A particular cell type spends 4 hours in G1 phase, 2 hours in S-phase, 2 hours in G2 phase, and 30 minutes in M-phase. If a pulse-chase experiment were performed with radioactive thymidine on an asynchronous culture of such cells, what percentage of mitotic cells would be radiolabeled 9 hours after the pulse?
a. 0 percent
b. 50 percent
c. 75 percent
d. 100 percent

Verified step by step guidance

1. Understand the problem: The question involves a pulse-chase experiment with radioactive thymidine, which labels DNA during the S phase. The goal is to determine the percentage of mitotic cells that would be radiolabeled 9 hours after the pulse. This requires understanding the cell cycle and the timing of each phase.
2. Calculate the total duration of the cell cycle: Add the durations of all phases (G1, S, G2, and M). The total cell cycle time is: \(4 \text{ hours (G1)} + 2 \text{ hours (S)} + 2 \text{ hours (G2)} + 0.5 \text{ hours (M)} = 8.5 \text{ hours}\).
3. Determine the time elapsed since the pulse relative to the cell cycle: After 9 hours, the cells have completed one full cycle (8.5 hours) and have entered the next cycle. Subtract the total cell cycle time from the elapsed time: \(9 \text{ hours} - 8.5 \text{ hours} = 0.5 \text{ hours}\). This means the cells are 0.5 hours into their second cycle.
4. Identify the phase of radiolabeled cells: Radiolabeled cells are those that were in the S phase during the pulse. These cells will progress through G2, M, and G1 before entering the next M phase. Calculate the time it takes for these cells to reach M phase: \(2 \text{ hours (G2)} + 0.5 \text{ hours (M)} + 4 \text{ hours (G1)} = 6.5 \text{ hours}\). Adding this to the 2 hours spent in S phase, radiolabeled cells will reach M phase \(2 + 6.5 = 8.5 \text{ hours}\) after the pulse.
5. Determine the percentage of mitotic cells that are radiolabeled: At 9 hours, the radiolabeled cells have already reached M phase, and the culture is asynchronous. Since the cell cycle is continuous and the timing aligns, all mitotic cells at this point will be radiolabeled. Thus, the percentage of radiolabeled mitotic cells is 100%.

Key Concepts

Cell Cycle Phases

The cell cycle consists of several phases: G1 (gap 1), S (synthesis), G2 (gap 2), and M (mitosis). Each phase has a specific duration and function, with G1 preparing the cell for DNA synthesis, S phase involving DNA replication, G2 preparing for mitosis, and M phase being the actual division of the cell. Understanding the timing of these phases is crucial for predicting how many cells will be in mitosis at a given time.

Pulse-Chase Experiment

A pulse-chase experiment involves exposing cells to a labeled compound (the pulse) for a short time, followed by a period without the label (the chase). This technique allows researchers to track the incorporation of the label into cellular components over time. In this context, radioactive thymidine is used to label cells in the S phase, and the timing of the chase helps determine how many cells have progressed to mitosis.

Asynchronous Cell Culture

An asynchronous cell culture contains cells at various stages of the cell cycle, rather than all being synchronized in the same phase. This diversity is important for understanding the overall behavior of a population of cells. In the context of the question, it means that at the time of the pulse-chase experiment, some cells will be in different phases, affecting the percentage of mitotic cells that become radiolabeled after a specific time.