# Definition : It is also called indirect cell division or somadtic cell division or equational division. In this, mature somatic cell divides in such a way that chromosomes number is kept constant in daughter cells equal to those in parent cell, so the daughter cells are quantitatively as well as qualitatively similar to the parental cell. So it is called equational division.
# Discovery : Mitosis was first observed by Strasburger (1875) and in animal cell by W.fleming (1879) term mitosis was given by Fleming (1882).
# Occurrence : Mitosis is the common method of cell division. It takes place in the somatic cells in the animals. Hence, it is also known as the somatic division. It occurs in the gonads also for the multiplication of undifferentiated germ cells. In plants mitosis occurs in the meristematic cells e.g. root apex and shoot apex.
# Duration : It ranges from 30 minutes to 3 hours time is species-specific but also depends upon type of tissues, temperature.
# Process of mitosis : Mitosis is completed in two steps
Karyokinesis : (Gk. karyon = nucleus; kinesis = movement) Division of nucleus. Term given by Schneider (1887).
Cytokinesis : (Gk –kitos = cell; kinesis = movement) Division of cytoplasm, Term given by Whitemann (1887).
# Karyokinesis : It comprises four phases i.e. Prophase, Metaphase, Anaphase, Telophase.
- (i) Prophase : It is largest phase of karyokinesis.
(a) Chromatin fibres thicken and shorter to form chromosomes which may overlap each other and appears like a ball of wool. i.e. Spireme stage.
(b) Each chromosome divides longitudinally into 2 chromatids which remain attached to centromere.
(c) Nuclear membrane starts disintegrating except in dinoflagellates.
(d) Nucleolus starts disintegrating.
(e) Cells become viscous, refractive and oval in outline.
(f) Spindle formation begins.
(g) Cell cytoskeleton, golgi complex, ER, etc. disappear.
(h) In animal cells, centrioles move towards opposite sides.
(i) Lampbrush chromosomes can be studied well.
(j) Small globular structure (beaded) on the chromosome are called chromomeres.
- (ii) Metaphase
(a) Chromosomes become maximally distinct i.e. size can be measured.
(b) A colourless, fibrous, bipolar spindle appears.
(c) Spindle is formed from centriole (in animal cells) or MTOC (microtubule organising centre) in plant cells successively called astral and anastral spindle.
(d) Spindle has 3 types of fibres.
Continuous fibre (run from pole to pole).
Discontinuous fibre (run between pole to centromeres).
Interzonal fibre (run between 2 centromere).
(e) Spindle fibre are made up of 97% tubulin protein and 3% RNA.
(f) Chromosomes move towards equatorial plane of spindles called congression and become arranged with their arms directed towards pole and centromere towards equator.
(g) Spindle fibres attach to kinetochores.
(h) Metaphase is the best stage for studying chromosome morphology.
- (iii) Anaphase
(a) Centromere splits from the middle and two chromatids gets separated.
(b) Both the chromatids move towards opposite poles due to repulsive force called anaphasic movement.
(c) Anaphasic movement is brought about by the repolymerisation of continuous fibres and depolymerisation of chromosomal fibres.
(d) Different shape of chromosomes become evident during chromosome movement viz. metacentric acrocentric etc.
(e) Chromosomes takes V, J, I or L shapes.
(f) The centromere faces towards equator.
(g) The chromatids are moved towards the pole at a speed of 1 m/minute. About 30 ATP molecules are used to move one chromosome from equator to pole.
- (iv) Telophase
(a) Chromosomes reached on poles by the spindle fibers and form two groups.
(b) Chromosomes begin to uncoil and form chromatin net.
(c) The nuclear membrane and nucleolus reappear.
(d) Two daughter nuclei are formed.
(e) Golgi complex and ER etc., reform.
Cytokinesis : It involves division of cytoplasm in animal cells, the cell membrane develops a constitution which deepens centripetally and is called cell furrow method.
In plant cells, cytokinesis occurs by cell plate formation.
# Significance of mitosis
(i) It keeps the chromosome number constant and genetic stability in daughter cells, so the linear heredity of an organism is maintained. All the cells are with similar genetic constituents.
(ii) It helps in growth and development of zygote into adult through embryo formation.
(iii) It provides new cells for repair and regeneration of lost parts and healing of the wounds.
(iv) It helps in asexual reproduction by fragmentation, budding, stem cutting, etc.
(v) It also restores the nucleo-plasmic ratio.
(vi) Somatic variations when maintained by vegetative propagation can play important role in speciation.
# Types of Mitosis
(i) Anastral mitosis : It is found in plants in which spindle has no aster.
(ii) Amphiastral mitosis : It is found in animals in which spindle has two asters, one at each pole of the spindle. Spindle is barrel-like.
(iii) Intranuclear or Promitosis : In this nuclear membrane is not lost and spindle is formed inside the nuclear membrane e.g. Protozoans (Amoeba) and yeast. It is so as centriole is present within the nucleus.
(iv) Extranuclear or Eumitosis : In this nuclear membrane is lost and spindle is formed outside nuclear membrane e.g. in plants and animals.
(v) Endomitosis : Chromosomes and their DNA duplicate but fail to separate which lead to polyploidy e.g. in liver of man, both diploid (2N) and polyploid cells (4N) have been reported. It is also called endoduplication and endopolyploidy.
(vi) Dinomitosis : In which nuclear envelope persists and microtubular spindle is not formed. During movement the chromosomes are attached with nuclear membrane.
