17.1: What is the Cell Cycle?
The cell cycle refers to the sequence of events occurring throughout a typical cell’s life. In eukaryotic cells, the somatic cell cycle has two stages: interphase and the mitotic phase. During interphase, the cell grows, performs its basic metabolic functions, copies its DNA, and prepares for mitotic cell division. Then, during mitosis and cytokinesis, the cell divides its nuclear and cytoplasmic materials, respectively. This generates two daughter cells that are identical to the original parent cell. The cell cycle is essential for the growth of the organism, replacement of damaged cells, and regeneration of aged cells. Cancer is the result of uncontrolled cell division sparked by a gene mutation.
Cell Cycle Checkpoints
There are three major checkpoints in the eukaryotic cell cycle. At each checkpoint, the progression to the next cell cycle stage can be halted until conditions are more favorable. The G1 checkpoint is the first of these, where a cell’s size, energy, nutrients, DNA quality, and other external factors are evaluated. If the cell is deemed inadequate, it does not continue to the S phase of interphase. The G2 checkpoint is the second checkpoint. Here, the cell ensures that all of the DNA has been replicated and is not damaged before entering mitosis. If any DNA damage is detected that cannot be repaired, the cell may undergo apoptosis, or programmed cell death. The M or spindle checkpoint ensures that all the sister chromatids are correctly attached to the spindle microtubules at the metaphase plate before the cell enters anaphase.
Cancer: When the Cell Cycle Goes Awry
Cell cycle checkpoints ensure that healthy cells move through the cell cycle in a regulated way. However, cancer cells often bypass these checkpoints. Each successive round of unchecked cell division produces more damaged daughter cells. Additionally, cancer cells in the human body can divide many more times than normal cells, which can only undergo about 40-60 rounds of division. Cancer cells express telomerase, an enzyme that repairs the wear and tear at the ends of chromosomes that is typically caused by cell division.