Cell Division Visualizer — Mitosis and Meiosis Step by Step

Mitosis vs Meiosis — Key Differences

How to Use the Cell Division Visualizer

1. Open the Cell Division Visualizer
2. Choose Mitosis or Meiosis from the mode selector
3. Click Next Phase to step through each phase one at a time
4. Read the phase description to understand what is happening to chromosomes, spindle fibers, and the nuclear envelope
5. Click Auto Play to animate through all phases continuously
6. Use Reset to start over from interphase

Mitosis Phase by Phase

Interphase (preparation, not division)

The cell spends most of its life in interphase. DNA is replicated during S phase, producing sister chromatids joined at the centromere. The cell also grows (G1 and G2 phases) and synthesizes proteins needed for division. Chromosomes are not yet condensed — they exist as diffuse chromatin.

Prophase

Chromosomes condense into visible X-shaped structures (each X = two sister chromatids). The nuclear envelope breaks down. The mitotic spindle — made of microtubules — begins to form from centrioles at opposite poles of the cell. Spindle fibers attach to kinetochores on each chromosome.

Metaphase

Chromosomes align along the cell's equatorial plate (metaphase plate). This is the most visually striking phase and the one used in karyotyping — cells are arrested here with colchicine to photograph chromosomes at their most compact. Spindle fibers from both poles are under equal tension.

Anaphase

The centromere splits. Sister chromatids are pulled apart to opposite poles of the cell by shortening spindle microtubules. Each pole now has a complete set of chromosomes. The cell begins to elongate. If non-disjunction occurs here (chromatids fail to separate), the resulting cells will have abnormal chromosome numbers.

Telophase and Cytokinesis

A nuclear envelope reforms around each set of chromosomes. Chromosomes decondense. In cytokinesis, the cytoplasm divides — in animal cells via a cleavage furrow (actin ring contracts), in plant cells via a cell plate built from vesicles. Two daughter cells are produced, each genetically identical to the parent.

Meiosis — Two Rounds of Division

Meiosis I — the reductive division

Prophase I is the critical phase: homologous chromosome pairs (one from each parent) come together in synapsis and exchange segments in a process called crossing over (recombination). This shuffles alleles between chromosomes, generating genetic diversity. Metaphase I, Anaphase I, and Telophase I follow, separating homologous pairs to different cells — chromosome number is halved.

Meiosis II — the equational division

Meiosis II resembles mitosis: sister chromatids are separated. No DNA replication occurs between meiosis I and II. The result is four haploid cells (n), each with a unique combination of alleles due to both independent assortment and crossing over. In females, three of the four cells become polar bodies and only one becomes an egg.

Common Questions

Why does non-disjunction cause Down syndrome?

Non-disjunction during meiosis I or II produces gametes with an extra chromosome (n+1). When an n+1 egg is fertilised by a normal sperm, the zygote has three copies of chromosome 21 (trisomy 21), which causes Down syndrome. Non-disjunction frequency increases with maternal age.

How does crossing over increase genetic variation?

During prophase I, homologous chromosomes exchange corresponding segments at points called chiasmata. This creates recombinant chromosomes — novel combinations of alleles that did not exist in either parent. Combined with independent assortment (which orientation each homologous pair faces is random), meiosis can theoretically produce 2²³ × 2²³ unique gametes from a single human.