Question

Compare and contrast the roles of the positive cell-cycle regulators and negative regulators.

a) Positive regulators promote cell-cycle progression, while negative regulators inhibit it
b) Positive and negative regulators have identical functions in the cell cycle
c) Positive regulators inhibit cell-cycle progression, while negative regulators promote it
d) Positive regulators are only active during G1, while negative regulators function in all cell-cycle phases

Answer 1

Positive cell-cycle regulators like cyclins and Cdks promote cell cycle progression, whereas negative regulators such as Rb, p53, and p21 halt the cycle in response to issues like DNA damage.

Step-by-step explanation:

The roles of the positive cell-cycle regulators and negative regulators are crucial for the proper progression of the cell cycle. The positive regulators, such as cyclins and cyclin-dependent kinases (Cdks), are responsible for promoting the progress of the cell through the different phases of the cell cycle. They are activated by a combination of internal and external signals, leading to a rise in cyclin protein levels and enabling the cell to pass through the various checkpoints. Once the cell has moved to the next phase, these cyclins are then degraded.

In contrast, negative regulators act as a brake on the cell cycle. Proteins such as retinoblastoma protein (Rb), p53, and p21 are critical negative regulatory molecules that halt the cycle, especially if DNA damage is detected during the G1 checkpoint. p53 can initiate DNA repair through apoptosis if the damage is irreparable. Additionally, p21 enforces the halt in the cycle dictated by p53 by binding to and inhibiting the activity of Cdk/cyclin complexes.