Virtual mitosis lab: count cells and calculate the mitotic index
Calculate mitotic index from classified cells, distinguish a snapshot proportion from a division rate, and plan a fair comparison in a virtual root-tip investigation.
Calculate the mitotic index by dividing the number of cells in mitosis by the total number of cells counted. Multiply by 100 if you report a percentage. Interphase cells belong in the denominator, even though they are outside mitosis. A snapshot proportion describes the observed population; it does not, by itself, measure how many divisions happen each minute.
Scivra's Mitosis & Cell Cycle investigation connects root-tip observations, stage classification, counts, and a model of the cell cycle. Use it to ask how evidence from many cells supports a statement about a population, rather than identifying one striking cell and treating it as representative.
Decide what counts before counting
Classify cells consistently as interphase, prophase, metaphase, anaphase, or telophase. The mitotic count includes the four mitotic stages. Keep the total count alongside those stage counts so you can reconstruct both the numerator and denominator.
Choose the root region and a sampling plan before looking for an effect. Different parts of the root contain different cell populations. A field chosen because it visibly contains many dividing cells does not provide the same comparison as fields chosen by a consistent plan. Record the location and sample identity as well as the classifications.
In the course's notebook, repeated observations of the same sampling window remain identifiable. They do not become independent new evidence simply because they were recorded again. Preserve those identities when combining observations across fields.
Turn classifications into a proportion
Add the prophase, metaphase, anaphase, and telophase counts to obtain the number in mitosis. Divide that number by all classified cells in the chosen sample, including interphase. Keep the fraction or decimal clearly distinguished from a percentage.
The course's ideal control reference at 25 degrees Celsius has a modeled total cycle of 809 minutes, with 137 minutes assigned to mitotic stages. Its theoretical mitotic proportion is therefore 137 divided by 809, approximately 0.169345, or 16.9%. Those are model reference durations and a theoretical proportion, not a requirement that every small field contains exactly that fraction of dividing cells.
A finite field is a sample. Compare its actual counts with the reference while acknowledging variation. Increasing the number of independent fields can make a population comparison more informative; recounting one field does not provide the same increase in information.
When can a snapshot suggest stage duration?
The model relates each stage's fraction of counted cells to that stage's fraction of the cycle. Multiplying a stage proportion by the control cycle duration gives a duration estimate under that assumption. This inference depends on how cells are distributed around the cycle; it is not a direct stopwatch measurement of a single cell.
The course explicitly notes that real growing tissue can contain more young cells and that sister cells can be neighbors in the same stage. Treat snapshot-based duration estimates as approximate, and keep the chosen reference conditions visible. A cycle animation can illustrate a sequence without establishing the real time spent at each stage.
Compare treatments with a fixed plan
For the lectin investigation, choose the number of fields before counting treated roots. Use matched sampling rules for the control and treatment groups, retain each field's identity, and complete the planned comparison once. Repeatedly adding fields until a statistical threshold is crossed changes the procedure you intended to evaluate.
The course's lectin scenario uses a shorter modeled interphase. It is a teaching scenario, rather than a measured universal effect of lectin on roots. Describe the observed difference within that scenario instead of generalizing its exact size to every preparation or organism.
If you use the course's contingency-table comparison, retain the underlying interphase and mitotic counts for both groups. A statistical summary is interpretable only with the sampling plan and counts that produced it.
Keep chromosome count separate from DNA amount
A cell can copy its DNA without immediately doubling its chromosome count under the centromere-based counting convention. Later, sister chromatids separate and are counted as individual chromosomes before cytokinesis distributes them into daughter cells. These distinctions help explain why a DNA-content reading and a chromosome count answer different questions.
Use the Inside view to connect that sequence with stage observations. Do not infer DNA amount from a cell's apparent size or assume interphase is an inactive waiting period.
Build an evidence-based conclusion
Report the root region, sampling plan, total cells, stage counts, mitotic index, and any conditions used for a duration estimate. State whether your claim concerns a snapshot proportion, an inferred duration, or a treatment comparison; those claims require different evidence.
Browse the AP Biology collection for related investigations, and compare how observations are collected in Cells Under the Microscope. Virtual classification and model practice support preparation and explanation, while required hands-on work and safety instruction remain separate responsibilities.