Question

A particular cell type spends 4 hours in \(\mathrm{G}_{1}\) phase, 2 hours in \(\mathrm{S}\) phase, 2 hours in \(\mathrm{G}_{2}\) 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

Short answer

The percentage of mitotic cells that would be radiolabeled 9 hours after the pulse in a pulse-chase experiment is 100% \((d)\), as all mitotic cells will be in the M phase and would be radiolabeled.

Step-by-step solution

Calculate the total cell cycle duration

Find the sum of the time spent in each phase: Total cell cycle duration = 4 hours (G1) + 2 hours (S) + 2 hours (G2) + 0.5 hours (M) = 8.5 hours

Calculate the period of the cell cycle after the pulse

Since the pulse-chase experiment was conducted 9 hours ago, we need to find how many cell cycles have occurred: Number of cell cycles = 9 hours / 8.5 hours per cell cycle ≈ 1.06 cell cycles

Determine if mitotic cells would be radiolabeled

Now, we have to determine which phase the cells would be in, 9 hours after the pulse: Remaining time in the current cell cycle = 9 hours - 1 complete cell cycle (8.5 hours) = 0.5 hours Since mitotic cells take 0.5 hours (30 minutes) to complete the M phase, the remaining time is sufficient to complete the mitosis process. Thus, these mitotic cells would be radiolabeled.

Calculate the percentage of mitotic cells radiolabeled

Since all mitotic cells would be radiolabeled in this scenario, the percentage of mitotic cells radiolabeled is 100%. Therefore, the correct answer is d. 100 percent.

Key concepts

G1 Phase

The G1 phase, or Gap 1 phase, is a period of cellular growth and preparation for DNA replication. During this stage, the cell increases in size, producing new proteins and organelles. It's a phase of metabolic changes that sets the foundation for the successful duplication of DNA.

Moreover, in G1, cells undergo a critical check at a point called the G1 checkpoint, which ensures that conditions are favorable for DNA synthesis. Any damage to DNA or unfavorable conditions can prevent the cell from progressing to the S phase, emphasizing the role of G1 in quality control.

Understanding the duration of the G1 phase is essential when studying the cell cycle, as it can vary greatly from cell to cell and can be indicative of cell function and type.

S Phase

The S phase, or Synthesis phase, is marked by the replication of DNA. Each chromosome's DNA is replicated, resulting in two sister chromatids that remain attached at a region called the centromere.

DNA replication is a complex process involving unwinding the DNA helix and using each strand as a template for the formation of a new complementary strand. This process ensures that each new cell will receive an identical copy of the genome. It's imperative for students to grasp that the integrity of the S phase is vital for genetic stability, and errors during this phase can lead to mutations.

G2 Phase

Following DNA replication, the cell enters the G2 phase, or Gap 2 phase. This phase is about further cell growth and the replication of organelles in preparation for cell division. Besides, during G2, the cell checks for any DNA damage and ensures all DNA has been accurately replicated.

Cells also produce proteins necessary for mitosis and cytokinesis during the G2 phase. The G2 checkpoint control mechanism ensures the cell is ready to enter the M phase, checking to confirm that DNA replication has completed successfully and that the cell size is adequate for division.

M Phase

The M phase, or Mitosis, is the climax of the cell cycle, where cell division occurs. This stage is sub-divided into several steps: prophase, metaphase, anaphase, and telophase, culminating in cytokinesis where one cell splits into two individual cells with identical DNA.

The M phase is critical as it ensures that each daughter cell inherits the correct number of chromosomes. Any errors during this phase, such as the incorrect distribution of chromosomes, can have significant impacts, including the development of cancer. Understanding the timing of the M phase, as seen in the exercise problem, helps in the analysis of cell division patterns and growth rates.