The cell cycle
What is the cell cycle?
The cell cycle is actually the life cycle of a cell. It is also the other way to interpret the mitosis. In the interphase of mitosis, the DNA should produce RNA, which then produce protein. However, in the middle of the interphase of mitosis, The DNA stops producing RNA, and starts to produce more DNA -- it replicates itself.
Howard and Pelc monitored the duplication of DNA by labelling one of its component -- thymidine with a radioactive isotope. They named this phase the synthesis (S) phase, because new DNA is synthesised. Soon, they also noticed that there is a gap after mitosis and before synthesis, which they called gap 1 (G1), and a gap after synthesis and before mitosis, which they called gap 2 (G2). These 4 phases together forms the cell cycle.
It should be noticed that the cell is diploid only in G1. The chromosomes divides in the S phase, which makes the cell tetraploid. G2 is also tetraploid. In the mitosis (M) phase, the 4 sets of chromosomes are pulled into the 2 daughter cells, making each of them diploid. There're also another interesting fact that is worth considering. In the S phase, the DNA replicates once, and only once. No more, no less. If the cell has too much or too less chromosomes, it will die.
Apart from all these, there is another stage, so called gap 0 (G0), which the cell enters when it feel it would be folly to grow (when it is deprived of nutrients). The cell would be synchronized in this stage. It would be ready for the egg to reset its clock (in the case of cloning by nuclear transfer of adult cells). This stage, although seemed to be unimportant at the beginning, proved to be extremely useful in the experiment of cloning by adult cells.
The Control of the cell cycle
After talking about all these phases and cycles, you may be wondering: a cell has no brain, how can it control it's cycles? Here shows the brilliance of nature yet again. To explain the control of all these, there are a few words that requires explanation.
MPF: Maturation-promoting factor
The MPF promote all visible changes including mitosis and meiosis. It causes NEBD, it causes the chromatin to condense, and it prompts changes in the cytoskeleton. MPF is made up of two factors: cyclin and cdc2. The amount of cyclin varies throngh the cell cycle. It is an important control.
The licensing factor is present in the cytoplasm, but normally not in the nucleus. it causes the chromosomes to replicate.
When the body thinks the cell should divide, it will produce a hormone, which stimulates the rise in cyclin. The cyclin will form MPF with cdc2, and will cause NEBD. After the cell divide, when it is in telophase, the licensing factors get into the nucleus when the nuclear envelop reforms. The licensing factor causes the chromosome to divide. This is quite a delicate control.