are the two major types of cell divisions. The basic stages in
both the types of divisions are almost identical.amitoSiS
• amitosis is a simple method of cell division described by
Flemming (1882). In this method, the nucleus elongates
and constricts in the middle to form two daughter nuclei.
This is followed by a centripetal constiction of the cytoplasm
to form two daughter cells.
• amitosis is not a regular method of division because it
does not divide the nuclear material equitably. It occurs in
meganucleus of Paramoecium, nuclei of internodal cell of
Chara, aging and diseased cells etc. It does not occur in
Amoeba where an intranuclear mitosis has been discovered.
Moneran cell division is sometimes included under amitosis
due to absence of spindle formation.
mechanism
• amitosis occurs in two steps, nuclear division and cytoplasmic
division.
• Nuclear division (karyokinesis) : Nuclear envelope
remains intact. Chromosomes do not differentiate. a spindle
is also not formed. The nucleus elongates and constricts in
the middle. It becomes dumb-bell shaped at first and then
divides into two daughter nuclei.
• Cytoplasmic division (cytokinesis) : It begins
simultaneously with the constriction of the nucleus. Plasma
membrane invaginates all around in the middle to form a
furrow. Cytoplasm constricts centripetally in the region so
that the circular furrow deepens and ultimately divides the
cell into two daughter cells each with a daughter nucleus.
Drawbacks
- elongated chromatin fibres do not
condense to form chromosomes
and break during constriction
of nucleus. - There is no mechanism for
separation and equitable
distribution of daughter
chromosomes. - amitosis causes unequal
distribution of chromatin material
amongst the daughter nuclei which results in structural
and functional abnormalities in the cells.mitoSiS
• Mitosis or mitotic cell division or equational cell division is
meant for multiplication of cells. Development of complete
organism from zygote is by means of mitotic cell division.
So mitosis is necessary for maintenance, growth, repair and
continuity of life.
• Mitosis generally takes place in vegetative or somatic cells.
• In this process, one parent cell divides into two daughter cells,
where chromosome number remains the same as in parent
cell, i.e., daughter cells exactly resemble with parent
cell both quantitatively as well as qualitatively. So
mitosis leads to increase in number of cells without any
change in genetic make-up.
• Mitosis can be easily observed in meristematic cells of plant.
Root tips (e.g., onion root tips) are considered to be the
best material for the study of mitosis.
• W. Flemming (1882) gave the term mitosis (Gr.: Mitos-
thread + osis-state). However, mitosis was first observed
in plants by E. Strasburger (1875) and in animals by
W. Flemming (1879).
• The actual mitotic-phase (M-phase) is completed in two
steps : karyokinesis, i.e., division of nucleus into two
and cytokinesis (follows karyokinesis), i.e., division of
cytoplasm into two cells.Karyokinesis
• Karyokinesis (division of nucleus) consists of four stages:
prophase, metaphase, anaphase and telophase.- Prophase
• Prophase which is the first stage of mitosis follows the S and
G 2 phases of interphase.
• Viscosity of cytoplasm increases. Chromatin fibres become
shorter and thicker due to the process of coiling and folding
and get condensed into distinct thread like chromosomes in
late prophase.
• each chromosome appears double and consists of two coiled
sister chromatids joined by a centromere. Their ends
are not visible in early prophase. Therefore,
the chromosomes appear like a ball
of wool. It is also called spireme
stage.
• The centriole, which had
undergone duplication during
S-phase of interphase, now
begins to move towards opposite
poles of the cell.
• Nuclear membrane breaks down
to form membranous vesicles.