Biology IA · D · Continuity and change · topic 22 of 25

Onion roots: does the mitotic index vary by time? · IB Biology IA idea: guide, sample graph and step-by-step plan

classic practical, new question :)

Everyone counts dividing cells at one time of day. You're going to ask whether it matters when you look.

Difficulty●●○ Medium
KitStandard: light microscope, root-tip stain, hydrochloric acid, water bath
TimeAbout 10 hours (roots need several days to grow)
FitsSL and HL
  • ✓ you like microscope work and careful counting
  • ✓ you can stick to a strict timetable
  • ✓ you want the classic mitosis practical, with a twist

The big idea

← fair counting is the whole IA

The mitotic index is the fraction of cells caught dividing: it estimates how much of the tissue is in mitosis at that moment. If cell division follows a daily rhythm, the fraction will change with the time the roots are fixed; if it does not, the index will stay similar. Your question is a good IA because it is controlled, quantitative and has an honest chance of a null result. Your job is to make the counting fair, make each root tip a replicate, and use a test that fits that design.

Make it yours: pick one twist

Copying a classic won't show your choices. Pick one of these (or invent your own) and your IA becomes yours.

Swipe for more twists →

Temperature instead of time

Grow roots at three or four temperatures, then fix at one time of day.

→ The independent variable becomes temperature; you need a way of holding it steady and recording it, and the background shifts to temperature and the cell cycle.
Compare phases, not just the index

Count each stage (prophase, metaphase, anaphase, telophase) and compare their proportions.

→ You get proportions of dividing cells in each phase; you must define your counting rules very clearly, and you can discuss how long each phase lasts.
Different species or varieties

Compare onion with garlic or chive, if you can get roots.

→ Species becomes the independent variable; keep the other conditions the same, and discuss why the index may differ.
A growth substance

Soak roots in a known weak solution (for example a plant growth regulator or a caffeine solution, with teacher approval) and compare the index.

→ The independent variable becomes concentration; safety and disposal need extra care, and your controls must include an untreated group.
Root length versus index

Fix roots of different lengths at the same time of day.

→ You can plot the index against length (a correlation); you must define length and where along the root you take the tip.
Change the context

Link it to cancer research, where the mitotic index is used to grade tumours, or to plant breeding.

→ Gives the introduction a real purpose; the method and the graph stay the same.

Your Biology IA research question: run the check

Write your own question first. Then make sure it ticks every box:

  • Names the independent variable and the specific times (or temperatures) you will test.
  • Names the dependent variable as the mitotic index and gives its formula (dividing cells ÷ all cells counted).
  • Says how many cells are counted for each root tip, and how many different root tips for each time.
  • Says exactly how a cell counts as dividing, and how prophase is told from interphase.
  • Says how bulbs, growth conditions, fixing, staining and squashing are kept the same.
  • Can realistically be done: fixing at several times of day on one or more days, then staining and counting in the time you have.

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The set-up

One way to organise it. Draw yours with your own times, stain and counting plan for the Method section.

stain + squash08:0010:0012:0014:0016:0018:0020:00cut tips at theset clock timefix every tip in thesame fixative500+ cells countedper root tip5+ different root tips at each time: each tip is one replicate

How your Biology IA could be laid out

The usual order for a topic like this, what each part needs to show, and roughly how much of your 3,000 words it takes. The words are all yours.

Research question and context~250
Your question; why the mitotic index matters; what you predict about time of day and why (or why not).
Background biology~400
The cell cycle and mitosis phases; what the mitotic index measures and what it cannot; anything known about timing of cell division in roots (cite it).
Variablestable
Independent, dependent and controlled variables with how each is held, measured or checked.
Method~500
Growing the roots; when and how tips were cut and fixed; hydrolysis, staining and squashing; how slides were chosen and cells counted and classified; how many tips per time.
Safety, ethics, environment~100
Specific: 1 mol dm⁻³ hydrochloric acid warmed in the water bath (goggles, no splashes); your stain (acetic orcein is corrosive, toluidine blue stains skin); an ethanol-acetic acid fixative is flammable and irritant; the hot water bath; how acid, stain and slides are disposed of.
Raw datatable
Dividing cells and total cells for every root tip at every time, with units and counting rules.
Processing~300
Mitotic index for each tip, then the mean and spread for each time; one worked example; how you decided on the number of cells counted.
Graphs and analysis~400
Mitotic index against time with SD error bars; ANOVA (or Kruskal-Wallis) across times, with each root tip as one replicate, and the conditions it needs checked.
Conclusion~400
Answer the question with values and spread; compare with published figures for onion root mitotic index; say what your times can and cannot show.
Evaluation~600, the biggest part!
Weaknesses ranked by their effect on the index: counting rules, number of cells, differences between tips and bulbs, squash quality, fixing time.
Referencesnot counted
Method, theory and any published mitotic index values.