# Definition : It is also called indirect cell division or somadtic cell division or equational division. In this, mature somatic cell divides in such a way that chromosomes number is kept constant in daughter cells equal to those in parent cell, so the daughter cells are quantitatively as well as qualitatively similar to the parental cell. So it is called equational division.
# Discovery : Mitosis was first observed by Strasburger (1875) and in animal cell by W.fleming (1879) term mitosis was given by Fleming (1882).
# Occurrence : Mitosis is the common method of cell division. It takes place in the somatic cells in the animals. Hence, it is also known as the somatic division. It occurs in the gonads also for the multiplication of undifferentiated germ cells. In plants mitosis occurs in the meristematic cells e.g. root apex and shoot apex.
# Duration : It ranges from 30 minutes to 3 hours time is species-specific but also depends upon type of tissues, temperature.
# Process of mitosis : Mitosis is completed in two steps
Karyokinesis : (Gk. karyon = nucleus; kinesis = movement) Division of nucleus. Term given by Schneider (1887).
Cytokinesis : (Gk –kitos = cell; kinesis = movement) Division of cytoplasm, Term given by Whitemann (1887).
# Karyokinesis : It comprises four phases i.e. Prophase, Metaphase, Anaphase, Telophase.
- (i) Prophase : It is largest phase of karyokinesis.
(a) Chromatin fibres thicken and shorter to form chromosomes which may overlap each other and appears like a ball of wool. i.e. Spireme stage.
(b) Each chromosome divides longitudinally into 2 chromatids which remain attached to centromere.
(c) Nuclear membrane starts disintegrating except in dinoflagellates.
(d) Nucleolus starts disintegrating.
(e) Cells become viscous, refractive and oval in outline.
(f) Spindle formation begins.
(g) Cell cytoskeleton, golgi complex, ER, etc. disappear.
(h) In animal cells, centrioles move towards opposite sides.
(i) Lampbrush chromosomes can be studied well.
(j) Small globular structure (beaded) on the chromosome are called chromomeres.
- (ii) Metaphase
(a) Chromosomes become maximally distinct i.e. size can be measured.
(b) A colourless, fibrous, bipolar spindle appears.
(c) Spindle is formed from centriole (in animal cells) or MTOC (microtubule organising centre) in plant cells successively called astral and anastral spindle.
(d) Spindle has 3 types of fibres.
Continuous fibre (run from pole to pole).
Discontinuous fibre (run between pole to centromeres).
Interzonal fibre (run between 2 centromere).
(e) Spindle fibre are made up of 97% tubulin protein and 3% RNA.
(f) Chromosomes move towards equatorial plane of spindles called congression and become arranged with their arms directed towards pole and centromere towards equator.
(g) Spindle fibres attach to kinetochores.
(h) Metaphase is the best stage for studying chromosome morphology.
- (iii) Anaphase
(a) Centromere splits from the middle and two chromatids gets separated.
(b) Both the chromatids move towards opposite poles due to repulsive force called anaphasic movement.
(c) Anaphasic movement is brought about by the repolymerisation of continuous fibres and depolymerisation of chromosomal fibres.
(d) Different shape of chromosomes become evident during chromosome movement viz. metacentric acrocentric etc.
(e) Chromosomes takes V, J, I or L shapes.
(f) The centromere faces towards equator.
(g) The chromatids are moved towards the pole at a speed of 1 m/minute. About 30 ATP molecules are used to move one chromosome from equator to pole.
- (iv) Telophase
(a) Chromosomes reached on poles by the spindle fibers and form two groups.
(b) Chromosomes begin to uncoil and form chromatin net.
(c) The nuclear membrane and nucleolus reappear.
(d) Two daughter nuclei are formed.
(e) Golgi complex and ER etc., reform.
Cytokinesis : It involves division of cytoplasm in animal cells, the cell membrane develops a constitution which deepens centripetally and is called cell furrow method.
In plant cells, cytokinesis occurs by cell plate formation.
# Significance of mitosis
(i) It keeps the chromosome number constant and genetic stability in daughter cells, so the linear heredity of an organism is maintained. All the cells are with similar genetic constituents.
(ii) It helps in growth and development of zygote into adult through embryo formation.
(iii) It provides new cells for repair and regeneration of lost parts and healing of the wounds.
(iv) It helps in asexual reproduction by fragmentation, budding, stem cutting, etc.
(v) It also restores the nucleo-plasmic ratio.
(vi) Somatic variations when maintained by vegetative propagation can play important role in speciation.
# Types of Mitosis
(i) Anastral mitosis : It is found in plants in which spindle has no aster.
(ii) Amphiastral mitosis : It is found in animals in which spindle has two asters, one at each pole of the spindle. Spindle is barrel-like.
(iii) Intranuclear or Promitosis : In this nuclear membrane is not lost and spindle is formed inside the nuclear membrane e.g. Protozoans (Amoeba) and yeast. It is so as centriole is present within the nucleus.
(iv) Extranuclear or Eumitosis : In this nuclear membrane is lost and spindle is formed outside nuclear membrane e.g. in plants and animals.
(v) Endomitosis : Chromosomes and their DNA duplicate but fail to separate which lead to polyploidy e.g. in liver of man, both diploid (2N) and polyploid cells (4N) have been reported. It is also called endoduplication and endopolyploidy.
(vi) Dinomitosis : In which nuclear envelope persists and microtubular spindle is not formed. During movement the chromosomes are attached with nuclear membrane.