Week 9 — Counting Division: The Root Tip Mitotic Index Report

Assignment 8 of the term — count the dividing cells in a root tip, put a number on how fast the tissue divides, and explain to a cousin why a rooted cutting is a copy of its parent plant.

This adaptive assignment uses a coaching dialogue to help you turn a first attempt into a clear, evidence-based report on this week's cell division. You remain the author. The coach asks questions, checks the reasoning you show, and waits for you to revise before it produces the required report. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles; this assignment is where you put the root tip count, the mitotic index, the hours on the cycle, and the chromosome count together.

The task

The onion root tip example is an illustrative dataset constructed for this course (not real data). Of 100 cells viewed under a microscope at the tip of an onion root:

• 80 are in interphase;
• 9 are in prophase;
• 4 are in metaphase;
• 3 are in anaphase;
• 4 are in telophase.

The mitotic index is the percent of counted cells that are in mitosis: cells in mitosis ÷ total cells counted × 100. A human body cell has 46 chromosomes.

Work these four problems, showing your arithmetic:

1. In the onion root tip example (illustrative), 9 cells are in prophase, 4 in metaphase, 3 in anaphase, and 4 in telophase. How many cells are in mitosis?
2. In the onion root tip example (illustrative), 100 cells were counted and 9 + 4 + 3 + 4 of them are in the four phases of mitosis. What is the mitotic index, in percent?
3. In the onion root tip example (illustrative), 20 of the 100 cells counted are in mitosis. On a 24-hour cell cycle, about how many hours does mitosis take?
4. A human body cell has 46 chromosomes. After the S phase, each chromosome is made of 2 sister chromatids. How many chromatids does the cell have after the S phase?

Beside each answer, add one brief interpretation:

• Problem 1: name the four phases of mitosis in order with one event for each, and explain why the interphase cells are left out and when the DNA is copied.
• Problem 2: say in words what the index means for this root tip, and explain what belongs on the bottom of the division, so that neither the interphase count nor the interphase share sneaks in.
• Problem 3: explain why the share of cells caught in a stage can estimate the share of time a cell spends there, and state how many hours interphase takes and what that says about a cell's life.
• Problem 4: explain how one X-shaped chromosome is counted, what happens to the sister chromatids in anaphase, and how many chromosomes each daughter cell receives, and contrast the two identical cells of mitosis with the four unique cells of meiosis.

Explain it to a non-expert

Your cousin, who has started working at a garden center, asks, "When I root a cutting from a houseplant, is the new plant a copy of the old one? How does one snipped stem turn into a whole plant?" Write a 3–5 sentence message that explains, in everyday words, how cell division builds the new plant and why it carries the same instructions as the parent, and names one limit, such as that the root tip numbers in your report are illustrative rather than measured, or that your chromosome count is for a human cell, not a plant. Explain any technical term in everyday words. More than one good answer is possible.

For a different, ungraded example, imagine a friend asking why lettuce goes limp a few minutes after it is salted. Model answer: "A lettuce leaf is made of tiny cells full of water. When you sprinkle salt on it, the outside of each cell ends up with more dissolved stuff than the inside. Water moves across the cell's border toward the saltier side, so it leaves the cells, and the leaf wilts. This explains the wilting, but it does not tell you how much salt it takes, which depends on the leaf and how long the salt sits."

Use that pattern for your cousin's question, with your own wording and evidence.

Part 1 — Student instructions

1. Draft all four answers, the four brief interpretations, and your message to your cousin before opening a chatbot.
2. Open any course-approved chatbot. Copy the entire Coach Prompt box, including its first and last marker lines, and paste it as one message. The coach will ask for your draft and then work one question at a time.
3. Answer in your own words. Show calculations when asked. Use the coach's feedback to revise at least one calculation or explanation, even if your first attempt was correct.
4. Continue until the prompt's exit condition is met.
5. Save the chat's share link and copy the final report exactly as the coach gives it.

The chatbot is a coach, not a ghostwriter. It must draw out your reasoning rather than write the report for you.

Coach Prompt

COPY EVERYTHING BELOW THIS LINE
ROLE: You are a careful learning coach for an introductory biology course. Draw out and challenge MY thinking about this week's cell cycle and mitosis. Do not write my assignment for me, do not invent work I did not show, and never invent a measurement, a study, or a percentage.

ASSIGNMENT: Work through these four problems in order, one at a time. Do not invent, replace, or reorder a problem. The data: the onion root tip example, an illustrative dataset constructed for this course (not real data). Of 100 cells counted: interphase 80, prophase 9, metaphase 4, anaphase 3, telophase 4. The mitotic index is (cells in mitosis / total cells counted) x 100. A human body cell has 46 chromosomes.
PROBLEM P1: In the onion root tip example (illustrative), 9 cells are in prophase, 4 in metaphase, 3 in anaphase, and 4 in telophase. How many cells are in mitosis?
PROBLEM P2: In the onion root tip example (illustrative), 100 cells were counted and 9 + 4 + 3 + 4 of them are in the four phases of mitosis. What is the mitotic index, in percent?
PROBLEM P3: In the onion root tip example (illustrative), 20 of the 100 cells counted are in mitosis. On a 24-hour cell cycle, about how many hours does mitosis take?
PROBLEM P4: A human body cell has 46 chromosomes. After the S phase, each chromosome is made of 2 sister chromatids. How many chromatids does the cell have after the S phase?
After the four answers, I must briefly interpret each one: for P1, the four phases of mitosis in order with one event for each, plus why the interphase cells are left out and when the DNA is copied; for P2, what the index means for this root tip in words, plus what belongs on the bottom of the division so that neither the interphase count nor the interphase share sneaks in; for P3, why the share of cells caught in a stage can estimate the share of time in that stage, plus how many hours interphase takes and what that says about a cell's life; for P4, how one X-shaped chromosome is counted, what happens to the sister chromatids in anaphase, how many chromosomes each daughter cell receives, and the contrast between the two identical cells of mitosis and the four unique cells of meiosis. I must then write a 3-5 sentence message to my cousin who works at a garden center, explaining in everyday words how cell division builds a rooted cutting into a new plant with the same instructions as its parent, and naming one limit. I must explain technical terms in everyday words.

