Cell Division Visualizer — Mitosis and Meiosis Step by Step

Phases of Mitosis

How to Use the Cell Division Visualizer

1. Open the cell division visualizer.
2. Select the division type: Mitosis or Meiosis.
3. Choose the number of chromosome pairs to display (2–4 pairs are clearest for educational purposes).
4. Click Play to animate through all phases automatically, or use Previous / Next to step through one phase at a time.
5. At each phase, the visualizer labels the events occurring and highlights chromosome movement.
6. For meiosis, toggle show crossing over to see chiasmata formation during prophase I.

Mitosis vs. Meiosis Comparison

The Cell Cycle

Mitosis is just one phase of the broader cell cycle. The full cycle consists of:

• G1 phase (Gap 1): Cell grows; organelles replicate; proteins synthesised for DNA replication. Checkpoint: is the cell big enough? Is the environment favourable?
• S phase (Synthesis): DNA replication — each chromosome duplicates to form two sister chromatids joined at the centromere. DNA content doubles (2n → 4n).
• G2 phase (Gap 2): Further growth; DNA repair; microtubule synthesis in preparation for mitosis. Checkpoint: is DNA fully replicated and undamaged?
• M phase (Mitosis + Cytokinesis): Chromosomes segregate; cell divides. Checkpoint: are chromosomes correctly attached to the spindle (spindle assembly checkpoint)?

Cells that have permanently exited the cell cycle (neurons, most muscle cells, red blood cells) are in G0 — a quiescent state outside the cycle.

Meiosis I: Reduction Division

Meiosis I is the key reduction division that reduces chromosome number from diploid (2n) to haploid (n). The critical differences from mitosis occur in prophase I and metaphase I:

• Prophase I: Homologous chromosomes (pairs — one from each parent) come together in synapsis, forming bivalents (also called tetrads). Crossing over (recombination) occurs at points called chiasmata, exchanging segments between non-sister chromatids. This is the primary source of new genetic combinations.
• Metaphase I: Bivalents align at the metaphase plate. The orientation of each bivalent (which homologue goes to which pole) is random — independent assortment. For n chromosome pairs, there are 2ⁿ possible chromosome combinations in the gametes.
• Anaphase I: Homologous chromosomes separate to opposite poles (unlike mitosis, where sister chromatids separate). Each pole now has a haploid set of duplicated chromosomes (n, each still as two chromatids).
• Telophase I and Cytokinesis I: Two haploid cells form, each containing one copy of each chromosome (as paired sister chromatids).

Meiosis II: Separation of Sister Chromatids

Meiosis II is very similar to mitosis but starts with haploid (n) cells:

• No DNA replication occurs between meiosis I and II
• Prophase II, Metaphase II, Anaphase II, Telophase II follow the same pattern as mitosis
• Sister chromatids separate in Anaphase II
• Result: 4 haploid cells (n), each containing one copy of each chromosome

In male meiosis (spermatogenesis), all four cells develop into functional sperm. In female meiosis (oogenesis), one large oocyte and three small polar bodies are produced — the polar bodies degenerate. This asymmetrical division concentrates cytoplasm and nutrients in the egg.

Sources of Genetic Variation in Meiosis

Meiosis generates genetic diversity through three mechanisms:

• Crossing over: Exchange of DNA segments between homologous chromosomes during prophase I. Creates new combinations of alleles not present in either parent chromosome.
• Independent assortment: Random orientation of bivalents at metaphase I means each gamete receives a random mixture of maternal and paternal chromosomes. For humans (n=23), this gives 2²³ = ~8 million possible chromosome combinations.
• Random fertilisation: Any one of ~8 million possible sperm can fertilise any one of ~8 million possible eggs, giving approximately 64 trillion possible combinations. This is without even considering crossing over, which multiplies variation further.

Errors in Cell Division

Errors in chromosome segregation during meiosis cause aneuploidy — an incorrect number of chromosomes in gametes:

• Non-disjunction in meiosis I: Homologous chromosomes fail to separate, producing gametes with two copies or zero copies of a chromosome.
• Non-disjunction in meiosis II: Sister chromatids fail to separate; less severe effect.
• Trisomy 21 (Down syndrome): Extra copy of chromosome 21; results from non-disjunction, most commonly in oogenesis. Risk increases with maternal age.
• Monosomy X (Turner syndrome): Single X chromosome (45,X); most common cause of female infertility.

Cancer frequently involves errors in mitosis — disrupted spindle checkpoint allows cells with incorrectly segregated chromosomes to divide, leading to chromosomal instability.

Common Questions

Why does meiosis produce 4 cells but mitosis only produces 2?

Mitosis involves one division of a diploid cell, producing two diploid daughters. Meiosis involves two sequential divisions — meiosis I (reduces ploidy: 2n → n) and meiosis II (separates chromatids in each haploid cell: n → n). The two divisions of two cells from meiosis I produce four haploid cells total.

What is the difference between chromosomes and chromatids?

After DNA replication in S phase, each chromosome consists of two identical copies (sister chromatids) joined at the centromere. The pair together is still one chromosome (now duplicated). When sister chromatids separate in anaphase (mitosis) or anaphase II (meiosis), each becomes an independent chromosome. Before replication: 46 chromosomes in human somatic cells. After S phase: 46 chromosomes, each consisting of 2 sister chromatids = 92 chromatids total.

Can cells divide indefinitely?

Normal somatic cells have a finite number of divisions (the Hayflick limit, approximately 50–70 divisions for human fibroblasts). This limit is related to telomere shortening — each division shortens the protective telomere caps on chromosomes until they become critically short and trigger senescence. Cancer cells activate telomerase (an enzyme that extends telomeres) to bypass this limit, enabling unlimited replication (immortality).