PATHOLOGY

Detailed explanation-1: -Some extracellular signal proteins, including PDGF, can act as both growth factors and mitogens, stimulating both cell growth and cell-cycle progression.

Detailed explanation-2: -The proteins that play a role in stimulating cell division can be classified into four groups-growth factors, growth factor receptors, signal transducers, and nuclear regulatory proteins (transcription factors).

Detailed explanation-3: -Cyclins are proteins that regulate the timing of the cell cycle in eukaryotic cells.

Detailed explanation-4: -Both internal and external regulators are proteins. The internal regulators are proteins inside of the cell while external regulators are proteins outside of the cell. Internal regulators will determine if the internal state of the cell is ready for cell division or not. If it is, then it allows division to proceed.