B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living ThingsMitosis Under the Microscope: Finding and Timing the Stages
B2.2 B2.1 A1.2 A1.6 A1.8 A1.10 A1.13About 75 minutes
Learning goals- I can explain why onion root tips and whitefish embryos are used to observe mitosis.
- I can identify interphase and each stage of mitosis in plant and animal cells from their visible features.
- I can organize cell counts in a table and graph, and use them to estimate how long each stage lasts.
- I can identify sources of error in a cell-counting investigation.
Words to know
meristem
A region of a plant where cells divide by mitosis, such as the tip of a root or shoot.
root cap
A layer of cells that protects the growing tip of a root as it pushes through soil.
blastula
A very early animal embryo: a ball of cells that are dividing quickly.
prepared slide
A permanent slide made in a lab, with a thin, stained slice of a specimen.
mitotic index
The percentage of cells in a sample that are in mitosis (not interphase).
qualitative / quantitative data
Qualitative data are descriptions ("chromosomes lined up"); quantitative data are numbers ("8 cells").
sample size
The number of observations (here, cells) used to draw a conclusion. Larger samples give more reliable results.
Catching cells in the act
Mitosis is happening in your body right now, but you can't watch it. Cells divide inside you, out of sight, and most of the time a cell is in interphase. So how do biologists study it? They look at tissues where cells divide a lot, take a thin slice, stain it, and "freeze" thousands of cells at whatever stage they happened to be in. It's a bit like a photo of a busy hallway between classes: everyone is frozen mid-step, and by counting how many people are at each spot, you can work out where people spend the most time.
The two classic specimens
Plant: the onion root tip. Roots grow at their tips. Just behind the protective root cap is the meristem, a zone where cells divide constantly. Farther back, cells stop dividing and elongate (stretch), then mature into specialized root cells, such as root hair cells. On a prepared slide, the meristem is where you find many cells in mitosis.
Animal: the whitefish blastula. A fish embryo a few hours or days old is a ball of cells dividing very quickly to build a new organism. Slices of whitefish blastulas show many cells in mitosis. Because they are animal cells, you can see round cells, no cell walls, a cleavage furrow in cytokinesis and, at high power, star-shaped spindles at each pole (around the centrioles).
How to find the stages (lab procedure)
- Carry the microscope with two hands. Start on the lowest power objective and use the coarse focus.
- Find the root tip, then move the slide so that the meristem (just behind the cap) is in the centre.
- Switch to medium, then high power (400×). Use only the fine focus on high power.
- Scan slowly. For each cell, decide: are the chromosomes visible? Where are they?
Clues for each stage
Use the table below to decide which stage each cell is in.
| Stage | Key things to look for under the microscope |
|---|
| Interphase | A clear, round nucleus with a membrane; often a dark nucleolus; no separate chromosomes |
| Prophase | Chromosomes visible as thick threads inside the nucleus area; nuclear membrane fading |
| Metaphase | Chromosomes in a single band across the middle of the cell |
| Anaphase | Two groups of chromosomes moving apart, often V-shaped, toward opposite ends |
| Telophase | Two groups at opposite ends; new nuclei forming; a cell plate (plant) or a furrow (animal) between them |
From counts to time: an investigation
Question: Which stage of the cell cycle takes the longest?
Hypothesis: If interphase is the longest stage, then most cells in a root tip will be in interphase, because a cell spends most of its time there and is most likely to be "caught" in it.
Method: count cells in each stage in several fields of view in the meristem. Record the counts in a table. Add class data to make the sample size bigger.
Key idea: the fraction of cells in a stage is about equal to the fraction of time a cell spends in that stage. So:
time in a stage ≈ (cells in that stage ÷ total cells counted) × total time of one cell cycle
This works when the cells are dividing independently and continuously, as in a meristem.
Worked exampleLet's estimate how long each stage lasts
Using the counts in the graph (200 cells in total) and assuming one cell cycle in an onion root tip takes about 24 hours, estimate the time spent in each stage.
