Describe Mitotic cell division or Mitosis or Somatic cell division in plants and its importance
Cell division
leading to the development of the vegetative body (soma) of the plant is known
as somatic cell division. It includes the division of the nucleus called
mitosis (mitose=thread) or karyokinesis (karyon=nut or nucleus)
(Kinesis=movement) or indirect nuclear division and the division of cytoplasm
is called cytokinesis (kytos=cell). It occurs in growing regions as in the root
tip and stem tip.
In Mitosis there
are four stages as under:
(1) PROPHASE
The first sign
of the prophase is the appearance of a number of separate slender, crooked
threads called chromosomes. The chromosomes are spirally coiled. The individual
chromosomes are always double with two threads called chromatids. Chromosomes
are composed of nucleoproteins. As prophase proceeds the chromosomes relex
their coils and thicken somewhat. Their double nature becomes more apparent. As
prophase advances a chromosomal substance accumulates in a sheath or matrix
round each chromosome and the chromatids become closely coiled in it. Each
chromatid divides longitudinally into two. Thus at this stage each chromosome
consists of four threads.
In well fixed
chromosomes some unstained gaps or constrictions are seen; those are the attachment
regions called centrosomes. The nucleoli lose their staining power and
disappear completely. The nucleus then rapidly passes into the next stage the
metaphase.
(2) METAPHASE
The nuclear
membrane and nucleolus disappear and a spindle like body called nuclear spindle
is formed. Spindle may be of nuclear origin or more probably of cytoplasmic
origin. Commonly in root tips it appears as two opposite polar caps outside the
nuclear membrane. The membrane then disappears and the spindle extends into the
nuclear area. The chromosomes move to the equatorial plane on the spindle and
there clearly apart from one another. At this stage the chromatids come even
more closely together. From the centrosome of each pair of chromatids fibre
like extensions called tractile fibres are formed towards the opposite poles
through the nuclear spindle. The number of chromosomes is normally constant for
a particular species of plants and this number is also normally even, expressed
as 2n (or 2x) or diploid. Chromosome numbers cover a wide range but 24 seem to
be a common figure.
(3) ANAPHASE
In the end of
metaphase the centrosomes of each pair of chromatids appear to repel each
other. They diverge and move ahead towards the opposite poles along the course
of tractile fibres. The chromatids soon become separated from each other.
Anaphase covers the shortest period in mitosis.
(4) TELOPHASE
At each pole the
chromatids (daughter chromosomes) form a close group. The nuclear spindle
disappears and so does the matrix. A nuclear membrane is formed round each
group of chromatids. The chromatids duplicate themselves into full chromosomes
which recognize themselves within the nuclear membrane. Nucleoli reappear at
definite points on certain chromosomes. The nuclear sap reappears and each
nucleus increases in size. It passes into metabolic stage or prepares for next
division.
Importance: In
this complicated process of nuclear division the constituents of chromosomes
are equally appropriated to two daughter nuclei and thus they become
qualitatively and quantitatively similar to the mother nucleus. Chromosomes are
the bearers of hereditary characters and because of even distribution of
chromosomal substances particularly DNA, two daughter nuclei possess all
characteristics and quantities of the mother nucleus.
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