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. Step 1: Understand the problem. The question is asking for the percentage of mitotic cells that would be radiolabeled 9 hours after the pulse in a pulse-chase experiment. This experiment involves labeling cells with a radioactive marker (in this case, thymidine) and then following the marker through the cell cycle.
2. Step 2: Calculate the total cell cycle time. The cell spends 4 hours in G1 phase, 2 hours in S phase, 2 hours in G2 phase, and 30 minutes (or 0.5 hours) in M phase. So, the total cell cycle time is 4 + 2 + 2 + 0.5 = 8.5 hours.
3. Step 3: Determine the time when the cells are labeled. The cells are labeled with radioactive thymidine during the S phase, which is 4 hours into the cell cycle. Therefore, cells that were in the S phase at the time of the pulse will be in the M phase 4.5 hours later (2 hours for the rest of S phase, 2 hours for G2 phase, and 0.5 hours for M phase).
4. Step 4: Calculate the time after the pulse when the cells are in the M phase. The cells are in the M phase 4.5 hours after the pulse. Therefore, 9 hours after the pulse, two cell cycles have passed and the cells are again in the M phase. This means that all cells that were in the S phase at the time of the pulse and survived two cell cycles are now in the M phase and are radiolabeled.
5. Step 5: Answer the question. Therefore, 100 percent of the mitotic cells would be radiolabeled 9 hours after the pulse.

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 being the growth phase, S phase involving DNA replication, G2 preparing for mitosis, and M being the actual division of the cell. Understanding the timing of these phases is crucial for determining how long cells spend in each stage and how this affects the overall cycle.

Pulse-Chase Experiment

A pulse-chase experiment is a technique used to study the dynamics of cellular processes, such as DNA synthesis. In this method, cells are exposed to a labeled compound (the pulse) for a short time, followed by a period without the label (the chase). This allows researchers to track the incorporation of the label into cellular components over time, providing insights into the timing of processes like DNA replication and cell division.

Radiolabeling and Mitotic Cells

Radiolabeling refers to the incorporation of a radioactive isotope into a molecule, such as thymidine, which is used to label newly synthesized DNA. In the context of the cell cycle, only cells that have undergone DNA replication during the S phase will be radiolabeled. By calculating the time spent in each phase, one can determine the percentage of cells that are mitotic and have incorporated the label after a given time, which is essential for answering the question.