by Madison Williams
The cell cycle is the process by which our cells grow and divide. This process happens during development in utero, childhood, and adulthood. Our cells are constantly dividing to replace cells that have died or to repair injuries. Because this process is so important, the cell cycle is divided into multiple phases that each have their own unique role in cell division.
The cell cycle has two main phases: interphase and the mitotic phase. You may have learned about the phases of mitosis in your biology classes, but most of the cell cycle is actually spent in interphase. In interphase, the cell grows and copies its DNA in preparation to divide. Interphase has three subphases: G1, S, and G2. In G1, the cell is growing and preparing to divide. In S phase, the new DNA is being created. In G2, the cells grow in preparation to split. Then, finally, the cell divides in the mitotic phrase, also known as M phase.
G1 of interphase is also known as a “gap phase,” and it simply provides the cell with extra time to grow before it makes new DNA and divides. At the end of G1, there is a checkpoint where the cell commits to going into S phase and synthesizing new DNA for the dividing cell. Special proteins called cyclins bind to enzymes (called cyclin-dependent kinases) to allow the cell to enter into S phase. If the cell does not grow as much as it needs to during this phase, it will not be able to enter S phase and make new DNA.
During S phase, the DNA is replicated in the nucleus of the cell so that it can be passed on during cell division. This phase is very important because if DNA synthesis goes wrong, the new cells could have problems with producing the proteins they need. There is a checkpoint in S phase that determines if there has been any damage or mutations to the DNA, and if so, there are proteins that can fix it. However, sometimes the DNA damage or mutation does not get fixed. If the mutation activates certain genes, it can promote uncontrolled cell division and lead to cancer.
G2 phase is another gap phase where the cell is continuing to grow before dividing. This is also the phase in the cell cycle where most of the organelles replicate and grow. Every cell has organelles that produce energy and transport proteins. If the new organelles do not form, the cell will likely not divide.
After interphase, M phase includes both mitosis and cytokinesis, and the cell divides in this phase. Like interphase, mitosis also has subphases, including: prophase, metaphase, anaphase, and telophase. During prophase, the chromosomes condense, and the membrane of the nucleus breaks down to allow the DNA out into the cell. Next is metaphase, and during this phase, the chromosomes line up in the middle of the cell. In anaphase, the chromosomes are pulled apart and move towards opposite ends of the cell. Lastly, in telophase the nuclear membrane begins to reform, which leads to cytokinesis. Cytokinesis is the final step in the cell cycle, and during this phase, the cytoplasm splits in two and the cell divides. One problem that can occur in mitosis is if the chromosomes do not move to the middle of the cell properly during metaphase, they will not move to the opposite ends of the cell in anaphase. This would result in the daughter cells having different numbers of chromosomes and would most likely kill the cell.
The cell cycle is incredibly complex, and scientists are still learning about its details, but this is something our cells do on a daily basis throughout our lives!
