It is essential that the daughter cells produced be exact duplicates of the parent cell. Moving forward from this initiation point, every parameter required during each cell cycle phase must be met or the cycle cannot progress.
THE DIVISION 2 CONTROL POINTS SERIES
Whatever the source of the message, the cell receives the signal, and a series of events within the cell allows it to proceed into interphase. The solution to this problem is to divide. Another factor that can initiate cell division is the size of the cell as a cell grows, it becomes inefficient due to its decreasing surface-to-volume ratio. Crowding of cells can also inhibit cell division. A lack of HGH can inhibit cell division, resulting in dwarfism, whereas too much HGH can result in gigantism. 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.
By the end of this section, you will be able to:
0 kommentar(er)
