External regulatory proteins that stimulate the growth and division of cells are called what?

• A. Growth factors
• B. Cyclins
• C. Kinases
• D. Checkpoints

Answer: (A)

Growth factors are external signaling molecules that bind to receptors on a cell and trigger pathways that promote growth and division. They come from other cells or the surrounding environment and act as mitogens to push a cell from a resting state into the cell cycle, often helping it move from G1 toward DNA replication. Inside the cell, progression is coordinated by cyclins and kinases, but those are internal regulators, not external signals. Checkpoints are internal surveillance points that halt the cycle if something is wrong. So the term that best fits “external regulatory proteins that stimulate growth and division” is growth factors.