M-phase

The process of cell division is found to be essentially

the same in all living organism and the events are
chiefly centered in the nucleus. Three type of cell
divisions have been distinguished:
➢ Amitosis or direct cell division
➢ Mitosis or indirect cell division
➢ Meiosis or reduction division

Mitosis and meiosis are the two major types of cell

division. The basic stage in both the types of divisions
are almost identical.

Amitosis is a direct division characterized by the

splitting of nucleus followed by that of cytoplasm.

Mitosis is a somatic cell division which takes place in

vegetative cells. It maintains the chromosome number.

Meiosis is a reduction division, occurring in the

reproductive cells. The chromosome numbers are
reduced to half
 Amitosis
Amitosis It is a method of direct cell division in which
the nucleus constricts into two daughter without
showing differentiation of chromosomes and
development of spindle. Nuclear division is followed by
cytokinesis ( division of cytoplasm)
➢ Amitosis was first described by Robert Remak
(1855 ) in red blood corpuscles of chick embryo.
The term was coined by Flemming ( 1882 )
Occurrence:
➢It occurs through cleavage or constriction example
cartilage cell degenerate cells meganucleus of
Paramecium, cells of foetal membranes of
vertebrates
➢ Moneran cell division is sometimes included under
amitosis due to absence of spindle.
Drawback:
➢ As Amitosis does not distribute chromatin
equitably, it results in structural and functional
abnormalities in the cell.
 Mitosis
Mitosis is a type of cell division in which chromosomes
of parent cells are duplicated ( by replication of DNA)
and equally distributed (quantitatively and qualitatively)
into two daughter nuclei.
➢ Mitosis was first observed by Strassburger in plant
cells (1870) and Boveri and Flemming in animal
cell ( 1879 ). The term was coined by Flemming in
1882. It is also known as equational division due to
equal distribution of chromosomes in daughter
nuclei. It is often known as somatic cell division
due to occurrence in somatic cells.
➢ In plants all meristematic regions are the sites of
mitosis e.g. root ape, shoot apex, intercalary
meristem, lateral meristem, leave, flowers, fruits,
embryo, seeds etc.
➢ In animals embryo, skin, bone marrow etc are the
sites of mitosis
➢ Mitosis is completed in two steps karyokinesis and
cytokinesis.
➢ Mitosis starts with the nuclear division of parent
cell known as karyokinesis The four phases of
karyokinesis are prophase, metaphase, anaphase
and telophase.
 Karyokinesis of mitosis

Prophase: Prophase is often divided into three

substages – early prophase, mid prophase and late
prophase. It is the first stage of mitosis proper. It is
the longest phase of Karyokinesis.

Figure 4: prophase under microscope

Early prophase:
➢ In this sub-stage nucleus and cell become spheroid
and nucleus appears as boll of wool.Chromatin
fibre condense to form elongated chromosome and
this increases viscosity and refractivity of
cytoplasm .In animal cells duplicated centrioles.
start to move towards opposite poles of the cell.
 Each centriole radiates out fine microtubular fibrils
called astral rays.

Figure 5:structure of early prophase

Late prophase:
➢ In this substage spindle fibres start appearing
around the nucleus. The size of chromosomes is
much reduced as compared to early prophase .
Spindle poles are formed without asters in plant
cells and with asters in animal cells.

Figure 5:structure of late prophase

Nucleolus and other cell organelles ( like mitochondria,
Golgi complex, ER, vacuoles etc) disappear. The
presence of the spindle is essential for mitosis. If cells
are treated with colchicines, which inhibits spindle
formation, anaphasic movement of the two groups of
chromosomes to the poles does not take place
Metaphase:
In metaphase discontinuous fibres radiate out from two
poles and get connected to the disc shaped structure at
the surface of the centromere called kinetochores. A
kinetochore is complex protein structure that is
analogous to ring for the microtubule hook; it is the
point where microtubules attach themselves to the
chromosome. Chromosomes or kinetochore fibres
contract and bring chromosome over equator this
phenomenon is called congression.

