Final answer:
The cell cycle in animals is primarily regulated by cyclins and cyclin-dependent kinases (Cdks), which act as positive signals for cell proliferation. External growth factors can also trigger these signals. Tumor suppressor proteins can negatively regulate the cycle to prevent division under certain conditions.
Step-by-step explanation:
Progress through the cell cycle in animals is regulated primarily by cyclins and cyclin-dependent kinases (Cdks) that signal cell proliferation. These molecules are critical for the cell to proceed through the various checkpoints in the cycle. Cyclins are proteins whose levels fluctuate throughout the cell cycle, increasing in concentration due to both external and internal signals before they are degraded once the cell moves on to the next stage. Together with Cdks, they act as positive regulators. On the other hand, negative regulation is mediated by tumor suppressor proteins, which can halt the cycle if conditions are unfavorable for division.
External signals that trigger cell division often come in the form of growth factors, which typically bind to cell-surface receptors leading to a cascade of signaling pathways that promote the cell cycle progress. Abnormalities in these regulatory mechanisms can lead to issues such as unregulated cell growth or cancer. Hence, the balance of these signaling molecules is crucial for normal cell division and growth.