A researcher studying the cell cycle observes a sudden increase in the activity of several enzymes, including dihydrofolate reductase and DNA polymerase. Which of the following processes most immediately precedes the observed increase in enzyme activity?

Question

A researcher studying the cell cycle observes a sudden increase in the activity of several enzymes, including dihydrofolate reductase and DNA polymerase.  Which of the following processes most immediately precedes the observed increase in enzyme activity?

Quizplus · Accepted Answer

11ec160b_39f1_28f4_9249_e931e15e5af1_MD0001_00 An increase in the activity of enzymes responsible for DNA synthesis (eg, dihydrofolate reductase, DNA polymerase) signifies that the cell is in the S phase of the cell cycle. Immediately prior to the S phase is the G1/S checkpoint, in which the cell ensures DNA integrity. Cells with damaged DNA are not allowed to enter the S phase to limit propagation of mutations. Retinoblastoma (Rb) protein is a regulator of the G1/S phase transition. The Rb protein has two forms, active (dephosphorylated) or inactive (phosphorylated). When the cell is stimulated by growth factors, activation of cyclin D, cyclin E, and the corresponding cyclin kinases (CDK 4 and 2) occurs and the Rb protein is phosphorylated (rendering it inactive). Phosphorylated Rb releases the E2F transcription factor, which allows progression through the G1/S checkpoint. After the cell divides, Rb protein is dephosphorylated (active) and remains so until the cell is ready to enter S phase again. In terminally differentiated cells, the Rb protein stays in the dephosphorylated (active) state bound to E2F transcription factors, resulting in inhibited transcription of genes necessary for G1/S transition. Therefore, active Rb protein stops the cell from dividing and allows the cell to enter a quiescent (G0) phase. (Choice A) Caspases are cysteine proteases, enzymes essential for apoptosis. Caspases are not associated with the G1/S transition in the cell cycle. (Choice B) Ras protein is a component of the MAP-kinase signal transduction system. It transmits growth signals from cell surface receptors into the nucleus, leading to the transcription of cyclin D complexes. Although involved in cell cycle progression, Ras protein signaling would occur before phosphorylation of RB protein and would not immediately precede the S phase. (Choice C) The p27 protein is a cell cycle inhibitor. It acts during the G1 phase by inhibiting cyclin-dependent kinases. Normal tissues (composed primarily of differentiated cells) contain high levels of p27, and malignant tissues contain very low levels of p27. Upregulation of p27 would stop the cell cycle. (Choice D) Protein kinase A is a component of the cAMP signaling system. It is not associated with the G1/S transition in the cell cycle. Educational objective: Dihydrofolate reductase and DNA polymerase are enzymes involved in DNA synthesis, which occurs during the S phase of the cell cycle. The Rb protein, in its active (dephosphorylated) form, regulates cell cycle progression by preventing the transition from the G1 phase to the S phase. Phosphorylation of the Rb protein inactivates it, allowing cells to progress through the G1/S checkpoint and proliferate.

A Researcher Studying the Cell Cycle Observes a Sudden Increase