2.

2 Mitosis

➢ Mitosis is the division in which chromosomes replicate and become equally

distributed both quantitatively and qualitatively into two daughter nuclei so that
the daughter cells come to have the same number and type of chromosomes as
are present in the parent cell.

➢ Since the number of chromosomes in the parent and progeny cells is the same, it
is also called as equational division.

➢ Mitosis occurs in formation of somatic body cells and is hence often named as
somatic cell division.

➢ Mitosis was first observed by Strasburger in plant cells.

➢ Boveri and Flemming observed it in animal cells.

➢ The term mitosis was coined by Flemming.

➢ Sites of mitosis in plants: Tips of root, stem and intercalary meristem.

➢ Sites of mitosis in Animals: During the formation of new body cells and formation
of gametes.

➢ Mitosis is the most common method of division which brings about growth in
multicellular organisms and increase in population of unicellular organisms.

➢ Mitosis consists of two steps:

a. Karyokinesis
b. Cytokinesis
➢ Karyokinesis is also called indirect nuclear division because the nucleus
passes through a complicated sequence of events before forming two
daughter nuclei.

➢ Cytokinesis is the division of protoplast of a cell into two daughter cells

after the nuclear division or karyokinesis.
➢ Mitosis is divided into the following four stages:
a. Prophase (pro = before and phasis = appearance)
b. Metaphase (meta = among, between and phasis = appearance)
c. Anaphase (ana = back, backward, and phasis = appearance)
d. Telophase (telos = end and phasis = appearance)

NOTE: Prophase is the longest phase of mitosis while Anaphase is the shortest phase
of Mitosis.
2.2.a. Prophase
➢ Prophase is marked by the initiation of condensation of chromosomal
material. The chromosomal material becomes untangled during the
process of chromatin condensation.

➢ The centrosomes, which had undergone duplication during S phase

now begins to move towards opposite poles of the cell.

➢ Centriole pairs radiate out fine microtubular fibrils called astral rays.

➢ Each group of astral rays along with its centriole pair is called aster.

➢ Plant cells lack centrioles but still, they are capable to form a mitotic
spindle from the centrosome area of the cell located just exterior to
the nuclear envelope.
➢ Since there are two asters, the spindle of animal cells is called amphiaster division
(centric spindle). In contrast, the spindle of plant cells is called anastral division
(acentric).

➢ Prophase is divided in three substages:

Early prophase
Middle prophase
Late prophase
1. What is the main function of the spindle fibers during mitosis?
a To repair damaged DNA
b To regulate cell growth
c To separate the chromosomes
d To synthesize new DNA
1. What is the main function of the spindle fibers during mitosis?
a To repair damaged DNA
b To regulate cell growth
c To separate the chromosomes
d To synthesize new DNA
Early Prophase

➢ DNA molecules condense to form elongated chromosomes and these

chromosomes are evenly distributed inside the nucleus.

➢ Viscosity of cytoplasm increases.

➢ Centrosomes begin to shift towards the opposite side.

Middle Prophase

➢ Chromosomes shift towards the periphery.

➢ Shortening and thickening of chromosomes results in appearance of

chromatids.
Late Prophase

➢ Nucleolus degenerates completely.

➢ The two spindle poles begin to get connected by fine fibers.

➢ The completion of prophase can thus be marked by the following

characteristic events:
▪ Chromosomal material condenses to form compact mitotic
chromosomes.
▪ Chromosomes are seen to be composed of two chromatids attached
together at the centromere.
▪ Initiation of the assembly of mitotic spindle, the microtubules occurs and
the proteinaceous components (tubulin protein) of the cell cytoplasm
help in the process.
▪ Organelles disappear.
▪ NM begins to disappear.
▪ Nucleus appears like ball of wool known as Spirene stage.
2.2.b. Metaphase
➢ The complete disintegration of the nuclear envelope marks the start of the
second phase of mitosis, hence the chromosomes are spread through the
cytoplasm of the cell.

➢ Condensation of chromosomes is completed by this stage, and they can be

observed clearly under the microscope.

➢ Metaphase is the stage at which morphology of chromosomes is most

easily studied.

➢ At this stage, the metaphase chromosome is made up of two sister

chromatids, which are held together by the centromere.

➢ Small disc-shaped structures at the surface of the centromeres are called

kinetochores.
➢ These structures serve as the sites of attachment of spindle fibers (formed by the
spindle fibers) to the chromosomes that are moved into position at the center of
the cell.

➢ Spindle fiber gets attached to kinetochore of chromosome.

➢ Due to movement in spindle fiber all chromosome lie at equatorial plane and
form equatorial plat / metaphasic plate.

➢ Movement of spindle fiber known as Congression.

➢ All chromosome now lie at equatorial plane. So, Metaphase is best phase to
▪ Count number of chromosome
▪ Study structure of chromosome
▪ Study morphology of chromosome
2. During which phase of mitosis do spindle fibers attach to the kinetochores of
chromosomes?
a Prophase
b Metaphase
c Anaphase
d Telophase
2. During which phase of mitosis do spindle fibers attach to the kinetochores of
chromosomes?
a Prophase
b Metaphase
c Anaphase

d Telophase
2.2.c. Anaphase
➢ The splitting of the sister chromatids marks the onset of anaphase.

