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Problem : What is the approximate cell cycle duration in fast-dividing mammalian cells? Is this the same for all cells?

24 hours. Different organisms have different cell cycle durations. Even within an organism, different cell types can have different cell cycle durations. Therefore, the cell cycle duration is not the same for all cells.

Problem : Which phase of the cell cycle—M phase, G1, S phase, or G2—has the most variability in its duration?

G1. The length of both G1 and G2 are both highly variable because their durations depend on certain cellular conditions. A cell in G1, however, has the option of entering a "sub-phase" called G0. A cell that enters G0 can remain in a paused state for years, thus significantly increasing the amount of time a cell spends in G1. Because G1 is the first phase that follows cell division, many cellular conditions must be met before the cell can continue on to S phase.

Problem : In cells that undergo very rapid cell division, which phases are decreased or eliminated?

G1 and G2 can be decreased or even eliminated in cells that undergo very rapid cell division.

Problem : In normal mammalian cells, which take longer: the processes that lead to chromosome duplication or those that lead to cell division?

The processes that lead to cell division occur during M phase, or mitosis. This phase lasts only 2 hours. The processes that lead to chromosome duplication occur during interphase which lasts about 18 hours. Therefore, the processes that lead to chromosome duplication take longer.

Problem : How can the amount of DNA present in a cell give an indication of where in the cell cycle a particular cell may be?

During S phase the amount of DNA in the cell doubles because chromosomes are duplicated. As a result, the amount of DNA found in a cell in G2 is significantly higher than that of a cell in G1. If one knows the amount of DNA that a normal cell has and finds that a particular cell has much higher levels of DNA that normal, the cell has likely already progressed through G1 and S phase, but not yet undergone division in M phase.
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