The cell division cycle and its control

In addition to the two mitogenic pathways outlined above, a third pathway can also be operational. This pathway involves the direct interaction of certain growth factors (e.g. epidermal growth factor) with their cognate plasma-membrane 

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]. 

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