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.
Figure %: Relative Duration of Cell Cycle Phases
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.
