Question

Which of the following are positive regulators of cell cycle?

• A. Cyclin and cyclin dependent kinases
• B. Retinoblastoma protein
• C. P53
• D. P21

Hint:

• Positive cell cycle regulators promote progression (e.g., Cyclins, CDKs).
• Negative cell cycle regulators inhibit progression (e.g., Rb, p53, p21, p16).
• Many negative regulators are tumor suppressor proteins.

Answer 1

The Correct Option is A

The cell cycle is controlled by a complex network of regulatory proteins.

• Positive Regulators: These proteins promote the progression of the cell cycle.
  • Cyclins and Cyclin-Dependent Kinases (CDKs) (Option a): Cyclins are proteins whose concentrations fluctuate throughout the cell cycle. They bind to and activate CDKs. The cyclin-CDK complexes then phosphorylate target proteins, driving the cell cycle through its various phases (G1, S, G2, M). They are key positive regulators.
• Negative Regulators (Tumor Suppressors/Cell Cycle Inhibitors): These proteins halt or slow down cell cycle progression, often in response to DNA damage or unfavorable conditions.
  • Retinoblastoma protein (Rb) (Option b): Rb is a tumor suppressor protein. In its active (hypophosphorylated) state, it binds to E2F transcription factors, preventing them from activating genes required for S phase entry. Phosphorylation of Rb by cyclin-CDK complexes inactivates it, releasing E2F and allowing cell cycle progression. So, active Rb is a negative regulator.
  • P53 (Option c): P53 is a crucial tumor suppressor protein, often called the "guardian of the genome." It is activated in response to DNA damage or cellular stress. Activated p53 can halt the cell cycle (e.g., by inducing p21) to allow for DNA repair, or it can trigger apoptosis (programmed cell death) if the damage is too severe. It is a negative regulator.
  • P21 (Option d): P21 (also known as WAF1/CIP1) is a CDK inhibitor (CKI). Its expression is often induced by p53. P21 binds to and inhibits the activity of cyclin-CDK complexes, thereby arresting the cell cycle, typically at the G1/S checkpoint or G2/M checkpoint. It is a negative regulator.

Therefore, cyclins and cyclin-dependent kinases are the positive regulators among the given options. \[ \boxed{\text{Cyclin and cyclin dependent kinases}} \]