It's fascinating that the entire set of 23 chromosome pairs can be copied during S phase. Thinking about all of the components that must be produced to facilitate this replication is complicated. For instance, in order for the DNA to wrap up into chromosomes, the quaternary histone protein and all of its substituents must be synthesized and present. In addition, all of the chromosomal arrangement must be orchestrated by a large number of proteins and chemical interactions must occur. Overall, the entire process is a lot to think about.
The interactions between CDKs and cyclins are extremely important to the cell cycle and highly organized. Since different phases of the cell cycle are dependent on the levels of cyclins, and consequently CDKs, there must be a large number of them. In addition, they must be produced at critical and specific moments throughout. The amount of involved proteins must be relatively high.
These processes must occur in a sequential order. If they somehow began too early or in the wrong order, the consequences within the cell would be devastating. For example, if M-phase began before S-phase was complete, the resulting daughter cells would be deprived of a number of chromosomes and become destined for cell-death. Perhaps overregulation of specific cyclins could lead to something like this.
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