Cyclins, mitotic cell cycle

Based on the functional analysis of Cdkl and Cdk2 complexes, we have proposed a model for the conversion of the mitotic cell cycle into an endoreduplication cycle (Fig. 1A Edgar Lehner 1996). Accordingly, this cell cycle conversion is dependent on elimination of Cdkl activity and periodic activation of cyclin E/Cdk2. Cyclin E is required for endocycles and a pulse of ectopic cyclin E expression is sufficient to trigger endoreduplication. Cdk2... 

In yeast, the MAP kinase Fus3 induces cell cycle arrest via the degradation of cyclins, Clnl and Cln2. The mitotic cyclins (CYCLIN) are cell cycle proteins that bind the protein kinase Cdc2 during interphase... 

The cell cycle is a key process that recurs in a periodic manner. Early cell cycles in amphibian embryos are driven by a mitotic oscillator. This oscillator produces the repetitive activation of the cyclin-dependent kinase cdkl, also known as cdc2 [131]. Cyclin synthesis is sufficient to drive repetitive cell division cycles in amphibian embryonic cells [132]. The period of these relatively simple cell cycles is of the order of 30 min. In somatic cells the cell cycle becomes longer, with durations of up to 24 h or more, owing to the presence of checkpoints that ensure that a cell cycle phase is properly completed before the cell progresses to the next phase. The cell cycle goes successively through the phases Gl, S (DNA replication), G2, and M (mitosis) before a new cycle starts in Gl. After mitosis cells can also enter a quiescent phase GO, from which they enter Gl under mitogenic stimulation. 

A development in our understanding of the possible role of protein kinases in cell cycle control stems from recent work with yeasts and other cell types [16]. The transition from G1 to S as well as from the G2 phase (which follows S) to the mitotic phase (M) appears to be controlled by the same protein kinase. The Gl/S kinase activity is associated with the accumulation of a novel family of G1 cyclins [17]. Cyclins are proteins that are synthesised and degraded at specific points in each cycle. The G1 cyclins are distinct from the cyclins required to activate the protein kinase for transition from the G2 phase to M[17]. 

The cyclin proteins appear to play a major role in cell cycle regulation. Cyclins are a family of proteins that accumulate in the G1 phase (G1-cyclins), and again during late S-G2 phases before mitosis (mitotic cyclins). After they reach a peak concentration, cyclin proteins are rapidly degraded. 

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