The cell cycle is a process in which a cell grows and divides to create a copy of itself. Some organisms reproduce through the cell cycle, and in complex multicellular organisms, the cell cycle is used to allow the organism to grow, and to replace cells as they grow worn out. In animals, the whole cell cycle takes around 24 hours from start to finish. Some cells, such as skin cells, are constantly going through the cell cycle, while other cells may divide rarely, if at all; neurons, for example, don't grow and divide once they are mature.

This process starts with interphase, in which the cell starts out in a resting state and then starts to prepare itself for division. In interphase, the cell makes two sets of its DNA, and the organelles in the cell also double themselves. As these processes occur, the cell also grows in size. At the end of interphase, the cell has twice the amount of material it needs, and it can start mitosis, in which the material migrates to opposite ends of the cell so that the cell can split in two, making two new cells.

Cells like the skin which are constantly shed must renew themselves through the cell cycle on a regular basis. The cell cycle is also used during fetal development to allow a single fertilized egg to develop into an entire organism. In this case, the daughter cells created through the cell cycle are not always identical, as the cells will eventually start to differentiate to form various structures in the body.

Every process of the cell cycle is regulated by proteins which tell the cell what to do. These proteins are also used during interphase to confirm that conditions are appropriate for cell division. If enough nutrients are not present or other issues are identified, these proteins will signal the cell to remain dormant and wait for conditions to improve, sort of like a crossing guard for cell division.

Errors can and do occur during cell division. Sometimes the information is not copied exactly during interphase, and errors in a cell's genome are created. These errors may be incompatible with life, causing the cells to die, or they may be harmless. They can also become problematic for the host organism, as in the case of an error which causes a cell to replicate and divide repeatedly, with no checks, forming a tumor as a cluster of cells grows out of control.