Cell Cycle and Cell Division Class 11 Notes Biology Chapter 10

 Page 1


Points To Remember
Cell cycle : 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.
Interphase (G
1
 phase + S phase + G
2
 Phase)
Phases of cell cycle  
 M Phase (Mitosis phases)
Interphase : (Resting Phase)
? G
1
 Phase : Cell metabolically active and grows continuously but does not
replicate DNA
? S Phase : DNA synthesis occurs, DNA content increases from 2C to 4C, but
the number of chromosomes remains same i.e., 2n.
? G
2
 Phase : Proteins are synthesised in preparation for mitosis while cell
growth continues.
M Phase (Mitosis Phase) : Starts with nuclear division, corresponding to
separation of daughter chromosomes (karyokinesis) and usually ends with division 
of cytoplasm, (cytokinesis).
Quiescent stage (G
0
) In adult animals cells that do not divide and exit  G
1
phase to enter an inactive stage called G
0
. Cells at this stage remain metabolically 
active but do not proliferate.
e.g., Heart cells
Karyokinesis (division of nucleus)
Cytokinesis (division of cytoplasm)
Page 2


Points To Remember
Cell cycle : 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.
Interphase (G
1
 phase + S phase + G
2
 Phase)
Phases of cell cycle  
 M Phase (Mitosis phases)
Interphase : (Resting Phase)
? G
1
 Phase : Cell metabolically active and grows continuously but does not
replicate DNA
? S Phase : DNA synthesis occurs, DNA content increases from 2C to 4C, but
the number of chromosomes remains same i.e., 2n.
? G
2
 Phase : Proteins are synthesised in preparation for mitosis while cell
growth continues.
M Phase (Mitosis Phase) : Starts with nuclear division, corresponding to
separation of daughter chromosomes (karyokinesis) and usually ends with division 
of cytoplasm, (cytokinesis).
Quiescent stage (G
0
) In adult animals cells that do not divide and exit  G
1
phase to enter an inactive stage called G
0
. Cells at this stage remain metabolically 
active but do not proliferate.
e.g., Heart cells
Karyokinesis (division of nucleus)
Cytokinesis (division of cytoplasm)
Mitosis
Since the number of chromosomes in the parent and progeny cells is the 
same, it is called as equational division. Mitosis is divided into four sub stages.
1. Prophase : (i) Replicated chromosomes, each consisting of 2 chromatids,
condense and become visible.
(i) Microtubules are assembled into mitotic spindle.
(iii) Nucleolus and nuclear envelope disappear.
(iv) Centriole moves to opposite poles.
2. Metaphase : (i) Spindle fibres attached to kinetochores (small disc-shaped
structures at the surface of centromere) of chromosomes.
(ii) Chromosomes line up at the equator of the spindle to form metaphase
 plate.
3. Anaphase : (i) Centromeres split and chromatids separate.
(ii) Chromatids move to opposite poles due to shortening of spindle fibres.
4. Telophase : (i) Chromosomes cluster at opposite poles.
(ii)	 Nuclear	en v elope	assembles	around 	chromosomes	clusters’.
(iii) Nucleolus, Golgi Complex, E.R. reforms.
Page 3


Points To Remember
Cell cycle : 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.
Interphase (G
1
 phase + S phase + G
2
 Phase)
Phases of cell cycle  
 M Phase (Mitosis phases)
Interphase : (Resting Phase)
? G
1
 Phase : Cell metabolically active and grows continuously but does not
replicate DNA
? S Phase : DNA synthesis occurs, DNA content increases from 2C to 4C, but
the number of chromosomes remains same i.e., 2n.
? G
2
 Phase : Proteins are synthesised in preparation for mitosis while cell
growth continues.
M Phase (Mitosis Phase) : Starts with nuclear division, corresponding to
separation of daughter chromosomes (karyokinesis) and usually ends with division 
of cytoplasm, (cytokinesis).
