Mitosis
Discovery of Mitosis
The first person
to observe mitosis in detail was a German biologist, Walther
Flemming (1843–1905), who is the pioneer of mitosis research and
also the founder of cytogenetics
Definition
“Mitosis is that step in the
cell cycle where the newly formed DNA is separated and two new cells are formed
with the same number and kind of chromosomes as the parent nucleus.
Explaination
Cell
division is the driving process of reproduction at the cellular level. Most
eukaryotic cells divide in a manner where the ploidy or the number of
chromosomes remains the same, except in the case of germ cells where the number
of chromosomes is halved.
Mitosis is essential for the growth of the cells and the replacement of worn-out cells. Abnormalities during mitosis may alter the DNA, resulting in genetic disorders.
Features of Mitosis
- In each cycle
of cell division, two daughter cells are formed from the parent cell.
- The cell is
also known as equational cell division because
the chromosome number in the parent cell and daughter cell is
the same.
- In plants,
mitosis leads to the growth of vegetative parts of the plant like root
tip, stem tip, etc.
- Segregation and
combination do not occur in this process.
The
processes occurring during mitosis have been divided into different stages.
Stages of Mitosis
Right
before prophase, the cell spends most of its life in the interphase, where
preparations are made before the beginning of mitosis (the DNA is copied).
However, since the actual process involves the division of the nucleus, the
prophase is technically the first stage of this process.
The different stages of mitosis occurring during cell division are given as follows
1. Interphase
Before
entering mitosis, a cell spends a period of its growth under interphase. It
undergoes the following phases when in interphase:
- G1 Phase: This is the period before the synthesis of
DNA.
- S Phase: This is the phase during which DNA
synthesis takes place.
- G2 Phase: This is the phase between the end of DNA
synthesis and the beginning of the prophase.
2. Prophase
Prophase
immediately follows the S and G2 phases of the cycle and is marked by condensation
of the genetic material to form compact mitotic chromosomes composed of two
chromatids attached at the centromere.
The
completion of the prophase is characterised by the initiation of the assembly
of the mitotic spindle, the microtubules and the proteinaceous components of
the cytoplasm that help in the process.
The
nuclear envelope starts disintegrating.
3. Metaphase
At
this stage, the microtubules start pulling the chromosomes with equal force and
the chromosome ends up in the middle of the cell. This region is known as the
metaphase plate. Thus, each cell gets an entire functioning genome.
4. Anaphase
The
splitting of the sister chromatids marks the onset of anaphase. These sister
chromatids become the chromosome of the daughter nuclei. The chromosomes are
then pulled towards the pole by the fibres attached to the kinetochores of each
chromosome. The centromere of each chromosome leads at the edge while the arms
trail behind it.
5. Telophase
The chromosomes that
cluster at the two poles start coalescing into an undifferentiated mass, as the
nuclear envelope starts forming around it. The nucleolus, Golgi bodies and ER
complex, which had disappeared after prophase start to reappear.
Telophase
is followed by cytokinesis, which denotes the division of the cytoplasm to form
two daughter cells. Thus, it marks the completion of cell division.
Functions
of Mitosis
Following are the two important functions of mitosis:
- Mitosis helps in the development of an organism. In
single-celled organisms, mitosis is the process of asexual reproduction.
- Mitosis helps in the replacement of damaged tissues. The cells
near the damaged cells begin mitosis when they do not sense the
neighbouring cells. The dividing cells reach each other and cover the
damaged cells
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