Week 9 — Lab / Scientific Inquiry · "Counting Mitosis: The Onion-Root-Tip Mitotic Index"

Course: Introduction to Biology — General Biology I (BIOL 101) · Silver Oak University (fictional sample) · Prof. Castellano
Objective: Objective 5 — identify the phases of the cell cycle in real cells, collect phase-count data, and compute a mitotic index · SLO A (scientific reasoning & data interpretation)
Worth 50 points · Labs group = 15% of the grade · Lab 9
Format: a free virtual microscope activity (no special equipment) — you'll count cells by phase in an onion root tip, build a data table, compute the mitotic index, and then catch the AI's mistakes when it interprets your results.

This is the course's signature weekly component. Every instructional week has one lab. This week's uses a free, browser-based virtual microscope. All lab resources are links to external sites — nothing to buy or download.

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Part 1 — The Big Picture

This week you learned that a cell spends most of its life in interphase (G1 → S → G2) and only briefly divides in the M phase (mitosis + cytokinesis). You also learned that biologists don't just describe division — they measure it with the mitotic index: the fraction of cells caught in the act of mitosis.

The classic place to see this is the tip of an onion root, which grows fast and is full of dividing cells. Slice it thin, stain it, and under a microscope you'll see cells frozen in every phase. By counting how many cells are in interphase versus each phase of mitosis, you can compute the mitotic index — a real number for how fast these cells are dividing.

Background (optional, ~8 min): Amoeba Sisters — "Mitosis: The Amazing Cell Process that Uses Division to Multiply!" (the PMAT phases): 🔗 https://www.youtube.com/watch?v=f-ldPgEfAHI

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Part 2 — Your Scientific Question & Hypothesis

The question: In a growing onion root tip, what fraction of cells are actively dividing (the mitotic index), and which phase of the cell cycle holds the most cells?

Before you start, write your hypothesis (an "if… then…" statement is perfect):

If most of a cell's life is spent in interphase, then when I count 100 root-tip cells, I expect __ (most / few) of them to be in interphase and the mitotic index to be _ (high / low / around %), because ______.

Write it down now — you'll compare it to your results at the end. (A "wrong" prediction is completely fine; science is about testing, not guessing right.)

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Part 3 — Materials & Procedure

You need (all free and online):
- A computer or phone with a browser. No microscope, slides, or onions required — the cells were photographed under a real light microscope and digitized for you.

The virtual activity (primary):
- University of Arizona — "Online Onion Root Tips" (The Biology Project): 🔗 https://biology.arizona.edu/cell_bio/activities/cell_cycle/cell_cycle.html
You'll examine stained onion-root-tip cells, classify each one into its phase of the cell cycle (interphase or one of P/M/A/T), and get feedback. Use it to practice identifying phases, then count a field of cells.

Procedure:
1. Open the virtual activity and read its short intro on how the cells were prepared.
2. Practice first: classify several cells until you can reliably tell interphase from prophase, metaphase, anaphase, and telophase. (Use the four-part question from class: condensing? lined up in the middle? splitting apart? two nuclei re-forming? If none of those — it's interphase.)
3. Count a field of 100 cells. Scan systematically (left to right, top to bottom) so you don't count a cell twice. For each cell, decide its phase and add a tally mark in your data table.
4. Stop when you have 100 cells tallied. (A larger count is more accurate; 100 makes the percentage math clean.)
5. Hold these the same (your controlled variables): the same image set / specimen, the same person classifying, the same phase criteria for every cell, the same total count (100).

No virtual activity loading? Any verified onion-root-tip or whitefish-blastula image set works the same way (e.g., the counting exercise in OpenStax Biology 2e §10.2, 🔗 https://openstax.org/books/biology-2e/pages/10-2-the-cell-cycle). Or use the model data table in Part 8 to practice the analysis — but counting real cells yourself is the point.

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Part 4 — Data Table (fill this in)

Count 100 cells and record how many fall in each phase. The phase counts must sum to 100.

Phase	Tally / count	Percent of 100
Interphase	______	______ %
Prophase	______	______ %
Metaphase	______	______ %
Anaphase	______	______ %
Telophase	______	______ %
Total	100	100 %

Now compute the mitotic index (show your arithmetic):
- Cells in mitosis = Prophase + Metaphase + Anaphase + Telophase = __
- Mitotic index = (cells in mitosis ÷ 100) × 100 = ____ %

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Part 5 — Identify Your Experiment's Parts

Answer in a sentence each:
1. What you measured (your data): __
2. The formula you used for the mitotic index: _
3. One controlled variable (kept the same for every cell you classified):
4. Why counting 100 cells is better than counting 5: ___

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Part 6 — Analysis Questions

1. Which phase held the most cells? Does that match what you learned about how cells spend their time? Explain.
2. What was your mitotic index? In plain words, what does that percentage mean about these cells?
3. Interphase and the 24-hour cycle: if these cells run a 24-hour cell cycle, the fraction of cells in a phase estimates the time spent there. Using the model data (80 of 100 in interphase), how many hours of a 24-hour cycle are spent in interphase? Show the arithmetic. (Hint: fraction × 24.)
4. Why is an onion root tip a good place to find dividing cells in the first place? (Think about where a root is growing.)
5. Connect it: a tumor has a high mitotic index. Based on this lab and this week's lecture, why would a doctor care about the mitotic index of a tissue sample, and what does a high value suggest?

