The Cell Cycle (and cancer) [Updated]

Living organisms are made of cells, and for multicellular organisms, cells work together in tissues, organs, and organ systems. Cell division, or reproduction, is how organisms grow and replace cells. This process, known as the cell cycle, is carefully regulated, and its malfunction can lead to serious health issues like cancer. Cancer involves cells dividing too frequently and uncontrollably.

Cancer cells divide uncontrollably and can have other problems like poor communication, inability to perform normal functions, and failure to anchor securely, potentially leading to metastasis. They can even secrete their own growth hormones, diverting nutrients from healthy cells. Cancer can be linked to genetic factors, exposure to toxins, radiation, or excessive UV light. Uncontrolled growth can form tumors, and ongoing research aims to develop better treatments that target frequently dividing cells.

The cell cycle consists of two main phases: interphase and M phase. Interphase, where cells spend most of their time, includes G1 (cell growth), S (DNA replication), and G2 (further growth in preparation for mitosis). M phase involves mitosis (nuclear division) and cytokinesis (cytoplasm splitting), leading to cell division. The frequency of mitosis varies; for example, hair follicle cells divide rapidly, which is why cancer treatments often affect hair growth.

Cell cycle checkpoints are crucial to prevent division of damaged cells. There are checkpoints in G1 (checking for growth, DNA damage, and resources), G2 (verifying DNA replication and resources), and M phase (ensuring chromosomes are correctly aligned for separation). If issues are found, the cell may pause to repair. If the damage is irreparable, the cell undergoes apoptosis, or programmed cell death, to prevent the propagation of faulty cells.

The cell cycle is regulated by proteins, some acting as positive regulators (like cyclin and Cdk, an enzyme) to advance the cycle, and others as negative regulators (like p53) to halt or initiate apoptosis. Cyclin levels fluctuate, binding with Cdk to signal progression through different phases. Additionally, some cells enter a resting phase called G0, where they perform functions but do not prepare to divide. Some cells enter G0 temporarily due to lack of resources, while others, like many neurons, remain in G0 permanently, explaining challenges in healing brain and spinal cord injuries.