Open with the story of Henrietta Lacks and HeLa cells. In 1951, cervical cancer cells taken from Lacks became the first human cell line to divide indefinitely in culture — HeLa cells double roughly every 24 hours, and today more HeLa mass has been grown than her original body weight. Ask students: what would a cell have to do, in order, every 24 hours to make an identical copy of itself? Take 2-3 quick student ideas on the board (grow, copy DNA, split). Tell them that today they will (1) put those steps in the right order and (2) measure how fast a real tissue — an onion root tip — is dividing, using the same mitotic-index method pathologists use to grade tumors. This frames the cycle as ordered, measurable, and clinically meaningful.

1. 8m

   The cell cycle is mostly interphase

   Content

   The eukaryotic cell cycle has two big parts: interphase (G1 → S → G2) and the mitotic phase (M = mitosis + cytokinesis). In a typical 24-hour human cell cycle, roughly G1 ≈ 11 h, S ≈ 8 h, G2 ≈ 4 h, and M ≈ 1 h — so interphase takes about 23 of every 24 hours. Interphase is NOT rest: in G1 the cell grows and builds organelles and proteins; in S phase DNA is replicated so every chromosome becomes two sister chromatids joined at a centromere; in G2 the cell continues growing and synthesizes the tubulin and other proteins needed to build the mitotic spindle. Cells that permanently stop dividing (mature neurons, cardiac muscle) exit into G0 — a separate quiescent state, not a phase of the active cycle.

   Delivery

   The slide shows a cell-cycle wheel with M as a thin slice — use it to hammer the proportion point: if you sampled 100 random cells from a rapidly dividing tissue, you would expect only a handful to be caught in mitosis. Pre-empt the two biggest misconceptions here: (1) cells do NOT spend most of the cycle dividing, and (2) interphase is NOT resting. Ask: 'If the whole cycle is 24 h and M is 1 h, what fraction of cells at any instant should be in M?' (~1/24 ≈ 4%). Tell them to remember that number — it will match their onion data.

2. 7m

   Chromosome bookkeeping: G1 → S → anaphase

   Content

   Track ONE chromosome through the cycle to fix the counting rules. In G1, one chromosome = one chromatid = one DNA molecule. During S phase, DNA replication produces two identical sister chromatids joined at a centromere — this still counts as ONE chromosome (chromosome number is counted by centromeres, not by chromatids). In G2 and through metaphase, the cell carries 2n chromosomes but 4c DNA content. At anaphase, the centromeres split: each sister chromatid is now its own chromosome, and one full set is pulled to each pole. After cytokinesis, each daughter cell has the same 2n chromosome number and 2c DNA content as the original G1 cell. For a human somatic cell: G1 has 46 chromosomes / 46 chromatids; after S, 46 chromosomes / 92 chromatids; after anaphase and cytokinesis, each daughter is back to 46 / 46.

   Delivery

   The slide shows the three-snapshot chromosome cartoon (G1 single, post-S paired sisters, anaphase separated) with the running chromatid count beneath. Walk students through the numbers out loud — this is the single most common AP exam trap. Directly confront the misconception 'DNA is copied during mitosis': no — DNA is copied in S phase, so a cell entering prophase already has duplicated chromosomes. Quick cold-call: 'A human cell in G2 has how many chromosomes? How many chromatids?' (46 and 92). Then: 'Immediately after anaphase, how many chromosomes are at EACH pole?' (46).

3. 6m

   Mitosis, cytokinesis, and the mitotic index

   Content

   Mitosis is the ordered segregation of the already-replicated chromosomes into two identical nuclei. Prophase: chromatin condenses into visible chromosomes, nuclear envelope breaks down, spindle forms. Metaphase: chromosomes align at the metaphase plate, with sister kinetochores attached to opposite spindle poles. Anaphase: cohesin is cleaved, sister chromatids separate and are pulled poleward. Telophase: nuclear envelopes reform around each set, chromosomes decondense. Cytokinesis then physically splits the cytoplasm — an animal cell pinches with an actin contractile ring; a plant cell (like onion) builds a cell plate down the middle because the cell wall prevents pinching. The mitotic index is the operational measurement of how much of a tissue is actively dividing: MI = (number of cells in any phase of mitosis) ÷ (total cells counted). Because M occupies only a small fraction of the cycle, MI is small even in fast-growing tissues — a root tip typically runs MI ≈ 0.15-0.30; a mature leaf, near zero; an aggressive tumor, high.