➢ At the onset of anaphase, each chromosome arranged at the metaphase

plate is split simultaneously.

➢ The two daughter chromatids, now referred to as chromosomes of the

future daughter nuclei, begin their migration towards the two opposite
poles along the path of their chromosomal fibers.

➢ They remain connected to each other by interzonal fibers.

➢ Sister chromatid separate from chromosome and move to opposite pole in

such a way that kinetochore will face pole and will remain behind.

➢ The movement of the chromatid is facilitated by a combination of

kinetochore movement along the spindle microtubules.
➢ To move one chromatid to opposite – 30 ATP used
8. Anaphase promoting complex (APC) is protein degradation machinery
necessary for proper mitosis of animal cells. If APC is defective in a human
cell, which of the following is expected to occur? (NEET-2017)
a Chromosomes will not condense
b Chromosomes will be fragmented
c Chromosomes will not segregate
d Recombination of chromosome arms will occur
8. Anaphase promoting complex (APC) is protein degradation machinery
necessary for proper mitosis of animal cells. If APC is defective in a human
cell, which of the following is expected to occur? (NEET-2017)
a Chromosomes will not condense
b Chromosomes will be fragmented
c Chromosomes will not segregate
d Recombination of chromosome arms will occur
2.2.d. Telophase
➢ Telophase is the final stage of mitosis.

➢ At the beginning of the telophase, the chromosomes that have reached their
respective poles decondense and lose their individuality.

➢ The individual chromosomes can no longer be seen, and chromatin material

tends to collect in a mass in the two poles.

➢ Chromosomes cluster at opposite spindle poles and their identity is lost as

discrete elements.

➢ The nuclear envelope assembles around the chromosome clusters.

➢ Nucleolus, Golgi complex and ER reform.

➢ The nuclear division, corresponding to the separation of daughter

chromosomes is karyokinesis.
Significance of Mitosis

➢ Mitosis or the equational division is usually restricted to the diploid cells

only.

➢ However, in some lower plants and in some social insects haploid cells also
divide by mitosis.

➢ Mitosis results in the production of diploid daughter cells with identical

genetic complement usually.

➢ The growth of multicellular organisms is due to mitosis.

➢ Mitotic divisions in the meristematic tissues – the apical and the lateral
cambium, result in a continuous growth of plants throughout their life.
➢ A very significant contribution of mitosis is cell repair. The cells of the upper layer
of the epidermis, cells of the lining of the gut, and blood cells are being constantly
replaced.
➢ Mitosis and restoration of nucleo-cytoplasmic ratio:
▪ Cell growth results in disturbing the ratio between the nucleus and the
cytoplasm.
▪ It therefore becomes essential for the cell to divide (mitosis) to restore
the nucleo-cytoplasmic ratio.
▪ Metabolically active cells are typically smaller due to higher nucleo-
cytoplasmic ratio and higher surface volume ratio.
▪ The nuclear-cytoplasmic ratio is a ratio of the size of the nucleus of a
cell to the size of the cytoplasm of that cell.

➢ The N-C ratio indicates the maturity of a cell.

➢ The N-C ratio allows nucleus to have better control on metabolic

activities.
➢ Cell size and volume limits are set mainly by the factors such as nucleo-
cytoplasmic ratio and surface area to volume ratio etc.

➢ Surface-volume ratio allow quicker exchange of materials between the cell and its
outside environment.
9. Which of the following is not a characteristic feature during mitosis in
somatic cells? (NEET-2016)
a Spindle fibers
b Disappearance of the nucleolus
c Chromosome movement
d Synapsis
9. Which of the following is not a characteristic feature during mitosis in
somatic cells? (NEET-2016)
a Spindle fibers
b Disappearance of the nucleolus
c Chromosome movement
d Synapsis
 Cytokinesis – division of cytoplasm

Animal Plant
(Cell Furrow Method) (Cell Plate Method)

Animal Cytokinesis Plant Cytokinesis

Spindle fibers degenerates soon after Spindle fibers usually persist during
anaphase cytokinesis
New cell membrane is usually derived New cell membrane is usually derived
from ER from vesicles of Golgi apparatus
Cytokinesis occurs by furrowing of Cytokinesis occurs mostly by cell plate
cytoplasm formation
Furrow extends centripetally Cell plate grows centrifugally
Difference between Animal and Plant Cell Mitosis:

Animal Mitosis Plant Mitosis

Occurs at several places. Generally, occurs in meristematic
regions
Cell often becomes spherical. Cell does not change shape prior to
division.
Cytokinesis occurs mostly by cell Cytokinesis occurs mostly by cell plate
furrow formation. formation.
Furrow extends centripetally. Cell plate grows centrifugally.
Centrioles are present, and the spindle Centrioles are absent, and the spindle
is astral (with one aster at each pole). is anastral (No asters are formed).