Quiescent stage (G
0
) In adult animals cells that do not divide and exit  G
1
phase to enter an inactive stage called G
0
. Cells at this stage remain metabolically 
active but do not proliferate.
e.g., Heart cells
Karyokinesis (division of nucleus)
Cytokinesis (division of cytoplasm)
Mitosis
Since the number of chromosomes in the parent and progeny cells is the 
same, it is called as equational division. Mitosis is divided into four sub stages.
1. Prophase : (i) Replicated chromosomes, each consisting of 2 chromatids,
condense and become visible.
(i) Microtubules are assembled into mitotic spindle.
(iii) Nucleolus and nuclear envelope disappear.
(iv) Centriole moves to opposite poles.
2. Metaphase : (i) Spindle fibres attached to kinetochores (small disc-shaped
structures at the surface of centromere) of chromosomes.
(ii) Chromosomes line up at the equator of the spindle to form metaphase
 plate.
3. Anaphase : (i) Centromeres split and chromatids separate.
(ii) Chromatids move to opposite poles due to shortening of spindle fibres.
4. Telophase : (i) Chromosomes cluster at opposite poles.
(ii)	 Nuclear	en v elope	assembles	around 	chromosomes	clusters’.
(iii) Nucleolus, Golgi Complex, E.R. reforms.
Cytokinesis : Is the division of protoplast of a cell into two daughter cells 
after karyokinesis (nuclear division)
Animal Cytokinesis :
Appearance of furrow in plasma membrane which deepens and joins in the 
centre, dividing cell cytoplasm into two.
Plant cytokinesis : Formation of new cell wall begins with the formation 
of a simple precursor — cell plate which represents the middle lamella between 
the walls of two adjacent cells.
? When karyokinesis is not followed by cytokinesis, a multinucleated condition
arises. This is called syncytium.
Significance of Mitosis :
1. Growth-addition of cells.
2. Maintenance of surface/volume ratio. Maintain Nucleo–cytoplasmic ratio.
3. Maintenance of chromosomes number.
4. Regeneration.
Page 4


Points To Remember
Cell cycle : 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.
Interphase (G
1
 phase + S phase + G
2
 Phase)
Phases of cell cycle  
 M Phase (Mitosis phases)
Interphase : (Resting Phase)
? G
1
 Phase : Cell metabolically active and grows continuously but does not
replicate DNA
? S Phase : DNA synthesis occurs, DNA content increases from 2C to 4C, but
the number of chromosomes remains same i.e., 2n.
? G
2
 Phase : Proteins are synthesised in preparation for mitosis while cell
growth continues.
M Phase (Mitosis Phase) : Starts with nuclear division, corresponding to
separation of daughter chromosomes (karyokinesis) and usually ends with division 
of cytoplasm, (cytokinesis).
Quiescent stage (G
0
) In adult animals cells that do not divide and exit  G
1
phase to enter an inactive stage called G
0
. Cells at this stage remain metabolically 
active but do not proliferate.
e.g., Heart cells
Karyokinesis (division of nucleus)
Cytokinesis (division of cytoplasm)
Mitosis
Since the number of chromosomes in the parent and progeny cells is the 
same, it is called as equational division. Mitosis is divided into four sub stages.
1. Prophase : (i) Replicated chromosomes, each consisting of 2 chromatids,
condense and become visible.
(i) Microtubules are assembled into mitotic spindle.
(iii) Nucleolus and nuclear envelope disappear.
(iv) Centriole moves to opposite poles.
2. Metaphase : (i) Spindle fibres attached to kinetochores (small disc-shaped
structures at the surface of centromere) of chromosomes.
(ii) Chromosomes line up at the equator of the spindle to form metaphase
 plate.
3. Anaphase : (i) Centromeres split and chromatids separate.
(ii) Chromatids move to opposite poles due to shortening of spindle fibres.
4. Telophase : (i) Chromosomes cluster at opposite poles.
(ii)	 Nuclear	en v elope	assembles	around 	chromosomes	clusters’.
(iii) Nucleolus, Golgi Complex, E.R. reforms.
