Calculating a mitotic index. In whitefish embryos (blastulas, as in the above activity), cells go through the cell cycle in approximately 24 hours. Below is a photomicrograph of an embryo of whitefish. We will calculate the approximate time each cell spends in each phase by doing the following: a. Examine the photomicrograph and recored the stage of cell cycle (interphase, prophase, metaphase, anaphase, or telophase) in the table below. b. Calculate the mitotic index for each stage using the formula: Mitotic index = (# cells in this phase) / (total cells observed) c. To estimate the length of time the fish cells spend in each time, multiply the phase by 24 hours, which is the duration of one cell cycle. Table: Stage #of cells Mitotic index Duration Interphase Prophase Metaphase Anaphase Telophase Total # cells 1.00 24 hours
The Correct Answer and Explanation is :
To calculate the mitotic index and estimate the duration of each phase of the cell cycle in whitefish embryos, we need to follow a few clear steps:
Step-by-Step Process:
- Examine the Photomicrograph:
- First, look at the photomicrograph of the whitefish embryo (blastula). Count the number of cells in each phase of the cell cycle (interphase, prophase, metaphase, anaphase, telophase).
- Record the number of cells in each phase in the table.
- Calculate Mitotic Index:
- The mitotic index is the proportion of cells in the process of mitosis compared to the total number of cells observed.
- Formula:
[
\text{Mitotic Index} = \frac{\text{# cells in phase}}{\text{Total cells observed}}
] - You will repeat this calculation for each of the mitotic stages (prophase, metaphase, anaphase, and telophase), and calculate the mitotic index for each.
- Estimate the Duration of Each Phase:
- Multiply the mitotic index for each stage by 24 hours, since the entire cell cycle takes approximately 24 hours.
- This will give you an estimate of how much time is spent in each stage of the cycle.
Example Calculation:
Let’s say you observed the following number of cells:
- Interphase: 50 cells
- Prophase: 10 cells
- Metaphase: 5 cells
- Anaphase: 3 cells
- Telophase: 2 cells
- Total Cells: 70 cells
Step 1: Mitotic Index for Each Stage
- Interphase:
Mitotic Index = ( \frac{50}{70} = 0.71 ) - Prophase:
Mitotic Index = ( \frac{10}{70} = 0.14 ) - Metaphase:
Mitotic Index = ( \frac{5}{70} = 0.07 ) - Anaphase:
Mitotic Index = ( \frac{3}{70} = 0.04 ) - Telophase:
Mitotic Index = ( \frac{2}{70} = 0.03 )
Step 2: Duration in Each Phase
Since the cell cycle is 24 hours, we multiply each mitotic index by 24 hours:
- Interphase: ( 0.71 \times 24 = 17.04 ) hours
- Prophase: ( 0.14 \times 24 = 3.36 ) hours
- Metaphase: ( 0.07 \times 24 = 1.68 ) hours
- Anaphase: ( 0.04 \times 24 = 0.96 ) hours
- Telophase: ( 0.03 \times 24 = 0.72 ) hours
Conclusion:
By following these steps, you can determine the mitotic index for each stage of the cell cycle and estimate how much time cells spend in each phase. The longest period is typically spent in interphase, where cells are preparing for division. The phases of mitosis (prophase, metaphase, anaphase, telophase) usually account for a smaller portion of the total cell cycle time. This method of calculating the mitotic index and estimating phase duration helps in understanding the dynamics of cell division in organisms such as whitefish.