Figure 2.

Sequential and reciprocal activation of upstream and downstream signalling pathways. (A) Normal mitogenic signalling involves the sequential activation of both upstream and downstream pathways. Note that, due to feedback loops, the distinctions between 'upstream' and 'downstream' are not always maintained, and the molecules indicated in the boxes are merely examples. (B) Classical growth arrest in G0 occurs when there is no activation of either upstream or downstream pathways. (C) The conflicting signal model of apoptosis involves the isolated overactivation of downstream pathways. (D) The conflicting signal model of senescence involves the isolated overactivation of upstream pathways.