The continuity of living organisms is a general law that manifests itself differently in prokaryotic and eukaryotic, unicellular and multicellular organisms. The dividing cells go through a regular series of events that represent the cell cycle.
The completion of the cycle requires variable periods of time, depending on the type of cell and external factors, such as the temperature or nutrients available. Whether it lasts an hour or a day, however, the amount of time spent in each of the phases is roughly the same.
As soon as a daughter cell separates, it begins a new life cycle, which will probably end with another mitosis.
The life cycle of the cell, if it does not end with death, is therefore between two mitoses. We can represent it with the symbols M-G1-S-G2-M, where the M represent the mitoses. The S phase (synthesis) of the cell cycle is the period during which the genetic material (DNA) is duplicated. The phases G (English gap = interval) precede and follow the phase S; the period G, follows the mitosis and precedes the phase S; the G2 period follows the S phase and precedes mitosis. The set of phases G and S is indicated as interphase. The G1 phase, between mitosis and chromosome synthesis, is above all a growth period of cytoplasmic materials, including the various organelles. Furthermore, during the G1 period according to the current hypothesis, substances are synthesized that can inhibit or stimulate the S phase and the rest of the cycle, thus determining whether the cell division must occur or not. During the G2 phase, the structures directly involved in mitosis, such as spindle fibers, are prepared. Some cells pass repeatedly through successive cell cycles. This category includes unicellular organisms and some cells in the growth zones of both plants and animals. Some highly specialized cells, such as nerve cells, lose the ability to divide when differentiated. A third category of cells retains the ability to divide, but manifests it only in particular circumstances. The human liver cells, for example, usually do not divide, but if a part of the liver is surgically removed, the remaining cells (although only about a third of the total remain) continue to divide until the liver has reached its original size; at this point they stop.
Edited by: Lorenzo Boscariol
