Not all cells adhere to the classic cell cycle pattern in which a newly formed daughter cell immediately enters the preparatory phases of interphase, closely followed by the mitotic phase. Cells in G0 phase are not actively preparing to divide. The cell is in a quiescent (inactive) stage that occurs when cells exit the cell cycle. Some cells enter G0 temporarily until an external signal triggers the onset of G1. Other cells that never or rarely divide, such as mature cardiac muscle and nerve cells, remain in G0 permanently.
Problem: How long does a cell spend in interphase compared to each stage of mitosis?
Background: A prepared microscope slide of blastula cross-sections will show cells arrested in various stages of the cell cycle. It is not visually possible to separate the stages of interphase from each other, but the mitotic stages are readily identifiable. If 100 cells are examined, the number of cells in each identifiable cell cycle stage will give an estimate of the time it takes for the cell to complete that stage.
Problem Statement: Given the events included in all of interphase and those that take place in each stage of mitosis, estimate the length of each stage based on a 24-hour cell cycle. Before proceeding, state your hypothesis.
Test your hypothesis: Test your hypothesis by doing the following:
- Place a fixed and stained microscope slide of whitefish blastula cross-sections under the scanning objective of a light microscope.
- Locate and focus on one of the sections using the scanning objective of your microscope. Notice that the section is a circle composed of dozens of closely packed individual cells.
- Switch to the low-power objective and refocus. With this objective, individual cells are visible.
- Switch to the high-power objective and slowly move the slide left to right, and up and down to view all the cells in the section (see the figure below). As you scan, you will notice that most of the cells are not undergoing mitosis but are in the interphase period of the cell cycle.
- Practice identifying the various stages of the cell cycle, using the drawings of the stages as a guide (see this figure from the previous lesson).
- Once you are confident about your identification, begin to record the stage of each cell you encounter as you scan left to right, and top to bottom across the blastula section.
- Keep a tally of your observations and stop when you reach 100 cells identified.
- The larger the sample size (total number of cells counted), the more accurate the results. If possible, gather and record group data prior to calculating percentages and making estimates.
Record your observations: Make a table similar to the table below in which you record your observations.
|Results of Cell Stage Identification|
|Phase or Stage||Individual Totals||Group Totals||Percent|
Analyze your data/report your results: To find the length of time whitefish blastula cells spend in each stage, multiply the percent (recorded as a decimal) by 24 hours. Make a table similar to the table below to illustrate your data.
|Estimate of Cell Stage Length|
|Phase or Stage||Percent (as Decimal)||Time in Hours|
Draw a conclusion: Did your results support your estimated times? Were any of the outcomes unexpected? If so, discuss which events in that stage might contribute to the calculated time.