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The cell cycle consists of sequential phases involving cellular structures and genetic material. Most cells are in interphase, followed by mitosis and cytokinesis. DNA replication occurs during the S phase, and errors can be corrected. Chromosomes become visible during prophase, and the final split is a separate stage.
A cell cycle phase is a phase in a continuous cycle that most cells in the body undergo. At each stage, different activities involving cellular structures and genetic material can take place. The phases occur sequentially and the cycle can take up to a day to complete. Most of the time, a cell is in interphase, during which it does not divide but undergoes checkpoint phases and deoxyribonucleic acid (DNA) replication. The chromosomes and nucleus divide during mitosis and the cell divides in two in a substep called cytokinesis.
The interphase and miotic phase, when cells divide, are characteristic of all cells in the human body, except those of the reproductive system. Most of the time, the split doesn’t happen. The cycle is typically dominated by processes involving the growth and replication of the intracellular structures that support the chromosomes. An intermediate step after division includes biological checks to make sure DNA replication can occur. Otherwise, a period of dormancy can last from a few days to years.
DNA replication occurs during a period of synthesis called the S phase. This is a phase of the cell cycle where growth often continues, while proteins and enzymes are created to replicate the genetic material. The chromosome count typically doubles and then another intermediate step occurs when checking for replicated DNA. Errors in the genetic code can be corrected and structures called microtubules are replicated in preparation for cell division. During this stage, there is usually time to correct problems and ensure that chromosomes are properly duplicated.
The chromosomes are scattered and usually cannot be seen under a microscope, until the prophase of the cell cycle begins. Centrioles are structures in which microtubules are replicated and two of these typically spread in opposite directions. Chromosomes also become visible during prophase, and various fibers form a network around the cell, forming a mitotic spindle. Another phase of the cell cycle is when the membrane of the nucleus disappears as 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 phase of the cell cycle has separate activities. Once two nuclei are formed, the chromosomes are dispersed within them. The final split is a stage of its own. A protein typically forms a ring around the point of division of the cell and closes until two separate rings are formed.
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