An event may be as simple as the death of a nearby cell or as sweeping as the release of growth-promoting hormones, such as human growth hormone (HGH). Regulation of the Cell Cycle by External Eventsīoth the initiation and inhibition of cell division are triggered by events external to the cell when it is about to begin the replication process. The timing of events in the cell cycle is controlled by mechanisms that are both internal and external to the cell. In early embryos of fruit flies, the cell cycle is completed in about eight minutes. In rapidly dividing human cells with a 24-hour cell cycle, the G 1 phase lasts approximately nine hours, the S phase lasts 10 hours, the G 2 phase lasts about four and one-half hours, and the M phase lasts approximately one-half hour. When fast-dividing mammalian cells are grown in culture (outside the body under optimal growing conditions), the length of the cycle is about 24 hours. There is also variation in the time that a cell spends in each phase of the cell cycle. In humans, the frequency of cell turnover ranges from a few hours in early embryonic development, to an average of two to five days for epithelial cells, and to an entire human lifetime spent in G 0 by specialized cells, such as cortical neurons or cardiac muscle cells. The length of the cell cycle is highly variable, even within the cells of a single organism. Describe the molecules that control the cell cycle through positive and negative regulation.Explain how the three internal control checkpoints occur at the end of G 1, at the G 2/M transition, and during metaphase.Understand how the cell cycle is controlled by mechanisms both internal and external to the cell. By the end of this section, you will be able to:
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