Final answer:
The cell cycle is regulated by internal factors such as checkpoints and molecules like cyclins and Cdks, and external factors like hormones and cell size. Checkpoints at the end of G1, G2/M transition, and during metaphase ensure the cell is ready for the next phase, while factors like HGH influence cell division externally.
Step-by-step explanation:
The cell cycle is highly regulated by internal and external factors to ensure proper cell division and function. Internal factors include a system of checkpoints that monitor and coordinate the various stages of the cell cycle. External factors involve signals from outside the cell, such as hormones or nutrients, which can promote or inhibit cell cycle progression.
The cell cycle contains three major checkpoints: at the end of G1 (growth 1 phase), the G2/M transition (growth 2/mitosis transition), and during metaphase. The passage through these checkpoints is regulated by molecules such as cyclins and cyclin-dependent kinases (Cdks), which are part of the positive regulation, allowing the cell to proceed to the next phase. In contrast, negative regulation involves molecules such as tumor suppressor proteins, which can pause the cell cycle until certain conditions are met.
External triggers for cell cycle regulation include factors like cell size, the death of nearby cells, and hormones such as human growth hormone (HGH). These elements can induce or inhibit cell division based on the physiological needs of the organism. For instance, overcrowding of cells generally inhibits further cell division to prevent excessive growth.
Overall, these regulatory mechanisms ensure that cells divide only when necessary and ready, preventing errors such as those that can lead to cancer.