   Delivery

   The slide shows the four mitotic phases side-by-side and the MI formula. Emphasize that mitosis produces two GENETICALLY IDENTICAL diploid daughters — this pre-empts the misconception that mitosis is where variation comes from (that's meiosis, unit 5). Set up the lab: 'You are about to be that pathologist. You'll count phases in an onion root tip and compute MI.' Give the working rule: any cell showing condensed, visible chromosomes counts as mitotic; any cell with a smooth intact nucleus counts as interphase.

1. 26m

   Investigation 7: Mitotic Index of Allium Root TipLab

   Group students in pairs at microscopes. Each pair gets one prepared Allium root-tip slide. They will scan to the meristematic zone (just behind the root cap — cells are small, square, and densely packed), count 100 cells across ~3 fields of view, classify each as interphase / prophase / metaphase / anaphase / telophase, and compute the mitotic index. They will then use their MI to estimate the duration of mitosis, assuming a 24 h cycle. Targets SP2 Visual Representations (identifying phases from micrographs), SP4 Representing and Describing Data (organizing counts in a table), SP5 Statistical Tests and Data Analysis (computing MI and inferring phase duration), and SP1 Concept Explanation (justifying what the MI means about the tissue). Teacher moves: (1) Model on the demo scope what a metaphase plate and an anaphase 'V' look like in onion — the two easiest phases to spot. (2) Walk the room and check that pairs are in the meristem, not the elongation zone (elongated cells = wrong zone). (3) Remind them: cells with a visible smooth nucleolus/intact nucleus and no condensed chromosomes = interphase. (4) After counting, have pairs write their MI on the board so the class can pool data for the Part 3 discussion. Student handout: Investigation 7 — Mitotic Index of Allium Root Tip Background. The tip of an onion root grows because a small region called the apical meristem divides rapidly. By counting how many cells are caught in mitosis in a random sample, you can estimate how fast the tissue is proliferating. This is the same logic pathologists use to grade tumors. Part 1 — Setup - Focus your prepared root-tip slide on ×10, then switch to ×40. - Scan up from the very tip. Skip the root cap (rounded, protective cells at the tip) and the elongation zone (long, rectangular cells further up). Stop in the meristem: small, roughly square, densely packed cells. - Choose your first field of view. Do NOT skip cells that look 'weird' — those are the ones in mitosis. Part 2 — Count 100 cells Classify every cell in your field. Move to a new field and continue until you reach 100 total. Use these ID rules: - Interphase: smooth, intact nucleus; no visible condensed chromosomes. - Prophase: nucleus contains visible thread-like condensed chromosomes; no clear equator alignment. - Metaphase: chromosomes lined up in a single row across the middle of the cell. - Anaphase: two separate groups of chromosomes pulling toward opposite ends (often a V-shape). - Telophase: two clusters of chromosomes at opposite ends, often with a cell plate forming between them. Data table - Interphase: ______ - Prophase: ______ - Metaphase: ______ - Anaphase: ______ - Telophase: ______ - Total counted: ______ (should = 100) - Cells in mitosis (P+M+A+T): ______ Part 3 — Calculate 1. Mitotic index: MI = (cells in mitosis) ÷ (total cells) = ______ 2. Assuming the full cell cycle in an onion root-tip cell lasts 24 hours, estimate the duration of mitosis: duration of M ≈ MI × 24 h = ______ hours 3. Which mitotic phase did you count the most of? ______ Why does that make sense in terms of how long that phase lasts relative to the others? ______ Part 4 — Reasoning (AP free-response style) - Claim: State whether your MI indicates this tissue is actively proliferating. - Evidence: Cite your MI value and your phase counts. - Reasoning: Explain why MI is a valid proxy for proliferation rate, and why it can never equal 1 in a normal healthy tissue. - Predict: A classmate counts a mature leaf epidermis instead of a root tip. Predict their MI and justify your prediction using what you know about G0 and the cell cycle.

