CHRIST SCHOOL
(Affiliated to the Council for the ICSE, New Delhi. Reg. No: KA260/2013)
ChristSchoolRoad,DharmaramCollegeP.O.,Bangalore560029,Karnataka
Cell Division – Notes
1. Cell division is a method of producing two or more new daughter cells from a
single mother cell.
2. Significance of cell division - Continuity of life depends on cell division. Cell
division also helps in
 Growth
 Replacement
 Repair
 Reproduction of an organism.
3. Cell division types – Amitosis, Mitosis, Meiosis
4. Amitosis
 Simple common process of cell division.
 Occurs in prokaryotic cells.
 Nucleus divides several times without the division of cytoplasm.
5. Mitosis
 Cell divides into two daughter cells.
 Each having the same chromosome number and genetic constitution.
 Ensures, all the cells of an individual is genetically identical to each
other.
6. Meiosis
 Cell divides into 4 daughter cells.
 Each having only half the number of chromosomes.
 Shows genetic variation.
 Ensures the production of gametes.
7. The sequence of events including duplication of chromosomes (DNA),
synthesis of other cell constituents, growth and division that a cell undergoes
from the time of its formation up to its division into daughter cells is called
CELL CYCLE.
8. A cell cycle consists of two phases.
 Interphase – Resting phase (Non-dividing phase) - Metabolically very
active.
 Mitotic phase – Divisional phase (Dividing phase)
9. Interphase
 A preparatory phase.
 Non-dividing phase.
 Cell grows in to its maximum size.
 Chromosomes duplication occurs.
 Growth phase between two successive divisions of the cell.
10. Interphase is divided into 3 phases.
 G1 – First growth (gap) phase/ Post mitotic phase.
 S – Synthesis phase.
 G2 - Second growth (gap) phase/ Pre mitotic phase.
11. Interphase is the non-dividing preparatory phase of cell cycle during which
a cell grows in size and duplication of chromosome take place.
12. G1 – First growth (Gap) phase/ Post mitotic phase.(10 hrs)
Longest phase of Interphase.
Also called first gap phase.
Preparatory phase for the oncoming cell division.
Major event
 Synthesis of RNA and proteins.
 Volume of cytoplasm increases.
 Cell organelles increases in number.
13. S – Synthesis phase.(8hrs)
 Duplication of chromatin fibres by the synthesis of histones
and DNA materials. ( So that they are equally distributed
among the daughter cells during cell division)
14. G2 - Second growth phase/ Pre mitotic phase.(5hrs)
 Synthesis of RNA and proteins continues.
 Replication of centrosomes.
15. Mitotic phase (M phase) 1hr.
 It is the phase of cell division - Dividing phase.
 Studied in two divisions.
 Division of nucleus (Karyokinesis) and Division of
cytoplasm (Cytokinesis).
MITOSIS
 Occurs in somatic cells.
 For growth, repair, replacement and asexual reproduction in unicellular
organisms.
 Maintain the chromosome number even after the division of the cell.
 Called Equational division.
 Produce two daughter cells.
 Each having the same chromosome number and genetic constitution.
 Ensures, all the cells of an individual is genetically identical to each other
 Maintain the diploid condition of the cell.
 A cell division in which two identical daughter cells are formed by the
division of one parent cell.
1. Mitosis is a type of cell division that take place in somatic cells, during which
the cell divides into two daughter cells, each containing the same and similar
number of chromosomes as present in the parent cell.
2. Karyokinesis occurs in 4 phases:
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


3.
Prophase
Metaphase
Anaphase
Telophase
Prophase
 Longest and most complex phase.
 Lasts about 50 min.
Major Events – Early Prophase











Condensation of chromatin fibres.
Nuclear membrane starts gradually disappearing.
Nucleolus also starts disappearing.
Centrosomes with centrioles start migrate to the opposite poles of the cell.
A number of fine radiating fibre like structures arise from the centrioles
called Astral rays
Centrioles along with astral rays are called Asters
Asters get extended between the centrioles which move towards the
opposite poles of the cell.
The very fine proteinaceous thread like structures that are developed
between the two asters are called Spindle fibres.
Asters along with spindle fibres are called spindle apparatus /mitotic
apparatus.
In animals spindle apparatus /mitotic apparatus formed as amphiastral –
Formed from asters.
In plants spindle apparatus /mitotic apparatus formed (without asters) as
anastral – Formed from cytoplasmic strands called microtubules.
Late prophase
 Nuclear membrane and nucleolus disappeared.
 Chromosomes can be seen prominently (as two
longitudinal chromatids joined by the
centromrere).
 Centrioles are reached the opposite poles with
spindle fibres.
 Chromosomes get attached to the spindle fibers
with the help of the centrosome.
4.
Metaphase
 Simple, Short, 2-10 min
 Chromosomes get aligned at the equator of the cell.
 Centromeres lie on the equator and the arms face
towards the poles.
 The arrangement of chromosomes on the equator in
such a way that their centromeres lie on the equator
and the arms face towards the poles is called
Metaphasic plate/ Equatorial plate.
5.
Anaphase
 Simple, Short, 2-3 min
 The centromere of each chromosome divides
(longitudinally) into two halves. (Chromatids
separated with its own centromere).
 Each daughter chromosome retains one of the two
chromatids.
 The daughter chromosomes move towards the
opposite poles (due to the contraction of spindle
fibres & the repelling force developed between
them)
Centromere
Chromosome
Spindle fibres
6.
Telophase
 Long, Complex, Reverse of prophase.
 The daughter chromosomes reach at the
opposite poles.
 Start uncoiling, thinning and elongatingchromatin network.
 Nuclear
membrane
and
nucleolus
reappear.
 Spindle fibres, astral rays disappear.
7. Karyokinesis is followed by Cytokinesis.
8. Cytokinesis:
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