What your Biology IA graph could look like

bars = spread between TIPS

Made-up data, to show you the form of a strong graph, not the answer. Real data may show no pattern at all, and that is a valid result. example only

810121416182045678910111213141516Time root tips were fixed / hour of the day (± 5 min)Mitotic index / % (mean ± SD, n = 5 root tips)daily rhythm (one possible shape)no change through the daymean of root tips, with SDdo the error bars overlap?then be careful with your claimILLUSTRATIVE · MADE-UP DATA
  • Each point is the mean of different root tips, and the bars are the spread between tips, not between fields on one slide.
  • Both a rhythm and a flat line drawn so the reader can judge which one the data support.
  • The axis starts at 4, not zero, so the change is visible. Say so in the text.
  • Times of day are not a continuous variable you can extrapolate: do not claim anything between or beyond your times.
  • Another graph to consider: the mitotic index of every tip as its own dot, grouped by time, which shows the spread and outliers.

How many cells is enough?

how many cells? decide with numbers

Counting only a few cells makes the index jumpy. This chart shows the uncertainty from counting alone, for a made-up root tip with an index of 9%. It does not include the real variation between tips, which is usually bigger.

100 cells counted2.86 pp200 cells counted2.02 pp500 cells counted1.28 pp1000 cells counted0.9 pp2000 cells counted0.64 ppaim for 500 or more per tipstandard error of the mitotic index, in percentage points (pp)ILLUSTRATIVE · MADE-UP NUMBERS

Your game plan

Tick steps off as you go. We'll remember on this device.

0 of 10 done

  1. Read and predictLearn what the mitotic index is, how roots are fixed and stained, and what is known about timing of cell division. Write down your prediction.
  2. Trial run (1 to 2 lessons)Prepare a slide with your method. Can you see cells clearly? Is prophase easy to call? How long does a count of 500 cells take? Fix problems now.
  3. Decide times, tips and countsChoose at least five times across the day, how many different tips for each, and how many cells per tip. Write down why.
  4. Grow the rootsSame variety, same water, same temperature. Roots of similar length. Randomise which bulb goes to which time.
  5. Fix at the set timesCut tips at your planned clock times and put them into fixative straight away. Record the exact time and the room temperature.
  6. Stain and squash togetherProcess all tips on the same day with the same time in acid and stain, so staining time does not line up with fixing time.
  7. Count blind if you canAsk a friend to label the slides with codes so that you do not know which time you are counting. Write down your rules for each phase first.
  8. Process and graphIndex for each tip, then mean and spread per time. Choose a graph and a test that treat each tip as a replicate, and check the test's conditions.
  9. Conclude and evaluateAnswer with values and uncertainty; compare with published values; rank your weaknesses by their effect on the index.
  10. Check before your teacher's draftRun the checklist below, then get your RQ or draft checked. Your teacher reads one draft only, so make it count.

Biology you'll need

Traps to avoid in this Biology IA

  • Fields from one slide treated as replicates: they all come from one root tip, so they are one data point.
  • Counting fewer than 500 cells per tip, so the index jumps around by chance.
  • Changing the rule for what counts as prophase partway through the count.
  • Staining at different times: stain time becomes a hidden variable.
  • Roots of very different lengths or bulbs that were not grown together.
  • Claiming a daily rhythm from a difference that is smaller than the spread between tips.
  • Safety copied from a worksheet, with nothing about the hot acid or the stain you actually use.

More Biology IA ideas like this

Make sure it's yours

This guide shows how a topic like this is usually approached. It is not an IA. Your question, choices, data, analysis and words must be your own: change something, tell your teacher where the idea came from, and follow your school's rules on outside help.

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