Cytokinesis : Is the division of protoplast of a cell into two daughter cells 
after karyokinesis (nuclear division)
Animal Cytokinesis :
Appearance of furrow in plasma membrane which deepens and joins in the 
centre, dividing cell cytoplasm into two.
Plant cytokinesis : Formation of new cell wall begins with the formation 
of a simple precursor — cell plate which represents the middle lamella between 
the walls of two adjacent cells.
? When karyokinesis is not followed by cytokinesis, a multinucleated condition
arises. This is called syncytium.
Significance of Mitosis :
1. Growth-addition of cells.
2. Maintenance of surface/volume ratio. Maintain Nucleo–cytoplasmic ratio.
3. Maintenance of chromosomes number.
4. Regeneration.
5. Reproduction in unicellular organisms, lower plants and some insects.
6. Repair and wound healing.
7. Vegetative reproduction in plants takes place by mitosis.
Meiosis :
? Specialised kind of cell division that  reduces the chromosomes number by
half. hence it is called reductional division.
? Occurs during gametogenesis in plants and animals.
? Involves two sequential cycles of nuclear and cell division called Meiosis I
and Meiosis II.
? It results in 4 haploid daughter cells.
? Interphase occurs prior to meiosis which is similar to interphase of mitosis
except the S phase is prolonged.
Meiosis I
Prophase I : Subdivided into 5 phases.
(i) Leptotene :
 ? Chromosomes make their appearance as single stranded structures.
? Compaction of chromosomes continues.
(ii) Zygotene :
 ? Homologous chromosomes start pairing and this process of association
is called synapsis.
 ? Chromosomal synapsis is accompanied by formation of Synaptonemal
complex.
 ? Complex formed by a pair of synapsed homologous chromosomes is
called bivalent or tetrad.
 (iii) Pachytene : Crossing over occurs between non-sister chromatids of 
homologous chromosomes. The enzymes involved in the process is 
‘recombinase’. Recombination between homologous chromosomes is 
completed. Exchange of genetic material.
 (iv) Diplotene : Dissolution of synaptonemal complex occurs and the recombined 
chromosomes separate separate from each other except at the sites of crossing 
over. These X-shaped structures are called chaismata. In oocytes of some 
vertebrates diplotene can last for month or years.
Page 5


Points To Remember
Cell cycle : 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.
Interphase (G
1
 phase + S phase + G
2
 Phase)
Phases of cell cycle  
 M Phase (Mitosis phases)
Interphase : (Resting Phase)
? G
1
 Phase : Cell metabolically active and grows continuously but does not
replicate DNA
? S Phase : DNA synthesis occurs, DNA content increases from 2C to 4C, but
the number of chromosomes remains same i.e., 2n.
? G
2
 Phase : Proteins are synthesised in preparation for mitosis while cell
growth continues.
M Phase (Mitosis Phase) : Starts with nuclear division, corresponding to
separation of daughter chromosomes (karyokinesis) and usually ends with division 
of cytoplasm, (cytokinesis).
Quiescent stage (G
0
) In adult animals cells that do not divide and exit  G
1
phase to enter an inactive stage called G
0
. Cells at this stage remain metabolically 
active but do not proliferate.
e.g., Heart cells
Karyokinesis (division of nucleus)
Cytokinesis (division of cytoplasm)
Mitosis
Since the number of chromosomes in the parent and progeny cells is the 
same, it is called as equational division. Mitosis is divided into four sub stages.
1. Prophase : (i) Replicated chromosomes, each consisting of 2 chromatids,
condense and become visible.
(i) Microtubules are assembled into mitotic spindle.
(iii) Nucleolus and nuclear envelope disappear.
(iv) Centriole moves to opposite poles.
2. Metaphase : (i) Spindle fibres attached to kinetochores (small disc-shaped
structures at the surface of centromere) of chromosomes.
(ii) Chromosomes line up at the equator of the spindle to form metaphase
 plate.
3. Anaphase : (i) Centromeres split and chromatids separate.
(ii) Chromatids move to opposite poles due to shortening of spindle fibres.
