Cell Cycle
The cell cycle describes the events that occur in a cell from one point in cell division until that cell reaches that same point in cell division. It does not describe the birth, life and death of a cell as you would see in a typical life cycle.The cell cycle is divided into two major phases, interphase and mitosis. Cells spend the majority of their time in interphase. Some cells, once differentiated never divide again. They continue to metabolize and function but do not divide. Cells in this non-divisional state are said to be in G0. Cell signals tell the cell when to begin the process of active cell division. Understanding these cell signals is key to turning cell division on and off and may hold the key to treating cancer, paralysis, blindness, etc.
Interphase is subdivided into three other phases: G1, S and G2. During the early days of cell biology, scientists had a difficult time understanding the cell process and what was going on in the cell. G1, stands for the first gap. Scientists knew something was going on, they just weren't sure what it was. Today we know that cells in G1 are making everything they need for 2 cells, more protein, more organelles, etc. The next phase is the S phase or the synthesis phase. During S the cell replicates its DNA. So in human cells, a skin cell for example, at the completion of the S phase the cell will contain 92 chromosomes. Once the DNA has replicated the cell has made the commitment to continue through cell division. There is no turning back. The chromosomes are still present as chromatin. DNA is not visible as chromosomes. G2 is the final subphase of interphase. During G2 the cell continues to grow and produce materials needed for 2 cells. Additionally the cell prepares for mitosis by organizing the machinery needed for cell division like, microtubules.
