A cell cycle phase is a stage of a continuous cycle that most cells in the body undergo. In each phase, various activities can take place involving cellular structures and genetic material. The phases occur sequentially, and it can take up to a day for the cycle to complete. Most of the time, a cell is in interphase, during which it is not dividing but undergoes checkpoint stages and deoxyribonucleic acid (DNA) replication. The chromosomes and nucleus divide during mitosis, and the cell divides into two in a sub-stage called cytokinesis.
The interphase and miotic phase, when cells divide, are characteristic of all cells in the human body, except for those in the reproductive system. Most of the time, division does not occur. The cycle is typically dominated by processes that involve growth and the replication of intracellular structures that support chromosomes. An intermediate phase after division includes biological checks to make sure DNA replication can occur. If not, then a period of dormancy can last anywhere from a few days to years.
Replication of DNA occurs during a synthesis period called an S phase. This is a cell cycle phase in which growth often continues, while proteins and enzymes for replicating genetic material are created. The chromosome count typically doubles and then another intermediate phase occurs as the replicated DNA is checked. Errors in the genetic code can be corrected and structures called microtubules are replicated to prepare for cellular division. During this phase, there is generally time for correcting problems and ensuring the chromosomes are properly duplicated.
Chromosomes are dispersed and usually cannot be seen under a microscope, until the cell cycle prophase begins. Centrioles are structures where the microtubules are replicated, and two of these typically spread in opposite directions. Chromosomes become visible during prophase as well, and various fibers form a network around the cell, forming a mitotic spindle. Another cell cycle phase is when the membrane of the nucleus disappears while proteins and microtubules interconnect. Fibrous structures then line up across the nucleus during metaphase and then structures called kinetochores pull chromosomes from each pair to the appropriate side.
Each cell cycle phase has separate activities. Once two nuclei form, the chromosomes disperse inside of them. The final division is a phase unto its own. A protein typically forms a ring around the dividing point of the cell and closes until two separate ones are formed.