Materials & Prep
- Prepared onion root tip slides, microscopes, and normal classroom microscope supplies.
- Projected unlabeled cell-cycle image set and a board diagram of interphase, prophase, metaphase, anaphase, telophase, and cytokinesis.
- One half-sheet per student with the root-tip count table: interphase 80, prophase 8, metaphase 5, anaphase 4, telophase/cytokinesis 3.
- Exit-ticket slips.
Safety reminder: Confirm local procedures for carrying microscopes, handling slides, and cleaning lenses before students begin.
Opening
DisplayProject four recognizable onion-root-tip cells at different points in the cell cycle. Students silently label each with the phase they already know from prior diagram work, then record one visible observation for each, such as “chromosomes are lined up in the middle” or “two groups are moving apart.”
Ask“A cell starts as one cell and ends as two. What must happen to its genetic information so both new cells can function?” Students predict individually, then compare with a partner. Do not confirm answers yet.
ConnectPoint out that their observations reveal a sequence, not six unrelated pictures. The cell is solving one problem: it must copy its DNA, move one complete set to each side, and physically separate without leaving either new cell missing instructions. Introduce the question for the day: What job does each phase perform in that larger accomplishment?
Direct instruction
DefineCell division produces two genetically similar daughter cells for growth, repair, and replacement. Mitosis distributes duplicated chromosomes into two nuclei. Cytokinesis divides the cytoplasm to make two separate cells. Interphase is part of the cell cycle but is not mitosis.
ModelUse a model cell with 4 chromosomes. In G1, it has 4 chromosomes. During S phase, DNA is copied, so it still has 4 chromosomes, but each consists of two identical sister chromatids. Say: “A frequent error is saying it has 8 chromosomes after replication. It has twice the DNA, but the paired chromatids are still counted as 4 chromosomes until they separate.”
Think aloudTrace the cell’s job through the phases on the projected diagram.
- Interphase: The cell grows, performs normal functions, and copies DNA during S phase so there is a complete set to distribute.
- Prophase: Chromosomes condense so they can be moved accurately; the spindle begins forming and the nuclear envelope breaks down.
- Metaphase: Duplicated chromosomes line up at the cell’s middle. This positioning helps each future cell receive one copy of every chromosome.
- Anaphase: Sister chromatids separate and move to opposite poles. Each side is now receiving a complete chromosome set.
- Telophase: New nuclear envelopes form around each complete set, creating two nuclei.
- Cytokinesis: The cytoplasm divides, producing two separate daughter cells.
AnalogyTreat the chromosomes like identical sets of instructions being packed for two new workstations. Copying the instructions is not enough. The cell must sort one full set to each workstation before the room can be divided.
ConfrontReturn to the opening prediction. Ask, “Does mitosis make daughter cells with half the chromosomes?” Students commit with a thumbs choice, then revise using the 4-chromosome model: after anaphase, each pole has 4 chromosomes, and each daughter cell ends with 4. Half-sized chromosome sets belong to meiosis, not mitosis.
Guided practice
Worked exampleProject a cell that begins interphase with 6 chromosomes. Ask students to complete a phase-to-job organizer with the class. Think aloud: “After S phase, there are still 6 chromosomes, each duplicated. At metaphase, all 6 duplicated chromosomes line up. At anaphase, sister chromatids separate, so 6 chromosomes move toward each pole. After cytokinesis, each daughter cell has 6 chromosomes.”
Check for understandingShow three phase images in a new order. For each, students hold up a phase card or write the phase and complete the stem: “This phase matters because the cell is so that .” Sample expected response: “Anaphase matters because the cell is separating sister chromatids so that each side receives a complete set of chromosomes.”
ClarifyCorrect answers that only name visible features. Press students to connect the feature to the cell’s overall task of making two genetically similar cells.
ScaffoldKeep the phase-to-job organizer projected with icons: copy, condense, line up, pull apart, rebuild nuclei, split cell. Provide the speaking and writing frame: “In , the cell ; this helps ensure .”
Independent investigation and discussion
StudentsIn pairs, view an onion root tip under the microscope. Each partner identifies and sketches one cell in interphase and one cell in a mitotic phase, recording only what is observed before naming a phase. Students then use the supplied count table from a class root-tip sample.
AnnotateStudents distinguish observation from inference. Observation: “80 of 100 cells have uncondensed-looking nuclei.” Inference: “Cells spend more time in interphase than in visible mitotic stages.”
WriteIndividually, write a short claim-evidence-reasoning response: “Cell division is more than a cell splitting in two.” Require students to name at least four phases in sequence, explain each phase’s contribution, and cite the microscope observation or count table as evidence that cell-cycle stages have different visible features and durations.
CirculateAsk students who list phases without explaining purpose, “What problem would occur if this step were skipped?” Ask students who claim chromosomes disappear after division to trace the 6-chromosome model.
ExtensionStudents explain why a cell with chromosomes that fail to line up properly at metaphase could produce daughter cells with unequal genetic information. They must link the error to anaphase and the final daughter cells.
Closing
Exit ticketA body cell begins the cell cycle with 8 chromosomes. In 4–6 sentences, explain what the cell accomplishes by dividing and describe what happens in interphase, prophase/metaphase, anaphase, telophase, and cytokinesis. Include what chromosome set each daughter cell receives.
Look forStudents should state that DNA is copied before mitosis, duplicated chromosomes are organized and separated, nuclei reform, the cytoplasm divides, and each daughter cell receives 8 chromosomes. Sort tickets into secure, developing, and reteach groups based on whether students explain the purpose of phases rather than merely label them.