SCORING CRITERIA: Score only evidence I supply: correct computations (40 points), biological reasoning (32 points), clarity for a non-expert reader (16 points), and showing work so it can be checked (12 points). For each of P1-P4, give 2, 1, or 0 evidence units per criterion; one unit is worth 5 computation points, 4 reasoning points, 2 clarity points, or 1.5 showing-work points. Sum all four problems within each criterion. Count my message to my cousin only with P4, not again with P1-P3.
COMPUTATION UNITS: A correct mitosis count, mitotic index, mitosis time, or chromatid count = 2; the right values and operation with only an arithmetic slip = 1; a missing result or the wrong values or operation = 0. Do not penalize an alternative valid method, such as converting the index to a fraction of the day first, or equivalent correct notation.
REASONING UNITS: For P1-P3 separately, an interpretation that meets both parts named for that problem = 2; exactly one part = 1; neither = 0. For P4, BOTH an accurate explanation of the chromosome count, the separation of the sister chromatids, the daughter cells' chromosome number, and the mitosis-versus-meiosis contrast AND a cousin message that explains the identical new plant in everyday words with a named limit = 2. If full P4 evidence is absent but at least one accurate substantive part appears, give 1. No accurate reasoning = 0; saying the DNA is copied during mitosis, counting an X as two chromosomes, saying mitosis makes four cells, putting the phases out of order, or reporting the interphase share as the index is not accurate. An arithmetic slip alone is not a reasoning error.
CLARITY UNITS: For each problem, labeled evidence understandable to someone new to biology, with technical terms explained = 2; intelligible evidence that needs rereading because of ambiguity or jargon = 1; absent or unintelligible evidence = 0. Include the cousin message with P4 if submitted. Missing reasoning alone does not lower clarity for otherwise clear submitted work.
SHOWING-WORK UNITS: For each problem, checkable values and steps = 2; a relevant but incomplete value or step = 1; a result alone or no method = 0. A visible method stays checkable despite an arithmetic slip.

Do NOT invent grading rules.

DIALOGUE RULES: Ask exactly one question per message and wait for my answer. Start by asking for my complete first attempt. Build each next question from my exact words. Probe an unsupported claim and ask me to examine its evidence. Keep the conversation focused on questions rather than completed work.

ENGAGEMENT GUARDS: A greeting, copied prompt, yes/no reply, or one-word answer is not a substantive exchange. Do not flatter or accept agreement as evidence. Praise only a concrete move I made. If evidence is missing, say what is missing and ask for it. Require a real revision plus a short reflection on why the revision is better. Do not issue the report early.

EXIT CONDITION: Continue until at least five substantive exchanges have occurred AND I have supplied all four answers and their brief interpretations AND written my message to my cousin AND revised at least one calculation or explanation AND reflected on the revision. Stop only after whichever requirement happens last.

REPORT: After the exit condition is satisfied, output exactly these labelled lines in this order. Replace X with the exact earned score; it may end in .5 and must not be rounded to a whole number. Base every line only on evidence in this chat; write "Not supplied" for missing evidence and never raise the score for polished chatbot prose.
STUDENT'S SCORE: X/100
EVIDENCE FOR THE SCORE: [specific evidence from my calculations and reasoning]
REVISION MADE: [what I changed and why]
STRENGTH: [one earned strength]
NEXT STEP: [one concrete improvement]
Then tell me to submit this report, my revised cell division report, and the share link.
COPY EVERYTHING ABOVE THIS LINE

Deliverables

Submit these three items in the assignment entry:

• your revised cell division report with all four answers, their brief interpretations, and your message to your cousin — written, or recorded as audio or video with a transcript, or as a labelled sketch of the four phases of mitosis with a short note; every form is graded by the same scoring criteria;
• the coach's five-line report; and
• the conversation share link.

Submit in any accessible file type or as a link; a recorded response may be submitted as audio or video with a transcript, or as text.

Equivalent route (no AI required)

This route assesses the same Week 9 skills — counting the cells in mitosis and naming the phases in order, computing and reading a mitotic index, turning a share of cells into hours on the cell cycle, and counting chromosomes and chromatids to explain what mitosis produces — for comparable time and effort, about 60 to 90 minutes, the same as the coached route. Complete the same four problems, the four brief interpretations, and the message to your cousin. Then audit your own first draft against the attached scoring criteria, criterion by criterion, and write a second draft. Add a 150–200 word revision memo that names one calculation or explanation you changed, the evidence that prompted the change, and one question you still have. Submit the revised cell division report, your completed criteria checklist, and the revision memo in the assignment entry; together they replace the share link and the coach's report. This route uses the same scoring criteria and the same points, with no penalty for choosing it.

How you'll be graded

The full analytic rubric is attached to this assignment in Canvas.

Academic integrity and AI use

The calculations, reasoning, revision, and final report must be your own. Do not ask the chatbot to create the assignment or add evidence you did not produce. Submit an honest share link from your own dialogue. The equivalent route keeps the same workload and evidence and requires no AI account.

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