The role of cell cycle control mechanisms in regulated and sustained cell proliferation

Sammanfattning: The cell cycle is the time a cell spends between two cell divisions. The cell cycle includes several parallel processes, all of which must be completed before a cell is mature for dividing. In the first place, all subcomponents (RNA, protein and membrane lipids) need to double in quantity and this occurs continuously through the cell cycle. Furthermore, the genome and some chromosomal proteins must double and this take place during a limited interval in the middle of the cell cycle (S-phase). On either side of the S-phase are two "gaps" which is called G1 and G2. When the cell has passed a point in the G-phase they are irreversibly programmed to progress through the remaining of the cell cycle and will undergo the next cell division. When a cell have passed a point in the G1 phase, approximately four hours after mitosis, a normal cell is in a state of indecision. During this phase (G1pm), the cell is affected by external factors, such as presence of certain growth factors and proximity to other cells to make a decision whether to continue toward S-phase or exit the cell cycle and enter a reversible resting stage-G0. G1pm phase is always of constant length (four hours), followed by a variable phase of G1 (G1ps) during which the cell builds its structural components with varying speed. A small cell spend a relatively longer time in G1ps than a larger one. The idea is that any size differences between cells will be adjusted before S-phase is initiated. Unlike normal cells, the transformed cells lack the ability to withdraw the G0 under suboptimal conditions. This article aims to summarize how normal and transformed cells relate to the G0-blocking and how individual factors determine the both cells relation to cell proliferation inhibition and "sustained proliferation."