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Part 7 — AI-Critique Moment (required — this is the BYOAI step)

Now bring in your approved chatbot (Gemini, Claude, or ChatGPT) and be the scientist who checks its work.

1. Paste your data table into the chatbot and ask it: "Here are my onion-root-tip phase counts. Calculate the mitotic index and tell me which phase has the most cells. Also, in mitosis, how many daughter cells form and is the chromosome number kept the same or halved?"
2. Check everything it says against your own work:
   - Did it calculate the mitotic index correctly? (Re-add your P+M+A+T cells and divide by your total yourself — watch for it dividing by the wrong number, or reporting the interphase percent as the mitotic index.)
   - Did it correctly say mitosis makes two identical cells with the same chromosome number — or did it confuse mitosis with meiosis (claiming four cells, gametes, or a halved chromosome number)? (Chatbots do this constantly.)
   - Did it put the phases in the right order (PMAT)?
3. Write 2–3 sentences reporting what the AI got right and at least one thing you had to correct or watch carefully. (If it happened to get everything right, say how you verified each claim — that's the skill.)

The habit all term: the tool drafts, you judge. A chatbot will confidently mis-compute a percentage or swap mitosis for meiosis — catching it is the point.

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Part 8 — What to Submit

Submit a single document (or text entry) with: your hypothesis, your completed data table with the mitotic index computed, your Part 5 answers, your Part 6 answers (including the 24-hour calculation), and your Part 7 AI-critique paragraph. Due Sunday, Nov 1, 11:59 p.m. (50 points).

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Instructor answer key & model data — REMOVE BEFORE PUBLISHING TO STUDENTS

Students count their own cells, so exact counts vary slightly. The model dataset below is the canonical one for grading the analysis and arithmetic; all numbers are pre-computed and independently verified (re-derived by the Week-9 Python quant-gate, which prints PASS).

Model data table (the canonical 100-cell count):

Phase	Count	Percent
Interphase	80	80%
Prophase	9	9%
Metaphase	4	4%
Anaphase	3	3%
Telophase	4	4%
Total	100	100%

• Cells in mitosis = 9 + 4 + 3 + 4 = 20. ✓
• Total cells = 80 + 20 = 100. ✓
• Mitotic index = (20 ÷ 100) × 100 = 20%. ✓
• Interphase time on a 24-h cycle = (80 ÷ 100) × 24 = 0.80 × 24 = 19.2 hours. ✓
• Mitosis time on a 24-h cycle = (20 ÷ 100) × 24 = 0.20 × 24 = 4.8 hours (19.2 + 4.8 = 24 ✓).

Expected answers:
- Part 4: cells in mitosis = 20; mitotic index = 20%.
- Part 5: (1) the number of cells in each phase (phase-count data); (2) mitotic index = (cells in mitosis ÷ total cells) × 100; (3) any one: same image set, same classifier, same phase criteria, same total of 100; (4) a larger sample reduces the effect of a single mis-classified or ambiguous cell and gives a more reliable, representative percentage.
- Part 6: (1) Interphase held the most (80) — yes, this matches that cells spend most of their lives in interphase. (2) Mitotic index = 20% → about 1 in 5 cells is actively dividing. (3) Interphase time = 0.80 × 24 = 19.2 hours of the 24-hour cycle (most of the day). (4) A root tip is a region of active growth, so cells there are constantly dividing — that's why so many are caught in mitosis. (5) A high mitotic index means cells are dividing rapidly; in a tissue sample it can flag fast-dividing (potentially cancerous) cells, which is one reason pathologists assess how much a sample is dividing.
- Part 7 (AI-critique): full credit for a specific catch — most commonly the AI confusing mitosis with meiosis (four cells / gametes / halved chromosome number), mis-computing the mitotic index (dividing by the wrong total or reporting the 80% interphase figure), or mis-ordering PMAT. Full credit also if the student verified each AI claim against their own arithmetic and reasoning.

Grading rubric — 50 points

Criterion	Full	Partial	None
Hypothesis — a clear, testable "if…then…" prediction with a reason (8)	8	4–6	0–2
Data table + mitotic index — phase counts sum to 100 and the mitotic index (20% in the model) computed correctly with arithmetic shown (15)	15	8–12	0–6
Variables & method (Part 5) — what was measured, the formula, a controlled variable, and why 100 cells (12)	12	6–10	0–4
Analysis (Part 6) — most-cells-in-interphase + meaning of the index + the 19.2-hour calculation + the tumor connection (10)	10	5–8	0–4
AI-critique (Part 7) — names a specific thing checked/corrected in the AI's interpretation (5)	5	3	0–2

Quality gate (self-checked): every number in the model dataset is pre-computed and independently re-verified — phase counts 80/9/4/3/4 sum to 100; cells in mitosis = 20; mitotic index = (20 ÷ 100) × 100 = 20%; interphase time = 0.80 × 24 = 19.2 h and mitosis = 4.8 h (sum 24). A tiny Python script in the scratchpad re-derived all of these and printed PASS — quantitative gate: PASS. The biology (mitosis makes two identical diploid cells; PMAT order; cells spend most of their time in interphase; high mitotic index = rapid division) is correct. No student-collected count is asserted as "the" answer — the key grades the analysis and the arithmetic method, not a specific tally.