Cell division and cell growth
In living cells a continuous transport of nutrients, signal molecules and degradation products occurs betwen the nucleus and the cytoplasm and between various compartments of the cytoplasm. Such transport is mainly realized by passive diffusion, but also targetted transportoccurs along memebranes and cytoskeletal elements. In the case cells would endlessly continue to grow, the distance and complexity of transport would become an hampering for the optimal functioning of the cell. Celldivision has arisen in the course of evolution as a solution to this problem. Cellulare proliferation like it occurs during the development of organisms, but also in malignant tissue expansion, is based on an alternation of division and growth phases (here schematically illustrated for one cell; D for division and Gr for growth).
Within a single cell cycle two main phases can be distinguished:
The M phase wihich is characterized by the actual chromosomal and cellular division (mitosis or meioisi) and
the M phase which is the interpahse between to two subsequent divisions.
During the interphase the cellcaries out its main tasks and getsprepared for the next division. The interphase comprises a G1, S and G2 phase. More here about in the table here below...
| Phases of the cell cycle |
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Schematical representation of the cell cycle. The length of each arrows along the circle gives an indication for the duration of each phase.
Flash animation on the cell cycle from the Bioplek (Dutch)
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| Mitotic division: M- and C-phase |
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In the proper division phase (M-phase of mitosis) a daughter cell identical to the mother cell is produced. The envelop around the nucleus disappears at the beginning of mitosis and the -duplicated- genetic material is divided equally over the two daughter nuclei. Mitotic division is concluded in a single round. During cytokinesis (C-phase) the cytosol and the organelles are divided over the daughter cells. The cytokinesis is sometimes regarded as part of mitosis.
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| Interphase: G1-, S- and G2-phase |
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During interphase most cell growth and nuclear activities take place. Interfase consists of the G1-phase (G from gap), the S-phase (S= synthesis) and the G2-phase.
The G1-phase covers the period between mitosis and DNA replication. In this phase cytoplasmic growth occurs and the cell is preparing its enzymatic machinery to be ready for the next stage (synthesis); The daughter cells become as large as the mother cell. the chromosomes are thread-like and unvisible. The duration of the cell cycle is prepoundarily determined by the extend of the G1-phase. Cells in a rapidly developing tissue have thus a short G-1 phase.
The S-phase is the period during which DNA replication (synthesis) takes place, thus, duplication of the genome.
The G2-phase is the period between DNA replication and mitosis. Durin gthis laspe rapid control of the replicated DNA is performed and the proper division is prepared.
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| Exit-phase: G0-phase and meiosis |
Cells can leave the mitotic cell cycle (cessation of periodic cell division) at the G1 to transit to the following two conditions:
In the first place such cells can enter in a nondividing state (a realtive "quiescence" or "rest" phase), the so-called G0 phase, in which a variable level of differentiation occurs. Distinction can be made between cells that keep the ability (potency) to further differentiate and cells that have accomplished and finished differentiation (for example muscle and nerve cells).
Some other cells can further develop to reproduction cells by entering meiosis. This special type of division, that then replaces mitosis in the cell cycle, is fundamental for introducing genetical variation over generations and in a population.
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