Week 9 — Counting Division: The Root Tip Mitotic Index Report — Traditional alternative (no AI)

Instructors: publish this instead of the adaptive version if you prefer, and unpublish or delete the adaptive one.

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 week you learned that the cell cycle is a loop — the G1, S, and G2 phases of interphase, then the M phase — and that mitosis runs prophase, metaphase, anaphase, telophase, in that order, to make two identical cells. In this assignment you work four short problems, interpret each one, and explain to someone who has never studied biology why a rooted cutting is a copy of its parent plant. This week's lecture tutorial, quiz, discussion, and Biology Lab & Inquiry practise the same ideas from other angles.

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.

Show your arithmetic. A reader should be able to reproduce every number you report from the counts in the problems without guessing which values or steps you used.

Deliverables

Submit a single report containing:

• the four answers, each with the arithmetic shown and the brief interpretation the task names for that problem;
• your message to your cousin, explaining in everyday words why a rooted cutting is a copy of its parent plant, with one limit; and
• a one-line restatement of the root tip counts you used, so your results are reproducible.

Keep it to roughly one page. Clear and correct beats long.

Choose how you show it. Present your report as written text, or as a recorded audio or video explanation of about three minutes 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. 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.

How you'll be graded

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

Academic integrity and AI use

This version is designed to be completed without AI: do the arithmetic, the interpretations, and the writing yourself, without a chatbot or other AI tool. You may use a calculator or a spreadsheet for the arithmetic. Submitting AI-generated work as your own, or copying another student's work, is an integrity violation under the course policy.

Built with The Course Maker · samples.thecoursemaker.com