Final answer:
Cell signaling pathways are always present but not always active, especially in the Go phase where cells are quiescent. Growth factors can activate these pathways, leading to cell division.
Step-by-step explanation:
In relation to cell division, cell signaling pathways are always present in the cell but are not always active.
Cells enter a quiescent stage known as the Go phase when they are not actively preparing to divide. This phase can be temporary until a cell is triggered to enter the G1 phase, or it can be permanent for cells that rarely or never divide, like mature cardiac muscle and nerve cells. Cell division is tightly regulated and cells typically require external signals, such as growth factors, which bind to cell-surface receptors known as receptor tyrosine kinases (RTKs). Activation of these RTKs leads to a cascade of events involving G-proteins like the RAS protein, which then activates the MAP kinase pathway, resulting in the stimulation of cell division.