   Materials

   • Prepared Allium (onion) root-tip slides, longitudinal section, one per pair
   • Compound light microscopes with ×10 and ×40 objectives
   • Handheld tally counters or pencil + paper
   • Calculators
   • Printed student handout (below), one per student

   Example outputs

   • Sample table: Interphase 82, Prophase 9, Metaphase 4, Anaphase 2, Telophase 3, Total 100, In mitosis 18. MI = 18/100 = 0.18. Estimated M duration ≈ 0.18 × 24 h ≈ 4.3 h. Most common mitotic phase = prophase, because prophase is the longest of the four mitotic subphases, so a random snapshot catches more cells in it.
   • Sample Part 4: 'MI = 0.18 shows the meristem is actively proliferating; 18% of cells were caught mid-division. MI cannot equal 1 because interphase (G1+S+G2) is far longer than M, so at any instant most cells must be in interphase. A mature leaf epidermis would have MI ≈ 0 because differentiated epidermal cells have exited into G0 and are no longer cycling.'

   No-equipment fallback

   If microscopes are unavailable, distribute a printed high-resolution onion root-tip micrograph with ~100 clearly visible cells and have pairs classify and count directly on the print with colored pencils, then complete Parts 3 and 4 unchanged.

1. A student examines a slide from a rapidly dividing tissue and counts: 168 interphase, 14 prophase, 6 metaphase, 4 anaphase, 8 telophase. Calculate the mitotic index and, assuming a 24-hour cycle, estimate the duration of mitosis in hours. Show your work. (Targets SP5 Statistical Tests and Data Analysis, SP4 Representing and Describing Data.)

   calculationTotal cells = 168 + 14 + 6 + 4 + 8 = 200. Cells in mitosis = 14 + 6 + 4 + 8 = 32. MI = 32 ÷ 200 = 0.16 (16%). Duration of M ≈ 0.16 × 24 h ≈ 3.84 h (~3.8 hours).

2. A human somatic cell (2n = 46) has just completed S phase. How many chromosomes and how many chromatids does it contain? Immediately after anaphase, how many chromosomes are moving toward EACH pole? (Targets SP1 Concept Explanation.)

   short answerAfter S phase: 46 chromosomes and 92 chromatids (each chromosome is now a pair of sister chromatids joined at a centromere). Immediately after anaphase, once centromeres split each chromatid is counted as its own chromosome, so 46 chromosomes are moving toward each pole.

3. Which statement best describes a cell in G2 phase? A) It is resting and metabolically inactive. B) It is actively replicating its DNA. C) It has already replicated its DNA and is synthesizing proteins needed for mitosis. D) Its sister chromatids have just been pulled to opposite poles. (Targets SP1 Concept Explanation.)

   multiple choiceC. In G2, DNA replication (which happened in S) is already complete; the cell is growing and making proteins such as tubulin needed to build the mitotic spindle. A is wrong because interphase is not rest; B describes S; D describes anaphase.

4. Two lab groups report mitotic indices from onion root tips: Group 1 MI = 0.22, Group 2 MI = 0.04. Group 2 argues their tissue is 'barely dividing.' Evaluate this claim — give one biological explanation and one methodological explanation for the low value. (Targets SP6 Argumentation, SP3 Questions and Methods.)

   short answerBiological: Group 2 may have sampled outside the apical meristem (e.g., the elongation or maturation zone), where cells have exited into G0 and are no longer cycling, so a genuinely low MI is expected there. Methodological: Group 2 may have misclassified early prophase cells (chromosomes not yet fully condensed) as interphase, undercounting the mitotic fraction. A valid comparison requires sampling the same zone and using consistent phase-identification criteria.

• ☐Prepared Allium (onion) root-tip slides, longitudinal section (1 per pair)
• ☐Compound light microscopes with ×10 and ×40 objectives
• ☐Immersion tissue / lens paper
• ☐Handheld tally counters or pencils and paper
• ☐Calculators
• ☐Printed student handout (Investigation 7)
• ☐Whiteboard space for pooled class MI values