The Cell Cycle

The cell cycle is the cornerstone of cell division and proliferation. Both normal and malignant cells undergo this process, which may last for approximately 25 to 30 hours. There are five phases to the process. In the first phase, Gap 0 (G0), a cell can stay in a dormant or latent state for months or even years until stimulated to move forward in the cycle. Because certain cells divide more rapidly than others, some rest in the G0 phase for a brief period, whereas others bypassthe G0 phase and enter the second phase, the Gap 1 (G1) phase, directly if the body needs the immediate production of a certain cell. The G1 phase occurs after mitosis, the birth of two daughter cells. During this phase, the cell synthesizes RNA and the proteins needed for DNA synthesis. The time a cell spends in this phase varies and can last from hours to days, depending on the cell type. After RNA and protein syn-theses occur, the cell then enters the third phase, the synthesis (S) phase, when RNA, protein, and DNA syntheses occur and DNA replicates.

DNA is an essential nucleic acid composed of deoxyribose, a phosphate, and four nitrogenous bases: adenine, guanine, cytosine, and thymine. Adenine and guanine are the purines, and cytosine and thymine are the pyrimidines. Chemical reactions occur between the two purines and also between the two pyrimidines, leading to the formation of the double-stranded DNA helix, which serves as the genetic template of the cell.

Generally, the S phase lasts 8 to 12 hours. The cell then enters the fourth phase, Gap 2 (G2), when more RNA and protein syntheses take place in preparation for mitosis. This phase tends to last 2 to 4 hours; then the cell enters the fifth or mitosis (M) phase. The M phase consists of the following orchestrated subphases: prophase, metaphase, anaphase,and telophase. As the cell progresses through these subphases, the cytoplasm and nucleus divide so that replication of the cell results in the birth of two daughter cells.

It is not clearly understood how the body maintains normal cellular homeostasis. What has been postulated is that the body possesses a feedback system that signals a cell to enter the G1 phase of the cell life cycle in response to cell death. In patients with cancer, this feedback system is dysfunctional, and the cancer cell enters the cell cycle independently of the body's feedback system.