- Find each fraction of the total: interphase 160 ÷ 200 = 0.80; prophase 20 ÷ 200 = 0.10; metaphase 8 ÷ 200 = 0.04; anaphase 4 ÷ 200 = 0.02; telophase 8 ÷ 200 = 0.04. (Check: 0.80 + 0.10 + 0.04 + 0.02 + 0.04 = 1.00.)
- Multiply each fraction by 24 h: interphase 0.80 × 24 = 19.2 h; prophase 0.10 × 24 = 2.4 h; metaphase 0.04 × 24 = 0.96 h; anaphase 0.02 × 24 = 0.48 h; telophase 0.04 × 24 = 0.96 h.
- Change small values to minutes: 0.96 h × 60 ≈ 58 min; 0.48 h × 60 ≈ 29 min.
- Round sensibly: the counts were only 200 cells, so give about two significant figures: interphase ≈ 19 h, prophase ≈ 2.4 h, metaphase ≈ 58 min (about 1 h), anaphase ≈ 29 min (about 0.5 h), telophase ≈ 58 min.
Interphase ≈ 19 h, prophase ≈ 2.4 h, metaphase ≈ 1 h, anaphase ≈ 0.5 h, telophase ≈ 1 h. Interphase is by far the longest; anaphase is the shortest.
Worked exampleLet's find the mitotic index
In the same sample, what percentage of cells are in mitosis? What would it mean if a different sample from the same plant had a much higher mitotic index?
- Cells in mitosis = prophase + metaphase + anaphase + telophase = 20 + 8 + 4 + 8 = 40.
- Mitotic index = 40 ÷ 200 × 100% = 20%.
- A higher mitotic index means a larger share of cells are dividing, so that tissue is growing faster (for example, a sample taken right from the meristem rather than from the elongation zone).
The mitotic index is 20%. A higher index means faster cell division in that tissue.
Analysing the evidence: sources of error
A good scientist asks, How much can I trust these numbers?
- Classification errors: early prophase looks a lot like interphase, and late anaphase looks like early telophase. Different people may classify the same cell differently. Agree on rules before counting.
- Sample size: 20 cells is not enough to measure a stage that has only 2% of cells. Count more cells and pool class data.
- Where you count: if you count cells outside the meristem, the share in interphase will be too high.
- The slice: a thin slice cuts through some cells at angles, so some chromosomes or nuclei are hidden.
- Double counting or skipping cells when moving the slide. Count in a fixed pattern.
State your conclusion with the evidence: "80% of cells were in interphase, so interphase is the longest stage. This supports the hypothesis." Then state your limits: "The times are estimates because ..."
Just for funBiology students counting onion root tip cells for an hour are the only people who come out of a lab saying, "I've seen a lot of phases today."
Hands-on activityWatch a root grow from the tip
You need: a few dried beans (kidney, mung or lima beans work) or a green onion with its root end, paper towel, a clear jar or zip-top bag, water, a fine waterproof marker and a ruler.
- Soak the beans overnight. Wrap them in damp paper towel inside the jar or bag. Keep the towel damp. In 2 to 4 days, roots appear. (For a green onion, stand the white root end in a little water.)
- When a root is about 15 to 20 mm long, gently lay it on a dry paper towel and make tiny ink dots every 2 mm from the tip back. Be very gentle; roots break easily.
- Put it back in the damp towel. After 24 hours, measure the distances between the dots again and record your data in a table: gap number, distance on day 1, distance on day 2.
- Graph the change for each gap.
What to look for: the dots near the tip spread far apart, and the dots farther back barely move. The fastest growth is in the elongation zone, just behind the meristem, where cells made by mitosis stretch. The very tip (the cap) does not stretch much. Raw beans are not for eating; throw them in the compost and wash your hands.
Your turn
1Look at the field of view diagram. Which stage is cell A in? Give the clue.
Prophase: thick chromosome threads are visible, and the nuclear membrane is fading (dashed).
2Which stage are cells B and F in?
Metaphase: the chromosomes are in a band across the middle of the cell.
3Which stage is cell C in?
Anaphase: two groups of V-shaped chromosomes are moving to opposite ends.
4Which stage is cell D in? What is the dashed line in the middle?
Telophase: two new nuclei are forming at opposite ends. The dashed line is the cell plate, which will become the new cell wall.
