Cell is the structural and functional unit life. Our cells have a definite lifespan and it varies between the different types of cells , therefore, cell division is essential to clean up the old cells and replace them with the new cells with the help of apoptosis. Our body cells are subjected to constant growth and repair. Each day, millions of cells die as a result of daily wear and tear. Furthermore, cells undergoes constant loss due to the bodily processes such as excretion, transportation, digestion etc . Newly formed cells must undergo cell cycle and cell division as a part of normal development. During the cell cycle, the cell undergoes constant replication, growth and differentiation which result in the formation of daughter cells. Cell cycle and cell division are inseparable from each other because cell division is one of the phase in the cell cycle. It is essential that every living cell in our body undergoes a systematic cell division process resulting in the replacement of inefficient and aged cells from the young and active ones. This article describes the phases of cell division and cell cycle.
Phases of Cell cycle
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; 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, G0 phase. Our body cells are replenished every 24 hours, however, this can vary from one species to another species, for example, yeasts undergo cell division every 90 minutes.
A. Growth phase or interphase
The initial phase-G1 phase refers to the short interval between mitosis and the beginning of DNA replication. To put it another way, this is the phase of constant preparation for cell division. During G1 phase, the cells are boosted both metabolically and chemically. S-phase or synthesis phase is the actual phase of DNA synthesis where DNA undergoes replication resulting in doubling of the amount of DNA while the actual number of chromosomes remains constant. Protein synthesis takes place during the G2 phase, consequently, there is cell growth. Some cells in the adult enter a resting phase as they do not participate in cell division (e.g. Heart cells). Quiescent stage (G0 stage ) of the cell cycle permits the cells to be metabolically active without undergoing cell division.
B. Mitotic phase( M-phase)
Mitotic phase is also known as, interphase or dividing phase. M-phase consists of a nuclear division- karyokinesis. It is a relatively shorter period of the cell cycle. M-phase is subdivided into prophase, prometaphase, metaphase, anaphase, and telophase. Mitosis is the process in which 2 identical sets of chromosomes are formed by the division of a single parental chromosome. During the mitosis, chromosomes are condensed and stick into microtubules. Microtubules help to pull the sister chromatids to their opposite poles of the cell. The detailed phases of mitosis are explained as under.
Prophase is the first stage of mitosis , here chromosome undergoes condensation. At this stage, the genetic material is loosely packed to form chromatin. At the beginning of prophase, each chromosome forms 2 identical chromatids joined by a centromere. Microtubules help in the formation of spindles fibres that help in the organization of chromosomes. During the mitotic phase, the 2 centrosomes slowly repel towards their opposite directions.
Prometaphase assist in finishing the chromosomal condensation to form a complete compact state. At this stage, the cell’s nuclear envelope is lysed allowing the mitotic spindles to attach to the chromosomes while the kinetochore is formed near the centrioles.
Metaphase is meant to the alignment of chromosomes at the center of the cell at an equal distance from each of the mitotic spindle poles. The kinetochore produced during the previous phase assist to push and pull the chromosomes so that they align properly to form the metaphase plate. Chromosomes on the metaphase plate are held there tightly by pushing and pulling forces from the microtubules.
Anaphase is equally spanned into 2 sub-phases called anaphase -1 and anaphase-2. During the anaphase, the replicated chromosomes are equally split while the newly formed daughter chromatids move to opposite poles of the cell.Therefore, anaphase is largely regarded as the phase of 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, daughter cells acquire complete cell-like features. Changes such as reconstruction of two nuclei and the formation of organelles becomes apparent. Telophase 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 necessary for the regulation and growth of daughter cells. Cytokinesis is not an independent phase by itself as it begins during anaphase and continues even after the telophase. The original cytoplasm is separated from the daughter cytoplasm through a cleft called cleavage furrow. It is formed at the centre of the cell resulting in the equal division of cytoplasmic contented between the 2 cells. Cytokinesis is also responsible for distributing organelles, cell membranes, and other components between the 2 cells.