S Phase Regulation
The G1/S transition is a major checkpoint in the regulation of the cell cycle. Depending on levels of nutrients, energy and external factors, cells must decide to enter the cell cycle or move into a non-dividing state known as G0 phase. This transition, as with all of the major checkpoint transitions in the cell cycle, is signaled by cyclins and cyclin dependent kinase (CDKs). The pulse of G1/S cyclins causes CLN3-Cdk1 to activate Cln1/2, (Start point (yeast)) as well as Clb5/6 at the initiation of S-phase. This pathway contains 2 positive feedback loops, allowing for rapid, unidirectional movement into S-phase. Redundant pathways like this are not uncommon because they allow for tuning the output of the system and often lead to faster genetic evolution.
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