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Discussion Starter · #1 ·
An external stimulus applied to a cell increases the activity of several enzymes, including dihydrofolate reductase and DNA polymerase. Which of the following immediately precedes the observed effect?


A. p27 protein up regulation
B. Rb protein phosphorylation
C. Caspase enzyme cleavage
D. GDP binding to Ras protein
E. Protein kinase A phosphorylation
 

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Wow - that's a good question! I chose D, then switched to B, then wondered about E...! It's one of those subjects that you know is probably important in lecture but forget it after the test... :notsure:

My best guess is B, although I think I would have chosen D if it had said GTP instead of GDP. Looking forward to the answer!
 

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I am guessing B, b/c the increase in enzymes involved in DNA synthesis so must have had to move from G1 to S phase, therefore Rb protein should be phosphorylated and therefore inactivated for DNA synthesis to occur.
 

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Is the answer D.. GDP- Ras --> MAP Kinase --> increase transcirption and prot synthesis. ??
Oh... I definitely got caught on that one.
I wasn't sure about the Rb phosphorylation but yes that has to be the answer, once it gets phos'lated it will release the binding transcription factor which will be free to stimulate the cell cycle and lead to division.
So ans is B, v good q!! I just hope now that the answer is actually b ....
 

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Discussion Starter · #7 ·
B) Retinoblastoma protein phosphorylation

That's right boys.... its B) Retinoblastoma protein phosphorylation.

Excellent! I'm impressed. :happy:

Let me give you the full explanation with a diagram,
World Happy Font Circle Art
click image to enlarge

An increase in the activity of enzymes responsible for DNA synthesis signifies that the cell is in the S phase of the cell cycle. The transition from GI to S phase is one of the two primary cell cycle checkpoints that ensure "quality control" of the DNA. Cells with damaged DNA are not allowed to enter the S phase.

Retinoblastoma (Rb) protein is a regulator of the G1 -.S phase transition. The Rb protein is present in one of two forms: active (hypo phosphorylated) or inactive (hyper phosphorylated). The resting cells in the G0 phase contain active (hypo phosphorylated) Rb protein.

Font Circle Auto part Diagram Illustration
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1. When the cell is stimulated by growth factor, activation of cyclin D, cyclin E and the corresponding cyclin kinases (CDK 4 and 6) occurs and the Rb protein is hyper phosphorylated (rendering it inactive). Hyper phosphorylated Rb releases E2F transcription factor, which allows the cell to progress through the G1 -.S checkpoint.

Facial expression Organism Font Happy Sharing
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2. If the Rb protein is hypo phosphorylated (active), it binds to the E2F transcription factor. This complex inhibits the transcription of genes necessary for G1 -S transition. Active Rb protein, therefore serves as a "brake" that stops the cell from dividing.
 
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