Mitosis | Cell Division | Class 12 Biology

Mitosis | Cell Division | Class 12 Biology
About This Video!

It is the type of cell division, which ensures the same number of chromosomes in the
daughter cells as that in the parent cells. In spite of slight diferences, major steps of
mitosis are similar in plants as well as in animals. However, to avoid the confusion
our statement will base on the animal cell. It can take place in haploid as well as in
diploid cells in nearly all parts of the body if and when required.
Mitosis is a continuous process, but conventionally it may be divided into two
phases, i.e., karyokinesis, which involves the division of nucleus and cytokinesis that
refers to the division of the whole cellKaryokinesis
At the beginning of the process in an animal cell, the partition of the centriole takes place,
which have been duplicated during interphase but present in the same centrosome. Early in the mitosis the two pair of centrioles separate and migrate to opposite sides
of the nucleus, establishing the bipolarity of the dividing cells.
Three sets of microtubules (ibers) originate from each pair of centrioles. One set the
astral microtubules, radiate outward and form aster, other two sets of microtubules
compose the spindle. The kinetochore microtubules attach to chromosomes at
kinetochores and polar microtubules do not interact the chromosomes but instead
interdigitate with polar microtubules from the opposite pole. These microtubules are
composed of a protein tubulin and traces of RNA.
This specialized microtubule structure including aster and spindle is called mitotic apparatus. This is larger than the nucleus, and is designed to attach and capture
chromosomes, aligning them and inally separating them so that equal distribution of
chromosomes is ensured.
Karyokinesis can further be divided into prophase, metaphase, anaphase and telophase for thorough understanding, though it is a continuous process.
Prophase.
During interphase (non-dividing phase) of the cell cycle the chromosomes are not
visible even with electron microscope, but using histologic stains for DNA, a network
of very ine threads can be visualized. This network is called as chromatin.
The chromatin material gets condensed by folding and the chromosomes appear as
thin threads (0.25mm - 50mm in length) at the beginning of prophase.
Chromosomes become more and more thick ultimately each chromosome is visible having two sister chromatids, attached at centromere. Towards the end of prophase,
nuclear envelope disappears and nuclear material is released in the cytoplasm, nucleoli
disappear. Mitotic apparatus is organized (as described above). Cytoplasm becomes
more viscous.
Metaphase
Each metaphase chromosome is a duplicated structure which consists of two
sister chromatids, attached at a point called centromere or primary constriction. The
centromere has special area, the kinetochore, with speciic base arrangement and
special proteins where kinetochore ibers of mitotic apparatus attach.
The kinetochore ibers of spindle attach to the kinetochore region (specialized area
in centromere) of chromosome, and align them at the equator of the spindle forming
equatorial plate or metaphase plate. Each kinetochore gets two ibers one from each
pole.
Anaphase
It is the most critical phase of the mitosis, which ensures equal distribution of
chromatids in the daughter cells. The kinetochore ibers of spindle contract towards
their respective poles, at the same time polar microtubules elongates exert force and
sister chromatids are separated from centromere. As a result, half sister chromatids travel towards each pole.
Telophase
Reaching of the chromosomes at opposite poles terminates anaphase and start
telophase. The chromosomes decondense due to unfolding, ultimately disappear as
chromatin. Mitotic apparatus disorganizes nuclear membrane and nucleoli reorganize, resulting two nuclei at two poles of the cell.

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shape of chromosomes appear in which phase ???

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