5Which stage is cell E in?
Interphase: a normal round nucleus with a nucleolus and no visible chromosomes.
6Why do biologists study mitosis in a root tip rather than in a mature leaf?
The root tip has a meristem where cells divide constantly. Most mature leaf cells have stopped dividing, so you would find almost no cells in mitosis.
7Name two ways the whitefish blastula cells look different from onion root tip cells during division.
Whitefish cells have no cell wall (rounder shape), they divide the cytoplasm with a cleavage furrow instead of a cell plate, and they have centrioles with star-shaped spindles at the poles.
More practice: warm-up
1Where is the meristem in a root?
Just behind the root cap, at the tip of the root.
2Which stage will you see most often on a root tip slide?
3Which objective lens should you always start with?
The lowest power objective.
4Which focus knob should you use on high power?
Only the fine focus knob.
5Is "8 cells in metaphase" qualitative or quantitative data?
Quantitative (it is a number).
More practice: on your own
1A student counts 50 cells: 42 interphase, 4 prophase, 2 metaphase, 1 anaphase, 1 telophase. What percentage are in prophase?
2Using the counts in the last question and a 20-hour cell cycle, estimate the time spent in metaphase.
2 ÷ 50 = 0.04; 0.04 × 20 h = 0.8 h = 48 min.
3Find the mitotic index for the same 50 cells.
Cells in mitosis: 4 + 2 + 1 + 1 = 8. 8 ÷ 50 × 100% = 16%.
4Why is it better to pool the whole class's counts?
A larger sample reduces the effect of chance and of one person's classification mistakes, so the percentages (and time estimates) are more reliable, especially for rare stages like anaphase.
5A student counted cells from the elongation zone and found 98% in interphase. Explain this result.
Cells in the elongation zone have mostly stopped dividing; they are growing longer. So almost none are in mitosis. The student should count in the meristem.
Challenge
1Two groups each counted 400 cells. Group 1 found 12 in anaphase; Group 2 found 4. Suggest two reasons for the difference and how to decide which is closer to the truth.
Reasons: different parts of the root (one closer to the meristem); different ways of classifying borderline anaphase/telophase cells; chance variation. To decide: pool more data from more slides, agree on clear classification rules, and count only in the meristem. The pooled average is more reliable than either group alone.
2Explain why the method "fraction of cells in a stage = fraction of time in that stage" would not work if all the cells in a tissue had started dividing at the same moment.
If all cells were in step (synchronized), they would all be in the same stage at the same time, so the counts would show only where they happened to be when the sample was taken, not how long each stage lasts. The method needs cells at random points in the cycle.
3A tissue has a mitotic index of 5%, and another has 25%. Which would you expect in a healing wound, and which in healthy adult skin that is not injured? Explain.
The healing wound would likely have the higher index (25%) because cells are dividing quickly to repair damage. Healthy, uninjured skin divides only to replace worn-out cells, at a steadier, lower rate (5%).
Quiz
1. Why are onion root tips used to observe mitosis?
2. In a sample, 6 of 200 cells are in metaphase. The cell cycle takes 20 h. About how long is metaphase?
3. A cell shows two V-shaped groups of chromosomes moving toward opposite ends. What stage is it in?
4. Which observation would you expect in a dividing whitefish blastula cell but not in an onion root tip cell?
5. Most cells on a root tip slide are in interphase. What does this tell you?
6. What is the best way to make cell-count results more reliable?
Big idea: expectation B2.2 asks students to examine cells under a microscope to identify the stages of mitosis in plant and animal cells. In school this is done with prepared slides of onion root tips and whitefish blastulas. At home, the diagram in this lesson stands in for a real slide; photos of these slides are also widely available online.
How to help: The bean-root activity is cheap and shows where growth happens. Help your teen keep a neat data table and make a graph. In the calculations, ask them to say the logic out loud: "If 10% of the cells are in this stage, then a cell spends about 10% of its time in it."
Common difficulties:
- Confusing early prophase with interphase.
- Forgetting to convert hours to minutes, or rounding too much or too little.
- Thinking a stage is "long" because it is complicated. Stage length is shown by how many cells are in it.