Cell proliferation refers to the process by which cells grow and divide to create new cells. This process is fundamental to the growth, development, and maintenance of all living organisms. In multicellular organisms, cell proliferation allows for growth and development, replaces dead or damaged cells, and helps in wound healing and immune responses.

The process of cell proliferation is tightly regulated by the cell cycle, a series of events that lead to cell division. The cell cycle consists of four main phases:

1. G1 phase (Gap 1 phase): During this phase, the cell grows and prepares for DNA replication. The decision to continue to the next phase or to exit the cell cycle and enter a resting state (G0 phase) is made at a checkpoint near the end of G1 phase.
2. S phase (Synthesis phase): During this phase, DNA replication occurs, creating two identical copies of the cell’s DNA.
3. G2 phase (Gap 2 phase): After DNA replication, the cell continues to grow and prepares for cell division. The cell also checks to make sure DNA replication was completed correctly at a checkpoint near the end of G2 phase.
4. M phase (Mitosis phase): During this phase, the cell divides its duplicated DNA and cytoplasm to create two new cells. M phase includes the processes of mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

The regulation of cell proliferation is crucial for maintaining tissue homeostasis. Uncontrolled cell proliferation can lead to diseases such as cancer. Conversely, insufficient cell proliferation can also cause diseases, such as certain types of immunodeficiency, and can contribute to the aging process. A variety of signals, both internal and external to the cell, influence whether a cell enters the cell cycle and proceeds to divide. These signals can include growth factors, hormones, nutrients, and signals from the extracellular matrix or neighboring cells.