Final answer:
Cell cycle progression in animals is regulated by proteins and signals like growth factors, cyclins, Cdks, and tumor suppressor genes, rather than by nutrient availability. These proteins oversee checkpoints and trigger or inhibit cell division. Incorrect regulation can lead to diseases like cancer.
Step-by-step explanation:
The passage through the cell cycle in animals is regulated primarily by signals for cell proliferation, rather than by the availability of nutrients. This regulation is through a complex interplay of growth factor proteins, cyclins, and cyclin-dependent kinases (Cdks), among other molecules. Growth factor proteins signal the cell to divide by binding to receptors on the cell's surface. Cyclins and Cdks are molecular signals that regulate the cell's transitions through various checkpoints within the cell cycle. Cyclins specifically help to advance the cell cycle to its next stage.
Conversely, a different class of genes known as tumor suppressor genes can send stop signals to halt the cell cycle if there are issues such as DNA damage or excessive cellular density, through a process known as contact inhibition.
Errors in cell cycle regulation can lead to cancer due to uncontrolled cell division. Moreover, signaling pathways include vital contributors to cell division, and are usually initiated by external agents called ligands or growth factors. When a cell divides, it is usually because it has received specific signals from other cells. However, cell division is also dependent on the cell's size and density, which can trigger division as a response to physiological inefficiency or to avoid overcrowding.