Question

A) Describe what generally happens to a cell when it enters G0 phase. B) For a cell in G0, would you expect the pRB protein to be underphosphorylated or overphosphorylated? Explain your answer.

Answer 1

The phases of cell cycle are divided in to G0, G1,S,G2 and M .

During G0 phase the cells does not undergo replication. It is the inactivated stage of cell cycle. Cell cycle is arrested in this stage. When there is absence of energy rich molecule such as ATP cell will enter to G0 phase. In this stage cell does not die but they are metabolically active but do not undergo growth and differentiation . So the cells in this stage act as reserve cells.

pRB protein is unphosphorylated in G0 phase but will start getting phosphorylated when entered in to G1 phase and in the rest of the phases it remains hyper phosphorylated. Only the unphosphorylated state of pRB protein is active.Rb stands for retinoblastoma and in normal cells it blocks cell cycle progression unless it is phosphorylated. Hence it prevents unwanted growth and differentiation of cells and is a tumor suppressor protein. E2F is an essential factor which is important for the transcription of the genes which are required for the later stage of cell cycle such as G2 and M. In G0 phase pRb will specifically bind to E2F protein and inhibit it thus the progression of cell cycle will be blocked. During late G1 phase pRb will be phosphorylated by cyclin D and CDK4/6 complex which prevents the binding of pRb to E2F and now E2F becomes free and will transcribe genes that are required for cell division.