The cell division cycle is the procedure by which larger and more complex eukaryotic genomes replicate. It consists of 5 phases; G1 - the primary growth phase of the cell and encompasses the major portion of the cell's life span, S - the cell synthesizes a replica of the genome, G2 - the second growth phase where preparations are made for genomic separation: organelles replicate, chromosomes condense, and microtubules assemble at spindle. G1, S and G2 make up interphase, which is very important for the completion of mitosis because of the growth and replication that occur. Proteins are synthesized and organelles are produced. After interphase come mitosis, when the cells are duplicated and cytokinesis, where the cytoplasm divides to form two daughter cells.
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The cell cycle is carefully controlled. The goal of controlling any cyclic process is to adjust the duration of the cycle to allow sufficient time for all events to occur. The cell cycle is regulated by a central set of "go/no-go" switches that are regulated by feedback from the cell.
The cell cycle is regulated at three principal checkpoints:.
1. Cell growth is assessed at the G1 checkpoint.
This checkpoint is located at the end of the G1 phase, just before entry into S phase. This checkpoint makes the key decision of whether the cell should divide, delay division, or enter Go. First studied in yeast, it was called START, the point when the cell begins to copy DNA if conditions are favorable for division, furthermore initiating the S phase. In complex eukaryotes, the G1 checkpoint is where the cell is arrested if environmental conditions are not conducive to division, or if the cell enters a resting state called Go for an extended period of time.
2. Success of DNA replication is assessed at the G2 checkpoint.
Located at the end of G2, this checkpoint triggers the M phase and initiates molecular processes that signal the beginning of mitosis.