4. Telophase : (i) Chromosomes cluster at opposite poles.
(ii)	 Nuclear	en v elope	assembles	around 	chromosomes	clusters’.
(iii) Nucleolus, Golgi Complex, E.R. reforms.
Cytokinesis : Is the division of protoplast of a cell into two daughter cells 
after karyokinesis (nuclear division)
Animal Cytokinesis :
Appearance of furrow in plasma membrane which deepens and joins in the 
centre, dividing cell cytoplasm into two.
Plant cytokinesis : Formation of new cell wall begins with the formation 
of a simple precursor — cell plate which represents the middle lamella between 
the walls of two adjacent cells.
? When karyokinesis is not followed by cytokinesis, a multinucleated condition
arises. This is called syncytium.
Significance of Mitosis :
1. Growth-addition of cells.
2. Maintenance of surface/volume ratio. Maintain Nucleo–cytoplasmic ratio.
3. Maintenance of chromosomes number.
4. Regeneration.
5. Reproduction in unicellular organisms, lower plants and some insects.
6. Repair and wound healing.
7. Vegetative reproduction in plants takes place by mitosis.
Meiosis :
? Specialised kind of cell division that  reduces the chromosomes number by
half. hence it is called reductional division.
? Occurs during gametogenesis in plants and animals.
? Involves two sequential cycles of nuclear and cell division called Meiosis I
and Meiosis II.
? It results in 4 haploid daughter cells.
? Interphase occurs prior to meiosis which is similar to interphase of mitosis
except the S phase is prolonged.
Meiosis I
Prophase I : Subdivided into 5 phases.
(i) Leptotene :
 ? Chromosomes make their appearance as single stranded structures.
? Compaction of chromosomes continues.
(ii) Zygotene :
 ? Homologous chromosomes start pairing and this process of association
is called synapsis.
 ? Chromosomal synapsis is accompanied by formation of Synaptonemal
complex.
 ? Complex formed by a pair of synapsed homologous chromosomes is
called bivalent or tetrad.
 (iii) Pachytene : Crossing over occurs between non-sister chromatids of 
homologous chromosomes. The enzymes involved in the process is 
‘recombinase’. Recombination between homologous chromosomes is 
completed. Exchange of genetic material.
 (iv) Diplotene : Dissolution of synaptonemal complex occurs and the recombined 
chromosomes separate separate from each other except at the sites of crossing 
over. These X-shaped structures are called chaismata. In oocytes of some 
vertebrates diplotene can last for month or years.
(v) Diakinesis : Terminalisation of chaismata.
? Chromosomes are fully condensed and meiotic spindles assembled.
 ? Nucleolus disappear and nuclear envelope breaks down.
Metaphase I : Bivalent chromosomes align on  the equatorial plate.
? Microtubules from opposite poles of the spindle attach to the pair of
homologous chromosomes.
Anaphase I : Homologous chromosomes, separate while chromatids remain 
associated at their centromeres.
Telophase I : 
? Nuclear membrane and nucleus reappear.
? Cytokinesis follows (diad of cells).
Interkinesis : Stage between two meiotic divisions, (meiosis I and meiosis II) 
generally short lived.
Meiosis II: (It resembles the normal mitosis).
Prophase II
? Nuclear membrane disappears.
? Chromosomes again become compact.
Metapahse II
? Chromosomes align at the equator.
? Microtubules from opposite poles of spindle get attached to kinetochores of
sister chromatids.
Anaphase II
? Simultaneous splitting of the centromere of each chromosome, allowing
them to move towards opposite poles of the cell.
Telophase II
? Two groups of chromosomes get enclosed by a nuclear envelope.
? Cytokinesis follows resulting in the formation of tetrad of cells i.e., 4 haploid
cells.
Significance of Meiosis
1. Formation of gametes :  In sexually reproducing organisms.
2. Genetic variability : Variations are very important for evolution.
3. Maintenance of chromosomal number : By reducing the chromosome
number in gametes. Chromosomal number is restored by fertilisation of
gametes.