Smaller chromosomes directed towards the centre while

larger ones are peripheral in position on equator. The
centromeres of all the chromosomes lie on the equator
forming an apparent plate called metaphasic or
equatorial plate while arms are directed towards the
poles
The kinetochores have two functions. The main
function apparently is that they serve for the attachment
of microtubules of the chromosomal spindle fibres.
They might also be involved in the formation of the
chromosomal spindle fibres during prometaphase and
metaphase by serving as centres for polymerization of
the protein of microtubules.

Metaphase is the best phase to count total number of

chromosomes in any species and details study of
morphology of chromosomes. Idiogram (arrangement
of chromosomes in a series of decreasing length) can be
drawn in this stage.

Figure6:structure of metaphase
 Figure7:Metaphase under microscope
Anaphase:
In anaphase chromosomes are arranged on the
equatorial plate for a short period. The centromeres of
chromosomes starts to divide into two,forming daughter
chromosomes with centromere in each. Daughter
chromosomes are repulsive so, migrate towards
opposite poles. Spindle fibres attached to the
centromeres shorten and pull the chromosomes to the
poles. The velocity of anaphasic movement does not
depend on the size of the chromosomes. In anaphasic
movement of chromosomes, the centromeres lead the
path while the limbs trails behind. So anaphasic
chromes, the centromeres lead the path while the limbs
trail behind. So anaphasic chromosomes appear as V-,
L-, J- and I- shaped.
At the end of anaphase two groups of chromosomes are
formed, one at each pole. The number and types of
chromosomes at each pole is the same as in the parent
nucleus.

Figure 9 :Structure of anaphase under microscope

Telophase:
During telophase of mitosis viscosity of cytoplasm
decrease. A new nuclear membrane is formedaround
each set of chromosomes. Chromosomes overlap one
another forming chromatin.
The nuclear organizer region of satellite chromosomes
produce nucleolus for each daughter nucleus.
Nucleoplasm surrounds in the area of chromatin. The
spindle disappears.
In this way two daughter nuclei are formed at the poles
of spindle. Hence this phase is just reverse of prophase.
Golgi complex and endoplasmic reticulum are
reformed. Cytokinesis starts either by cleavage or
constriction.
 Figure10: Telophase under microscope
Cytokinesis:
Mitosis ends with division of cytoplasm known as
cytokinesis. It is derived from greek word “cytos”
means hollow or cell, “kinesis: means movement. It
starts towards the middle of anaphase and is completed
with the telophase. It is different in animal and plants. If
nuclear divison takes place without cytoplasmic
division, a syncytium is formed animal cytokinesis.

The furrow forms and deepens centripetally and finally

cleaves the parent cell into two daughter cells. This
method of cytokinesis is known as cleavage method. All
cell organelles ( mitochondria, Golgi complex,
lysosomes., ER, ribosomes etc) are also distributed
between two daugheter cells.

Figure 11: Cytokinesis under microscope

 Significance of mitosis

Growth and development - A single cell zygote grows

into full blown baby ( 6×1022 cells) by repeated
mitosis. Plants are able to grow throughout their life due
to mitotic division in their apical and lateral meristems.
Increases in tissue mass, results from increase in cell
number called hyperplasmic. Hence, mitosis is essential
for growth and development of a multicellular
organism.
➢ Maintenance of cell size: An overgrown somatic
cell is induced to divide so that mitosis helps in
maintaining a proper surface volume ratio. It has
also a high nucleocytoplasmic ratio which is
brought back to efficient level through divisions.
These ratios are important for proper functioning of
cell distribution of all the chromosomes. This helps
in proper co-ordination among daughter cells.
➢ Healing and regeneration – For healing of wounds
new cells are produced by Mitosis some organism
are able to regenerate missing part of body also
whole organism though mitosis
➢ Reparing – the mechanism for replacing old or
worn out cells is called repairing. In human body
roughly 5 ×109 cells are lost from skin surface,
 lining of alimentary canal, blood cell etc. these are
replaced by new cells formed through mitosis.
➢ Evidence of basic relationship – The mechanism of
mitosis are similar in the majority of organism,
showing basic similarity and relationship among
them

Figure 12: stages of mitosis in epithelial cell