Division of cytoplasm
Begins during late Anaphase.
Completed soon after the telophase.
Different in animal and plant cell.
In animal cell:
 A furrow or constriction appears in the middle of the cell membrane,
parallel to the equator.
 Deepens towards the centre of the cell - CENTRIPETAL
In plant cell:
 A cell plate is formed at the centre of the equator.
 Extends towards the periphery - CENTRIFUGAL
9. Differences between mitosis in plant cell and animal cell.
Animal cell
Cytokinesis By the appearance of
furrow or constriction.
10.
Plant cell
By the formation of cell plate.
It is Centripetal –
Proceeds from periphery
towards the centre.
It is Centrifugal -Proceeds
from centre towards the
periphery.
Asters
Formed from the
centrioles. (Due to the
presence of centrosome)
Not formed (Centrosome &
centrioles are absent)
Spindle
fibres
Formed as amphiastral –
Formed from asters.
Formed as anastral – Formed
from microtubules
Occur
Most tissues throughout
the body.
Mainly at the growing tips.
Significance of Mitosis:
 Keeps the number of chromosomes constant in the daughter cells.(genetic
stability)
 Responsible for the growth of an organism.(increases the body size)
 Helps in repairing damaged or wounded tissues.
 Helps in the replacement of old and dead cells – lost during normal wear
and tear.
 Method of reproduction in unicellular organism – Asexual reproduction.
11.
Meiosis (Reduction division)
 Produces sex cells or gametes.
 Occurs in the reproductive organs – reproductive cells. (Testis – Sperm,
Ovary – Ova/Egg in animals & Anthers – pollen grains, Ovary – Ovules in
flowering plants)
 Occurs in diploid cell – A cell which has full set of chromosomes
corresponding to the chromosome number of that particular species is
termed as diploid cell (2n).
 Cell divides into 4 daughter cells.
 Each having only half the number of chromosomes. (Haploid cell) – A cell
which has only half the number of chromosomes corresponding to the
chromosome number of that particular species is termed as haploid cell (n).
 Meiosis is a reduction division as the chromosome number is reduced to
half in the daughter cells when compared to the parent cell.
 Meiosis is completed in successive cell division – Meiosis I and Meiosis II.
 Meiosis I – Reduction division.
 Meiosis II – Equational division
 Meiosis I is called Reduction division because the number of chromosomes
in this type of division is reduced to half in the daughter cells.
 Meiosis I is followed by Meiosis II.
 Meiosis II is called Equational division because it is just similar to the
mitosis.
12. Meiosis is a type of cell division in which the cell undergoes two successive
cell division producing four daughter cells, each containing half the number of
chromosomes as compared to the parent cell.
13.
Meiosis I
 Condensation of chromosomes.
 Homologous chromosomes start to pair up.
 [A pair or of chromosomes which are similar in size, shape, position of
centromere and gene loci is termed as Homologous chromosomes].
 A pair of homologous chromosomes is called BIVALENT.
 The process of pairing up of homologous chromosomes is called
SYNAPSIS.
 The four chromatids of a homologous chromosome or a bivalent are called
TETRAD.
 Pairs of homologous chromosomes line up on the metaphase
plate.
 Mutual exchange of genetic material between the non- sister chromatids of
the corresponding parts of the homologous chromosomes takes place. This is
called CROSSING OVER.
 The point at which crossing over occurs where the homologous chromosome
remain attached is called CHIASMA (Chiasmata [p])
14. Significance of Crossing Over:
 It results in genetic combination. (Paternal and Maternal chromosomes get
mixed up)
 Provides variation in the progeny.
 Variations lead to the evolution.
15. The homologous chromosomes separate completely and move towards the
opposite poles.
16. Significance of Meiosis
 Helps in the formation of gametes.
 Helps to maintain the chromosome number constant generation after
generation.
 Crossing over helps to produce variations and thus leads to evolution.