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The Cell Cycle
We have already discussed how the two main events of cellular reproduction are the copying of cellular components and the cleavage of the cell. These two events, copying and cleaving, represent the two larger phases of the cell cycle, interphase and the M phase. The M phase is the part of the cell cycle when the cell prepares for and completes cell division. During interphase, appropriate cellular components are copied. Interphase is also a time of checkpoints to make sure that the cell is ready to proceed into mitosis. Both of these two phases have further sub-divisions. Since the cell cycle is a "cycle" it has no distinct beginning or ending. Most cells are continually entering and exiting the various phases of the cycle. Other cells stop dividing and enter what is called G0. The M phase will be covered in depth in the next SparkNote; here we will focus on what goes on during interphase.
Interphase
Figure 4.04: Interphase and Mitosis
Interphase
We will begin our discussion of the events that take place during interphase with those that occur immediately after a cell has successfully divided during the M phase. This phase is called G1.
G1 phase
G1 is an intermediate phase occupying the time between the end of cell division in the M-phase and the beginning of DNA replication during the S phase. During this time, the cell grows in preparation for DNA replication. Before a cell begins DNA replication, it must ensure that it is biologically ready to take on such a process. G1 is the phase when this cellular monitoring takes place.
During G1, the cell reviews the cellular environment and the cell size to ensure that the conditions are appropriate to support DNA replication. Not until the cell is ready does it leave G1. If all is not ready to undergo DNA replication, cells can pause during G1 and enter a phase called G0. Depending on a cell's preparedness to continue in the cell cycle, G0 can last days, weeks, or even years. When the cell has reached an appropriate size and is in a supportive environment for DNA replication, it will exit either G1 or G0 and enter the next phase of interphase called the S phase.
S phase
The S phase, or synthesis, is the phase of the cell cycle when DNA packaged into chromosomes is replicated into two sister chromatids. This event is an essential aspect of the cell cycle because replication allows for each cell created by cell division to have the same genetic make-up. (The specifics of how this replication takes place is covered in the Unit 5.) During the S phase a number of events additional to chromosome replication take place. Cell growth continues through the S phase, as does the rate of synthesis of a number of proteins and enzymes that are involved in DNA synthesis. Once DNA replication is complete the cell contains twice its normal amount of genetic material and becomes ready to enter the phase called G2.
G2 phase
Similar to G1, G2 is an intermediate phase, a time for the cell to ensure that it is ready to proceed in the cell cycle. Occurring between the end of DNA replication in the S phase and the beginning of cell division in mitosis, G2 can be thought of as a safety gap during which a cell can check to make sure that the entirety of its DNA and other intracellular components have been properly duplicated. In addition to acting as a checkpoint along the cell cycle, G2 also represents the cell's final chance to grow before it is split into two independent cells during the M-phase
Cell Cycle Summary
Interphase is made up of three distinct phases: G1, S phase, and G2. The G1 and G2 phases serve as checkpoints for the cell to make sure that it is ready to proceed in the cell cycle. If it is not, the cell will use this time to make proper adjustments that can include cell growth, correction or completion of DNA synthesis, and duplication of intracellular components. S phase involves the replication of chromosomes. All three stages of interphase involve continued cell growth and an increase in the concentration of proteins found in the cell.
