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BIO 171 · Human Anatomy & Physiology I · Lab

Mitotic Index: Counting Cells in a Root Tip

A field of 100 cells from the growing tip of an onion root. Find the ones that are dividing, work out what fraction of its life a cell spends in each phase, and see how long each phase actually lasts.

Step 1 — pick a phase, then click the cells that match it

    0 of 100 cells tagged
    Field ID ——— — write it down. Everyone is given a different field, so your numbers will land close to your neighbour's without matching them.

    What you are looking for

    • Interphase — a round nucleus with soft, blurry, evenly spread chromatin and one or two dark nucleoli. No separate chromosomes visible.
    • Prophase — the chromatin has pulled itself into short dark threads, still inside a fading nuclear boundary. No nucleolus.
    • Metaphase — sharp dark chromosomes packed into a single narrow band across the middle of the cell.
    • Anaphase — two separate groups of sharp chromosomes with a clear gap between them, pulling apart.
    • Telophase — two dense clumps at opposite ends, softening at the edges, with a new wall starting to form across the middle.

    Simulated field — drawn to look like a stained onion root tip under a light microscope, at about 400×. It is an illustration, not a photomicrograph, but the proportions behave the way a real root tip does. A new field is dealt every time this page loads, so no two students are counting the same hundred cells. Each cell can be tagged with a phase; every cell also carries a written description of what is visible inside it.

    Step 2 — your tally
    PhaseCells% of cellsTime in phase

    Time in phase = (cells in that phase ÷ 100) × the length of the cycle. This works because you are looking at a snapshot: the more of its life a cell spends in a phase, the more likely you are to catch it there.

    Mitotic index

    —

    Tag some cells and this fills in. The mitotic index is the percentage of cells caught in mitosis rather than interphase.

    On your worksheet Record your tally in the table, then answer Q1–Q8. You will need your mitotic index and your calculated phase times.