Final answer:
Positive cell cycle regulators, such as cyclins and Cdks, promote progression through cell cycle checkpoints, while negative regulators like Rb, p53, and p21 halt the cycle in response to issues like DNA damage to either allow repair or initiate cell death to prevent replication of flawed DNA.
Step-by-step explanation:
The roles of positive and negative cell cycle regulators are crucial for maintaining the balance of cell division. Positive regulators of the cell cycle include proteins like cyclins and Cdk (cyclin-dependent kinases), which help to advance the cell through the different phases by promoting progression at checkpoints. Their levels rise and fall predictably within the cycle, reacting to both internal and external signals, and are degraded once they have served their purpose to ensure the cycle progresses accurately.
Negative regulators, such as the retinoblastoma protein (Rb), p53, and p21, work to halt the cell cycle if there's an issue such as DNA damage. These proteins are involved primarily at the G1 checkpoint. A key feature of p53 is its role in arresting the cell cycle to allow DNA repair or, if needed, to initiate apoptosis to prevent damaged DNA from being replicated. Likewise, p21 supports p53 by inhibiting the activity of Cdk/cyclin complexes, further enforcing a stop in the cycle when required.
Importantly, negative regulators show a type of negative feedback in the cell cycle, in contrast to positive regulators which display positive feedback mechanisms. This balance ensures cells only divide when appropriate and that damages are remedied or cells are terminated if they cannot be repaired, which is critical in preventing uncontrolled cell proliferation that can lead to cancer.