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Mitosis refers to a process of division of a single cell into two identical daughter cells in which total chromosome number is maintained. Mitosis is also referred to as an “equational division”. Hugo von Mohl discovered cell division in green algae namely Cladophora glomerata and exclaimed that the multiplication of cells occurs via cell division.
Programmed cell death refers to the death of a cell due to events occurring inside the cell like apoptosis or autophagy. During an organism’s lifestyle programmed cell death possesses few advantages like for e.g. the differentiation of fingers and toes of a developing human embryo is mainly due to the cells that are present in between the fingers apoptose. Thereby, resulting in separate digits. It helps in the development of both plants and animals.
Mitosis
It is the division of a single cell into two genetically identical daughter cells. Usually the process of mitosis is preceded via S phase of interphase (where DNA replication takes place) and it is followed via telophase and cytokinesis (which helps in division of cytoplasm, organelles and cell membrane of one cell into new cell comprising of equal quantity of all these cellular components). The various phases of mitosis are collectively known as mitotic phase or as M phase of the animal cell cycle.
During mitosis the duplicated chromosomes get condensed and attached to the spindle fibres, which pulls each copy of chromosomes towards the opposite ends of the cell. This leads to the formation of two genetically identical daughter nuclei. We can view different stages of mitosis with the help of live cell imaging. In some cases more than two daughter cells are produced; this condition is referred to as tripolar mitosis or multipolar mitosis. In eukaryotic cells mitosis takes place whereas prokaryotes lack nucleus and divides by the process known as binary fission.
Phases of Mitosis
Mitosis is divided into five different phases:
Interphase: it is a longer phase of cell cycle as compared to shorter mitosis phase. During this phase the cell prepares itself for the process of cell division. Interphase is further divided into three phases: G phase or G1 (first gap), S phase (synthesis) and G2 phase or (second phase). All these phases proceed along with the formation of proteins and cytoplasmic organelles. During S phase chromosome replication takes place. The cell starts dividing from G1 phase, replicates its chromosomes during S phase, grows more and prepares itself for mitosis, finally divides in the M phase. All these phases are regulated by cyclins and cyclin dependent kinases. The phases are accompanied by “checkpoints” that direct the cell to proceed from one phase to another. Few cells leave the cell cycle and enter the G0 phase and stop dividing. This phase prepares the cell for mitotic divisions.
Prophase: this phase occurs after G2 phase the chromosomes gets condensed and helps in initiating mitotic spindle formation. During this phase the chromatin fibres get condensed into chromosomes that are visible in high magnification via a light microscope. Chromosomes can be visible as long, thin and thread-like. Every chromosome bears two chromatids that are joined at the centre via centromere. The membrane around the nucleus dissolves with the release of chromosomes at the end of this phase. Mitotic spindle containing microtubules and some proteins extends across the cell between the centrioles as they move towards opposite poles of the cell.
Metaphase: alignment of chromosomes at the centre (i.e. equator) of the cell is seen during this phase. The centrioles move towards the opposite ends of the cell along with the mitotic spindle fibres extending from them. In order to ensure equal distribution of chromosomes at the end of mitosis the metaphase checkpoints help in guaranteeing that the kinetochores are tightly attached to the mitotic spindle, and also the chromosomes are aligned along the metaphase plate. Once the cell passes through the metaphasic checkpoints it is bound to enter the anaphase.
Anaphase: during this phase the sister chromatids are cleaved resulting in the formation of two identical daughter chromosomes. As the kinetochores shorten, the microtubule pulls away the newly formed daughter chromosomes towards the opposite poles of the cell. During late anaphase the chromosomes reach their overall maximum condensation level in order to help chromosome segregation and also reformation of the nucleus.
Telophase: this phase is a complete reversal of early and late prophases. During telophase the polar microtubules elongate and lengthen. A new nuclear envelope is formed using the membrane vesicles of the parent cell’s old nuclear envelope. This new set is formed around each of the separated daughter chromosomes and also the nucleolus reappears. Each daughter nuclei possesses an identical set of chromosomes.
Importance of Mitosis
Mitosis helps in maintaining an equal number of chromosomes in each separated daughter cell after division.
This process is responsible for the growth and development of multicellular organisms.
Mitosis repairs all the damaged tissues, and in recovery of damaged organs.
It helps the multi-cellular organisms to maintain their skin, blood and tissue cells.
It plays a very important role in sexual reproduction.
The number of chromosomes do not change during mitosis.
Mitosis helps in growth and development of embryos.
Cell death
It refers to an event of a biological cell in which the cell stops to carry out its functions. It can be due to the natural process of old cells dying and being replaced via new ones or can be the result of any disease or any localized injury.
Conclusion
Mitosis is very essential in life as it provides new cells for growth and replacement of worn out cells. This process may take about minutes or hours depending upon the kinds of cells. It is influenced by temperature and chemicals.
