The length of the cell cycle is important because it determines how quickly an organism can multiply. For single-celled organisms, this rate determines how quickly the organism can reproduce new, independent organisms. For higher-order species the length of the cell cycle determines how long it takes to replace damaged cells. The duration of the cell cycle varies from organism to organism and from cell to cell. Certain fly embryos sport cell cycles that last only 8 minutes per cycle! Some mammals take much longer than that--up to a year in certain liver cells. Generally, however, for fast-dividing mammalian cells, the length of the cycle is approximately 24 hours.
Most of the differences in cell cycle duration between species and cells are found in the duration of specific cell cycle phases. DNA replication, for example, generally proceeds faster the simpler the organisms. One reason for this trend is simply that prokaryotes have smaller genomes and not as much DNA to be replicated. Across species and organismal complexity, embryonic cells have an increased need for rapidity in the cell cycle because they need to multiply for the development of the embryo. Early embryonic cell cycles often omit G1 and G2 and quickly proceed through successive rounds of S phase and mitosis. For these cells, the main concern is not the regulation of the cell cycle (which occurs largely in G1 and G2), but rather in the speed of cell proliferation.
In this section, we will discuss the breakdown of the durations of mitosis, G1, S phase, and G2 for the general 24 hour cell cycle found in most cells. As we discussed in the previous section, the lengths of G1 and G2 vary in cells based on the individual cell's level of preparedness for proceeding in the cell cycle. Remember, cells can enter G0 for extensive amounts of time during G1 before continuing on to S phase. If a cell has quickly undergone sufficient cell growth or DNA replication, the time spent in G1 and G2 will be decreased.
G1 is typically the longest phase of the cell cycle. This can be explained by the fact that G1 follows cell division in mitosis; G1 represents the first chance for new cells have to grow. Cells usually remain in G1 for about 10 hours of the 24 total hours of the cell cycle. The length of S phase varies according to the total DNA that the particular cell contains; the rate of synthesis of DNA is fairly constant between cells and species. Usually, cells will take between 5 and 6 hours to complete S phase. G2 is shorter, lasting only 3 to 4 hours in most cells. In sum, then, interphase generally takes between 18 and 20 hours. Mitosis, during which the cell makes preparations for and completes cell division only takes about 2 hours.
It is possible to determine the time a cell spends in different phases of the cell cycle and its specific location in the cycle by feeding cells with molecules that are only taken into the cell at a specific point in the cell cycle. For example, thymidine is only incorporated into a cell during S phase, and scientists will often use thymidine as a tool to mark the onset of S phase. The amount of DNA present in a cell is also a good indication of where a cell stands in the cell cycle. During S phase, DNA is replicated and, as a result, cells in G2 have higher levels of cellular DNA than cells in G1.
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