Cell Cycle And Cell Division
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Introduction :
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.
Cell division is of two types :–
(1) Mitosis (2) Meiosis
MITOSIS
* Term mitosis was proposed by Flemming. Mitosis produced genetically identical cells, which are similar to
mother cell.
Cause of mitosis :-
(I) Kern plasm theory : Hertwig proposed kern plasm theory. According to this theory mitosis occurs
due to disturbance in Karyoplasmic Index (KI) or Nucleocytoplasmic ratio of cell.
Karyoplasmic Index :
KI =
V
V V
n
c n - Vn = Volume of nucleus
Vc = Volume of cell
Vc – Vn = Volume of cytoplasm
* Karyoplasmic Index of small cell is high as they have less cytoplasm.
Nucleus efficiently controls the acitivity of cytoplasm in small cells.
* In a large cell nucleus fail to control the activity of cytoplasm. To attain
the control of nucleus on metabolism a large cell divides into two cells.
(II) Surface-volume Ratio :
Surface-volume ratio is also considered as a cause of cell division. When a cell grows in size its volumes
increases more than its surface. So a stage will reach when the surface area becomes insufficient to draw
the material. At such critical stage, division of cell started.
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Pre-Medical : Biology ALLEN
CELL CYCLE AND CELL DIVISION
Introduction :
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.
Cell division is of two types :–
(1) Mitosis (2) Meiosis
MITOSIS
* Term mitosis was proposed by Flemming. Mitosis produced genetically identical cells, which are similar to
mother cell.
Cause of mitosis :-
(I) Kern plasm theory : Hertwig proposed kern plasm theory. According to this theory mitosis occurs
due to disturbance in Karyoplasmic Index (KI) or Nucleocytoplasmic ratio of cell.
Karyoplasmic Index :
KI =
V
V V
n
c n - Vn = Volume of nucleus
Vc = Volume of cell
Vc – Vn = Volume of cytoplasm
* Karyoplasmic Index of small cell is high as they have less cytoplasm.
Nucleus efficiently controls the acitivity of cytoplasm in small cells.
* In a large cell nucleus fail to control the activity of cytoplasm. To attain
the control of nucleus on metabolism a large cell divides into two cells.
(II) Surface-volume Ratio :
Surface-volume ratio is also considered as a cause of cell division. When a cell grows in size its volumes
increases more than its surface. So a stage will reach when the surface area becomes insufficient to draw
the material. At such critical stage, division of cell started.
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
PHASES OF CELL CYCLE
A typical eukaryotic cell cycle is illustrated by
human cells in culture. These cells divide once in ap-
proximately every 24 hours.
Yeast can progress through the cell cycle in only about
90 minutes. The time period of cell cycle is varied
from organism to organism and also from cell type
to cell type.
Cell cycle involves two stages :-
(1) Interphase
(2) Division phase/M-phase
1. Interphase :-
This is phase between two successive M-phase. In interphase cell grows in size and prepares
itself for next division. Interphase is most active phase of cell cycle. The interphase last more than 95% of the
duration of cell cycle.
A series of metabolic changes occurs during interphase in cell. These changes were not visible under micro-
scope, So some scientist termed interphase as resting phase. It is the time during which cell is preparing for
division by undergoing both cell growth and DNA replication in an orderly manner.
Howard and Pelc classified interphase into three sub stages :-
(i) G1 – phase or Pre DNA synthesis phase (Ist Gap phase)
* G1 phase corresponds to the interval between mitosis and initiation of DNA replication. During
G1 phase the cell is metabolically active and continuously grows
* During G1-most of cell organelles increases in cell and cell rapidly synthesizes different types of
RNA and proteins. Due to availability of protein, synthesis of new protoplasm takes place in cell and it
starts growing in size. Cell grows maximum in G1 stage.
(ii) S – phase (DNA synthesis phase) :
* Replication of nuclear DNA and synthesis of histone protein takes place in s-phase. Replication of
cytoplasmic DNA may occur in any stage of cell cycle.
* 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 G1, even after S phase the number of chromosomes remains the same, i.e., 2n.
* S-phase marks the phase of DNA replication and chromosome duplication (DNA content in a chromosome
In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in
the cytoplasm.
(iii) G2 – phase (2nd Gap phase) or Post DNA synthesis phase (Pre mitosis phase)
* Actual preparation (Final preparation) of M-phase occurs during this phase. Special materials
required for M-phase are synthesized in G2 phase. eg. Tubulin protein. –(Required for formation of
spindle fibres). Cell growth continues.
G0 phase -
* 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 G1 phase to enter an inactive stage called quiescent stage (G0)
of the cell cycle.
* Cells in this stage remain metabolically active but no longer proliferate (divide) unless called on to do so
depending on the requirement of the organism.
Division phase :
* Division phase or M–phase or mitotic phase lasts for only about an hour in the 24 hour duration of cell cycle
of a human cell.
* The M-phase represents the phase when the actual cell division or mitosis occurs.
* In animals, mitotic cell division is restricted or only seen in diploid somatic cell except in some social insects.
Against this, the plants can show mitotic division in both haploid and diploid cells.
* 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.
* The M-phae start with nuclear division, corresponding to the separation of daughter chromosome (Karyokinesis)
and usually ends with division of cytoplasm (cytokinesis).
Mitosis is divided into the following four stages :-
Prophase
Metaphase
Anaphase
Telophase
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