The retinoblastoma protein, prb, is a tumor-suppressor protein that controls the g1/s cell-cycle checkpoint. while prb is present in all phases of the cell cycle, regulation of its activity is important in the progression from g1 phase to s phase. sort each statement depending on whether it is is true of cells that remain at the g1 checkpoint or of cells that progress from g1 to s phase.1. prb phosphorylated 2. prb not phosphorylated 3. prb bound to e2f 4. prb not bound to e2f 5. e2f bound to dna 6. cdk4 bound to cyclin d1

1. pRB phosphorylated: The cell will not remain in G1 phase i.e. it will enter S phase because pRB will be degraded.

2. pRB not phosphorylated: The cell will remain in G1 phase i.e. will not enter S phase because pRB will remain bound to E2F.

3. pRB bound to E2F: The cell will remain in G1 phase i.e. will not enter S phase.

4. pRB not bound to E2F: The cell will not remain in G1 phase i.e. it will enter S phase.

5. E2F bound to DNA: The cell will not remain in G1 phase i.e. it will enter S phase.

6. CDK4 bound to Cyclin D1:  The cell will not remain in G1 phase i.e. it will enter S phase.

Explanation:

Retinoblastoma protein, pRB supresses tumor by regulating cell cycle progression in G1 phase. When a transcription factor known as E2F is bound to pRB the cell remains in G1 phase and did not proceed further to enter S phase. But as soon as pRB becomes phosphorylated, E2F becomes free and causes gene expression of cyclin E and and cyclin A which are required in progression of cell cycle from G1 phase to S phase.

Signaling involved in cell cycle progression from G1 phase to S phase.

When a cell is unstimulated, pRB is bound to E2F but G1 phase specific growth factors induce the expression of cyclin D through Ras-MAP kinase pathway.

Synthesis of cyclin D leads to the activation of CDK-4 which is a serine-threonine kinase. This kinase causes phosphorylation of pRB which results in the degradation of pRB.

Because of degradation of pRB, E2F becomes free and leads to the expression of cyclin E and and cyclin A which are S phase specific cyclins. This is how cell progresses from G1 phase to S phase.

E2F is a transcription factor which causes expression of S phase specific cyclins i.e. cyclin E and and cyclin A. So, E2F bound to DNA will result in cell cycle progression from G1 phase to S phase.