Principles of Cell Cycle Control

The different phases of the cell cycle include a number of highly ordered processes that ultimately lead to duphcation of the cell. The various cell cycle events are highly coordinated to occur in a defined order and with an exact timing, requiring precise control mechanisms. 

The ordered sequence of cell cycle events is ensured by different control loops that have an inhibitory or promoting effect on the progress(ion ) of the cell cycle. These are monitoring mechanisms that register the completion of important cell cycle events (e.g., complete DNA synthesis) and allow the transition to the next event (e.g., entry into mitosis) to occur. 

The control systems of the cell cycle ensure that the various phases are executed completely and in the correct sequence. Entry into a new phase can only take place when the preceding phase has been completed. In addition, the system allows coupling of processes that are not adjacent in the cell cycle sequence. Thus, there are close mutual regulation mechanisms between M and S phase. Examples of the various control mechanisms are shown in Fig. 13.3. 

Many of the control mechanisms of the cell cycle are of intrinsic nature and are constitutive, i.e., they are operational in every cell cycle and ensure the ordering of the individual steps. However, other control mechanisms exist that are not active in every cell cycle these are only induced when defects are detected in central cell cycle events. These control mechanisms are known as checkpoints. An example of a checkpoint that is only activated when required is the DNA damage checkpoint (see 13.7). This is a biochemical pathway that detects DNA damage and creates a signal that arrests cells in the Gl, S or G2 phase of the cell cycle. 

In addition to the built-in protection and control mechanisms, the cell is also subject to a number of external controls, which ensure that cell division occurs in balance with the overall development of the organism and with external growth conditions. This is a kind of social control of cell division that regulates the progress of the cell cycle, with the help of circulating signal molecules or via cell-cell interactions. 

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