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Cell Cycle – Summary Notes
Every organism begins its life as a single cell. The cell divides repeatedly to produce a cluster
of cells. These cells form tissues and the tissues form organs to perform different activities of
life. Cell division is essential for growth, replacement of old cells, repair of injury and
reproduction of living organisms.
Cell cycle is the sequence of events occurring between the cell formation and its division. It
consists of two phases – Interphase and Mitotic phase (M-phase).
Interphase is the interval between two successive cell division. It is divided into G 1 phase, S-
phase, G 2 phase.
The cell division involves three major steps. They include replication of DNA, division of
nucleus or karyokinesis and division of cytoplasm or cytokinesis.
Cell division is also essential for repair of injuries and reproduction.
There are two types of cell division – Mitosis and Meiosis. The cell division leading to growth,
development, repair and replacement is called mitosis. The cell division leading to production
of gametes is called meiosis.
Mitosis is the cell division which occurs in four phases namely prophase, metaphase, anaphase
and telophase.
In prophase, the chromosomes become short and thick which move towards the equator of
the cell. Each chromosome gets duplicated to form two chromatids which are attached to
each other at a small region called centromere.
In metaphase, the chromosomes arrange themselves on the equatorial plane. Each
chromosome gets attached to a spindle fibre at its centromere.
In anaphase, the centromere divides and the two sister chromatids of each chromosome
separate and are drawn apart towards opposite poles and form two groups of chromosomes.
Telophase is the last phase in which the chromatid becomes thinner and the nuclear
membrane reappears. Nucleolus also reappears in each daughter nucleus.
Mitosis is followed by the division of cytoplasm (cytokinesis). At the end of telophase, a
furrow appears in the cell membrane at the centre, which deepens and finally splits the
cytoplasm into two, thus producing two new cells.
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Meiosis is the kind of cell division that produces sex cells or the gametes. In this division, the
number of chromosomes in the cells is halved.
Meiosis is also called reduction cell division since the four daughter cells formed have half the
number of chromosomes than the mother cell.
There are two divisions of meiosis – meiosis I (reduction division) and meiosis II (equational
division).
Significance of Mitosis
Mitosis plays an important role in vegetative growth, wound healing, repair and
regeneration.
It maintains the same chromosome number in the daughter cells as well.
It maintains the proper size of a cell.
It helps in asexual reproduction, e.g. binary fission in Amoeba. It helps in asexual
reproduction in unicellular organisms.
Significance of Meiosis
It plays an important role in sexual reproduction, i.e., in the formation of gametes (sperms
and ova).
It maintains the constant number of chromosomes in a species.
It brings about recombination between maternal and paternal genes thereby bringing in
genetic variations are responsible for evolution.
Chromosomes are long thread-like structures made of DNA and proteins.
DNA molecule is made up of a double chain of nucleotides in the form of a helix. A nucleotide
is a sugar molecule joined to a phosphate group and an organic base.
Differences between mitosis and meiosis
Mitosis Meiosis
1. It occurs in vegetative cells. 1. It occurs in reproductive cells.
2. The cell divides only once. 2. It involves two successive divisions.
3. Two daughter cells are formed from a parent 3. Four daughter cells are formed from a parent
cell on completion of the division. cell on completion of the division.
4. The daughter cells are genetically similar to 4. The daughter cells are different from parent
the parent cells. cells.
5. The number of chromosomes in the daughter 5. The number of chromosomes in the daughter
cells is equal to that found in the parent cell. cells is half of that in the parent cells.
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Differences between mitosis in animal cell and plant cell
Mitosis in animal cell Metosis in plant cell
1. The spindle formed is amphiastral. 1. The spindle formed is anastral.
2. Cytokinesis occurs by constriction or furrow 2. Cytokinesis occurs by the formation of cell
method. plate.
3. Cytokinesis proceeds from outside to centre. 3. Cytokinesis proceeds from centre to the
4. Centrosome is present. periphery.
5. Two asters are formed. 4. Centrosome is absent.
5. No asters are formed.
The specific part of the chromosomes, which are composed of DNA are called genes. They
determine hereditary characters. Genes are responsible for transmission of characters from one
generation to another.
There are four types of chromosomes – metacentric, sub metacentric, acrocentric, telocentric.
Metacentric: Centromere lies in the centre and two arms of chromatid are equal.
Telocentric: Centromere is present at one end of chromosome.
Sub metacentric: Centromere slightly away from centre and one arm of chromatid is
slightly larger than other.
Acrocentric: Centromere is present near one end of chromatid and one arm is very large
than the other.
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