continue to grow
synthesised in preparation for mitosis while cell growth continues. 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 m e
r i s t e m a t i c tissues?
CELL CYCLE AND CELL DIVISION
10.1 Cell Cycle Are you aware that all organisms, even the largest, start their life from a single cell?
10.2 M Phase 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.
10.3 Significance of
All cells reproduce by dividing into two, with each parental cell giving rise to two
Mitosis
daughter cells each time they divide. These newly formed daughter cells can
10.4 Meiosis 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
10.5 Significance of
cycles of growth and division allow a single cell to form a structure consisting of
Meiosis
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 163
ase
Figure 10.1 A diagrammatic view of cell cycle indicating
formation of two cells from one cell
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:
z Interphase
z M Phase (Mitosis phase)
(cytokinesis). The
The M Phase represents the phase when the actual
interphase, though called
cell division or mitosis occurs and the interphase
the resting phase, is the
represents the phase between two successive M
time during which the
phases. It is significant to note that in the 24 hour
cell is preparing for
average duration of cell cycle of a human cell, cell
division by undergoing
division proper lasts for only about an hour. The
both cell growth and
interphase lasts more than 95% of the duration of cell
DNA replication in an
cycle.
orderly manner. The
The M Phase starts with the nuclear division, corresponding to the separation of
interphase is divided into
daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm
three further phases:
, z G1 phase (Gap 1)
z S phase (Synthesis)
z G2 phase (Gap 2)
G1 phase corresponds to the interval between mitosis and initiation of DNA
replication. During G1 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
How do plants and
the centriole duplicates in the cytoplasm. During the G 2 phase, proteins are
animals continue to grow
synthesised in preparation for mitosis while cell growth continues. 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 m e
r i s t e m a t i c tissues?
