Question

Rb is a negative regulator that blocks the cell cycle at the G1 checkpoint until the cell achieves a requisite size. What molecular mechanism does Rb employ to halt the cell cycle?

Short answer

It involves phosphorylation and dephosphorylation mechanism.

Step-by-step solution

Step 1. What is Rb?

Rb stands for Retinoblastoma protein. It is a tumor suppressor protein which is malfunctional in many major cancers. pRb prevent inadequate cell growth by hindering cell cycle progression until a cell is prepared to divide.

Step 2. Explanation to the answer

Rb is operated when it is dephosphorylated. In it's active stage it will bind to transcription factor i.e. E2F which is not only essential for transcription but also for translation of molecules for G1/S change.

E2F cannot decipher certain genes when it is bound to Rb.

As the size of the cell grows, it will lead to phosphorylation of Rb due to which it will get disabled and will free the E2F.

This E2F once released can aid transcription of gene which it controls and hence finally the Transition protein will be produced.