Mitosis is a short, precise dance. Chromosomes condense, line up on the equator, and their sister chromatids are pulled to opposite poles by the spindle, so each new nucleus gets exactly one copy of everything. Learn to see each stage down a microscope.
In prophase the chromosomes condense and become visible as two sister chromatids, the nucleolus disappears, the spindle forms (from centrosomes in animal cells) and the nuclear envelope breaks down. In metaphase spindle fibres attach at the centromeres and line the chromosomes up on the equator. In anaphase the centromeres divide and the chromatids are pulled to opposite poles, centromere first. In telophase the chromatids reach the poles and uncoil, nuclear envelopes re-form and the spindle breaks down. Cytokinesis then divides the cell: a cleavage furrow in animal cells, a cell plate in plant cells.
Press Play, or click the stage chips, and rotate the cell as it divides. The model has four chromosomes (two homologous pairs, red and blue). Watch the chromosome count in the panel, and switch between animal cell and plant cell for cytokinesis.
| Stage | Chromosomes | Nuclear envelope | Spindle |
|---|---|---|---|
| prophase | condense (coil), become short and thick; visible as two chromatids joined at the centromere; nucleolus disappears | breaks down at the end of prophase | centrosomes move to opposite poles; spindle forms |
| metaphase | line up on the equator; each centromere attached to fibres from both poles | absent | fully formed; fibres attach at the kinetochores |
| anaphase | centromeres divide; sister chromatids separate and move to opposite poles, centromere first (V shapes) | absent | fibres shorten, pulling the chromatids apart |
| telophase | chromatids (now chromosomes) reach the poles and uncoil; nucleoli reappear | re-forms around each group | breaks down |
In anaphase it is the sister chromatids that separate, not homologous chromosomes (that is meiosis). And the chromosome number does not halve in mitosis: a cell with 46 chromosomes gives two cells with 46 each. Count centromeres to count chromosomes.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the 3D mitosis model, at which stage do the chromosomes line up on the equator of the spindle?
The model cell has four chromosomes. How many chromosomes does each new nucleus have in telophase?
What happens to the nuclear envelope during mitosis?
How does cytokinesis differ in a plant cell compared with an animal cell?
In an exam photomicrograph or a stained slide you cannot watch the process, so you identify each stage from a single still image. Use these clues.
| Stage | What you see in a stained cell |
|---|---|
| interphase | a round, evenly stained nucleus, often with a nucleolus; no separate chromosomes |
| prophase | the nucleus still present but containing visible, thread-like chromosomes |
| metaphase | chromosomes in a line across the middle of the cell |
| anaphase | two groups of V-shaped chromosomes moving to opposite ends |
| telophase | two dense groups at opposite ends, often with a new cell plate or furrow between them |
To place a set of drawings in order, follow the chromosomes: visible inside a nucleus, then on the equator, then separating, then in two new nuclei.
Click cells in the stained onion root tip, name each stage, and watch the mitotic index build up. Name at least 30 cells, then switch from the meristem to the zone of elongation and compare.
The mitotic index counts cells in prophase, metaphase, anaphase and telophase only, never interphase, and divides by the total number of cells, not by the number in interphase. Count a large sample (at least 30, ideally more) from several fields of view.
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A photomicrograph shows a cell with two groups of V-shaped chromosomes close to the middle of the cell, moving apart. Which stage is this?
In a root tip squash, 600 cells were counted: 450 in interphase, 72 in prophase, 33 in metaphase, 21 in anaphase and 24 in telophase. What is the mitotic index?
Using the same root tip data and a cell cycle of 20 hours, how long does metaphase last?
Which event ensures that the two daughter nuclei are genetically identical?
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