The cell cycle governs the entire life of a cell, including growth, division and death. There are defined phases of the cell cycle, during which the cell grows, duplicates its DNA and eventually divides into two new cells. Checkpoints at each phase of the cycle verify the progress so the cell may enter the next phase. Failure to pass these inspections can result in arrest of the cell cycle and death; however, failure to properly regulate the cell cycle can lead to cancer.
Growth Phase I
A young cell initially expands in response to the presence of proteins called growth factors. If an insufficient amount of growth factors are present, the cell ceases to grow and exits the cell cycle in a phase known as Growth Phase Zero. This arrest can be temporary, with the cell returning to Growth Phase 1 when appropriate growth factors become present. If the growth factors fail to become present, the cell undergoes programmed cell death. Certain types of cells do not divide in adulthood and permanently exit the cell cycle, remaining in Growth Phase Zero for long periods of time. This is observed in neurons, the longest-living cells in the human body.
After a cell has been in Growth Phase I for a sufficient amount of time, it becomes too large and needs to divide into two smaller cells. The first step toward this division is duplication of the DNA. The cell synthesizes a copy of its DNA so that two complete copies are present in the cell. Mistakes are often made during the duplication process, and the cell needs to correct these mistakes before it can progress to the next phase of the cycle.
Growth Phase II and Mitosis
Next, the cell enters the relatively short Growth Phase II. This phase gives the cell a chance to grow a little more before dividing. Much like the first growth phase, appropriate amounts of growth factors are needed to progress into the next phase, which is called mitosis. During this phase, the two sets of chromosomes line up and are separated, so that the two copies move to different sides of the cell. The cell then splits down the middle, resulting in two cells, each containing one copy of the DNA.
If the cell doesn't pass checkpoint inspections, it may undergo programmed cell death. A failure to maintain regulation of the cell cycle leads to uncontrolled cell growth and division, and eventually cancer. Cancer cells lose the ability to be regulated by the cell cycle and continue to divide, even in the absence of certain growth factors, which is what makes killing cancer cells difficult.