Our body cells undergo a process of constant growth and repair. Each day, millions of cells die as a result of daily wear and tear. Most of our bodily processes produce a lot of cell changes in the form of wear and tear. In order to compensate the cell death, the cell cycle and cell division are essential. During the cell cycle, the genome undergoes a constant replication, growth and differentiation to form daughter cells. Cell cycle and cell division are inseparable from each other. It is essential that every living cell in our body undergoes a systematic cell division process resulting in the death of inefficient and aged cells and the formation of new daughter cells. This article is designed to explain the process of cell division and cell cycle.
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2. Cell cycle in human beings
4. Growth phase or interphase
5. Mitotic phase( M-phase)
Cell cycle in human beings
For the first time, biologists Howard and Pelc had explained the cell cycle concept in the year 1953. The cell cycle is mainly divided into 2 major phases; they are, interphase( growth phase) and M-phase (Dividing phase/Mitotic phase). Interphase is again divided into the G1 phase, S phase, and G2 phase and a special phase known as the G0 phase. A healthy human cell gets divided once every 24 hours, however, this can vary from species to species, for example in the yeasts, the cell cycle takes place every 90 minutes.
A. Growth phase or interphase
The initial phase of interphase is the growth phase. During this phase, the cells undergo many changes at different time periods. G1 phase refers to the brief interval between mitosis and the beginning of DNA replication, in other words, this is the phase of constant preparation for cell division. During G1 phase, the cells get boosted by themselves both metabolically and chemically. S-phase or synthesis phase is the actual phase of DNA synthesis. DNA undergoes replication but the actual number of chromosomes remains constant instead, the amount of DNA gets doubled In each cell. During the G2 phase, protein synthesis takes place to help in cell growth. Some of the cells in the adult enter a resting phase and they do not participate in cell division (e.g. Heart cells). Quiescent stage (G0 stage ) of the cell cycle enables the cells to be metabolically active, hence the cells grow well without undergoing cell division.
B. Mitotic phase( M-phase)
Mitotic phase is also known as, interphase or dividing phase. Ideally, M-phase consists of a nuclear division or karyokinesis. It is a relatively short period of the cell cycle. M-phase has sub-phases like prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis is the term used to describe the process by which 2 identical sets of chromosomes are formed within the nucleus by the division of a parental chromosome. During the mitosis process, chromosomes get condensed and stick into microtubules that pull the sister chromatids to the opposite poles of the cell. The detailed phases of mitosis are explained as under.
Prophase is the first stage of mitosis during which there is chromosome condensation. At this stage, the genetic material is loosely packed to form chromatin. ut, for the mitosis to happen, the material must be tightly packed that helps in proper separation in anaphase. So, at the beginning of prophase, each chromosome form 2 identical chromatids joined by a centromere. At the same stage, mitotic spindles are formed, these spindles are made up of microtubules that help in the organization and arrangement of chromosomes. Each cell in mitosis has two centrosomes and the centromeres slowly repel towards 2 opposite directions.
Prometaphase help in finishing the chromosomal condensation that forms a complete compact state. At this stage, the cell`s nuclear envelope gets lysed allowing the mitotic spindles to attach to the chromosomes. There is a special structure formed near the centromeres called the kinetochore.
Metaphase is a phase in which the chromosomes align upon a theoretical line. The centrosomes get aligned equally in a particular direction on their opposite ends of the cell so this phase has a task of arrangement of centromeres with their kinetochores. This will ensure there is a correct and equal split between daughter cells. Any genetic error in the alignment or in a spindle attachment can hinder the normal division of daughter cells.
Anaphase is equally spanned into 2 sub-phases called anaphase -1 and anaphase-2. During this phase, the replicated chromosomes get equally split and the newly formed daughter chromatids will be moved to opposite poles of the cell. So, anaphase is mainly concerned with the movement of split daughter cells into 2 opposite poles.
Telophase is the phase of the elongation of the cell. The cell gets elongated and is nearly finished dividing. During this phase, complete cell-like features start to appear in the daughter cells. A few of the changes like the reformation of two nuclei and formation of organelles start getting clearly visible. This phase is also equally spanned into 2 sub-phases called telophase -1 and telophase-2.
As the name suggests, cytokinesis is the formation of the cytoplasm that can produce 2 new cells. Although cytokinesis is not an independent phase of itself. It begins in either anaphase or telophase and continues even after the telophase. There is a formation of a ring of a protein called actin ring which can form a cleavage furrow by pinching the cytoplasm from one direction, this will eventually divide the cytoplasm equally between the two cells. Cytokinesis is also responsible for dividing organelles, cell membranes, and other components into two cells containing roughly equal shares of these cellular components.