Cell BiologyBuilder16 minLesson 2 of 7

Meiosis

Every sperm and egg is the product of meiosis — two back-to-back divisions that halve the chromosome count and shuffle the genetic deck. Learn how one diploid cell becomes four unique haploid gametes — then build some yourself and see how many different ones a single cell can make.

Build it

How many different gametes can one cell make?

This cell has three pairs of chromosomes — one copy of each from the mother, one from the father. Choose how the pairs line up, decide whether crossing over happens, and divide. Every new combination you find goes into your collection.

From mother (A B D E)From father (a b d e)Metaphase plate · pairs 1–3, left to right
ABABababDDddeeEE

1 · Line up the pairs

Choose which copy of each pair faces the top of the cell.

2 · Cross over

3 · Divide

Your four gametes

Set up the cell, then press Run meiosis to make gametes.

Gamete collection · 0 / 16

Every different gamete this cell can make.

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Challenges · 0 / 4

  • Not yet done: Make a gamete carrying only the mother's chromosomes.

  • Not yet done: Make a gamete with a recombinant chromosome — one that's part mother, part father.

  • Not yet done: Find 8 different gametes.

  • Not yet done: Collect all 16 possible gametes.

How it works

Step through meiosis I and II

Meiosis I
Meiosis II
Chromatin(loosely coiled DNA)2n = 4diploid — 2 pairs
drag to explore phases
Crossing over

← drag right to advance phases →

Interphase
4 Haploid Cells

Click a phase tab or drag the cell to explore

Meiosis I
Phase 1 of 8

Interphase

S Phase Complete

The cell has completed DNA replication during S phase — every chromosome now consists of two identical sister chromatids. The cell grows and duplicates its centrosomes in preparation for two rounds of division.

  • DNA replicated: each chromosome is now 2 sister chromatids
  • Homologous chromosome pairs each contain 4 chromatids total
  • Cell will divide TWICE to produce 4 haploid cells

Two very different divisions

  • Meiosis I — Reductional

    Homologous chromosomes separate. Chromosome number halves: 2n → n. Sister chromatids stay joined. This division is unique to meiosis.

  • Meiosis II — Equational

    Sister chromatids separate — essentially mitosis on haploid cells. Chromosome count stays at n. Two cells → four cells.

Why meiosis matters

Meiosis is the engine of sexual reproduction and genetic diversity. Without it, chromosome counts would double with every fertilization and allele combinations would never shuffle — evolution would grind to a halt.

  • 2n → n

    Meiosis halves the chromosome count from diploid (2n = 46) to haploid (n = 23), so that fertilization restores the diploid number without doubling it every generation.

  • Crossing Over

    In Prophase I, non-sister chromatids of homologous chromosomes exchange segments at chiasmata — physically shuffling alleles and creating chromosomes that are neither fully maternal nor fully paternal.

  • 8 Million+

    Independent assortment alone yields 2²³ ≈ 8 million possible chromosome combinations per gamete. Combined with crossing over, no two human gametes are genetically identical.

Exam trap

Meiosis vs. Mitosis

Mitosis produces 2 identical diploid cells for growth and repair — no mixing, no reduction. Meiosis produces 4 genetically unique haploid cells for reproduction. The key differences: meiosis has two divisions, includes synapsis and crossing over in Prophase I, and separates homologs (not sister chromatids) in the first division. Never confuse them on an exam.

Quick recap

  1. 1

    Interphase — DNA replicated; centrosomes duplicated — ready for two divisions.

  2. 2

    Prophase I — Homologs pair (synapsis); crossing over shuffles alleles.

  3. 3

    Metaphase I — Bivalents align at equator; orientation is random (independent assortment).

  4. 4

    Anaphase I — Homologs separate to poles; chromosome number halves (2n → n).

  5. 5

    Telophase I — Two haploid cells form — each chromosome still has 2 chromatids.

  6. 6

    Metaphase II — Individual chromosomes align in both cells — like mitosis.

  7. 7

    Anaphase II — Sister chromatids finally separate in both cells.

  8. 8

    4 Haploid Cells — Cytokinesis II → 4 genetically unique haploid gametes.