The growth factor would bind to its receptor. This receptor would initiate a cell signalling pathway which would eventually turn on the expression of the cyclin gene. The resulting cyclin protein would be particular one to activate a Cdk which phosphorylates proteins leading the cell into S phase.
Work Step by Step
Recall that cell signalling involves a ligand binding to a receptor. This in turns causes a chain of steps which do things like turn on kinases, produce second messengers, and ultimately alter gene expression. Since the question asks about the eventual production of cyclin protein, the growth factor is the ligand, and it must have a receptor. Then, describe the basic idea of a signalling pathway, making sure to highlight the change in gene expression, part of the pathway, leading to the cyclin protein which will kick the cell into S phase.