DAT Review · Biology
Mitosis and Meiosis
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In 30 seconds
- Mitosis: somatic cells, 2n → 2n, one division, genetically identical daughters, no crossing over. Purpose: growth and repair.
- Meiosis: germ cells, 2n → n, two divisions, genetically unique daughters, crossing over in Prophase I. Purpose: gamete production.
- Meiosis I separates homologous chromosomes; Meiosis II separates sister chromatids. Nondisjunction in either division causes aneuploidy (e.g., trisomy 21).
The college version
Core Review
Key Terminology
- Chromosome: A single DNA molecule with associated proteins. Humans have 46 chromosomes (23 homologous pairs).
- Chromatid: One of two identical copies of a replicated chromosome, joined at the centromere. After replication and before separation, a chromosome consists of two sister chromatids.
- Homologous chromosomes (homologues): A pair of chromosomes — one from each parent — carrying genes for the same traits at the same loci. They are similar but NOT identical (different alleles).
- Diploid (2n): Two sets of chromosomes (one from each parent). Human somatic cells: 2n = 46.
- Haploid (n): One set of chromosomes. Human gametes: n = 23.
The Cell Cycle
- G₁ phase: Cell grows, performs normal functions.
- S phase (Synthesis): DNA replicates — each chromosome becomes two sister chromatids.
- G₂ phase: Final growth and preparation for mitosis. Centrosomes replicate.
- M phase: Mitosis (nuclear division) + cytokinesis (cytoplasmic division).
Mitosis
Prophase: Chromatin condenses into visible chromosomes (each with two sister chromatids). The nucleolus disappears. The mitotic spindle begins forming from the centrosomes. In animal cells, centrioles organize the spindle; plants lack centrioles but still form spindles.
Prometaphase: The nuclear envelope breaks down. Kinetochores (protein structures at the centromere) attach to spindle microtubules.
Metaphase: Chromosomes align at the metaphase plate (equator of the cell). Each chromosome's kinetochores are attached to microtubules from opposite poles. This alignment ensures each daughter cell receives one copy of each chromosome.
Anaphase: Sister chromatids separate — they are now individual chromosomes. The spindle pulls them to opposite poles. The cell elongates.
Telophase: Nuclear envelopes re-form around each cluster of chromosomes. Chromosomes decondense. Nucleoli reappear. The spindle disassembles.
Cytokinesis: In animal cells, a cleavage furrow forms (actin-myosin ring contracts). In plant cells, a cell plate forms at the equator (vesicles from the Golgi fuse) and becomes the new cell wall.
Result of Mitosis: Two genetically identical diploid daughter cells (2n → 2n + 2n).
Meiosis
Meiosis consists of two consecutive divisions (Meiosis I and Meiosis II) with only one round of DNA replication.
Meiosis I — The Reduction Division (2n → n):
Prophase I (long and complex):
- Leptotene: Chromosomes condense.
- Zygotene: Homologous chromosomes pair up in a process called synapsis — held together by the synaptonemal complex. Each pair is a bivalent (or tetrad: 4 chromatids total).
- Pachytene: Crossing over occurs — non-sister chromatids of homologous chromosomes exchange segments at chiasmata. This is the molecular basis of genetic recombination.
- Diplotene: Synaptonemal complex disassembles; chromosomes remain attached at chiasmata.
- Diakinesis: Chromosomes fully condense; nuclear envelope breaks down.
Metaphase I: Homologous pairs (bivalents) align at the metaphase plate. Microtubules attach to kinetochores — each homologue's kinetochore faces OPPOSITE poles. Independent assortment occurs here — the orientation of each pair is random (2²³ possible arrangements in humans).
Anaphase I: Homologous chromosomes separate (sister chromatids remain attached). This is the reduction division — chromosome number halves.
Telophase I and Cytokinesis: Two haploid cells are formed (each chromosome still consists of two sister chromatids).
Meiosis II — The Equational Division (n → n): Essentially mitosis of haploid cells. Prophase II, Metaphase II (chromosomes align at equator), Anaphase II (sister chromatids separate), Telophase II.
Result of Meiosis: Four genetically unique haploid daughter cells (2n → n + n + n + n).
Mitosis vs. Meiosis — Comparison Table
| Feature | Mitosis | Meiosis |
|---|---|---|
| Cell type | Somatic cells | Germ cells |
| Number of divisions | One | Two (Meiosis I and II) |
| Ploidy change | 2n → 2n | 2n → n |
| Daughter cells | 2 | 4 |
| Genetic identity | Identical to parent | Unique (crossing over + independent assortment) |
| Crossing over | No | Yes (Prophase I) |
| Homologous pairing | No | Yes (Prophase I) |
| What separates in Anaphase | Sister chromatids | Anaphase I: homologues; Anaphase II: sister chromatids |
| Function | Growth, repair, asexual reproduction | Sexual reproduction (gametes) |
Nondisjunction
Nondisjunction is the failure of chromosomes or chromatids to separate properly during cell division.
In Meiosis I: Homologous chromosomes fail to separate. Result: two gametes with an extra chromosome (n+1) and two gametes missing a chromosome (n−1).
In Meiosis II: Sister chromatids fail to separate. Result: two normal gametes (n), one gamete with an extra chromosome (n+1), one gamete missing a chromosome (n−1).
Consequences: Trisomy (2n+1) — e.g., Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), Trisomy 13 (Patau syndrome). Monosomy (2n−1) — e.g., Turner syndrome (XO, the only viable human monosomy). The risk of nondisjunction increases with maternal age.
Common Traps
- "Sister chromatids separate in Anaphase I": No! Homologous chromosomes separate in Anaphase I. Sister chromatids remain together until Anaphase II.
- "Crossing over occurs in mitosis": Never. Crossing over is unique to Prophase I of Meiosis I.
- "Meiosis produces 2 cells": Students often confuse the products — meiosis produces 4 haploid cells, not 2.
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine your body has two recipe books for making you — one from mom, one from dad (homologous chromosomes). Mitosis is like photocopying both books and giving identical sets to two new libraries. Meiosis is much fancier. First, the two books shuffle pages (crossing over) — swapping some mom recipes for dad recipes so each book becomes unique. Then, in the first split (Meiosis I), you put one whole shuffled book into each of two new libraries (separating the books, not the copies). In the second split (Meiosis II), you separate the photocopies (sister chromatids) so each of the four final libraries gets one unique book. Nondisjunction is when a book goes to the wrong library — one library gets two books, another gets none. That's how Down syndrome happens: an extra copy of chromosome 21.
Key takeaways
- Meiosis I separates homologues; Meiosis II separates sister chromatids. This is the most tested distinction.
- Crossing over = Prophase I of Meiosis I. Not mitosis, not meiosis II.
- Independent assortment = Metaphase I. The random alignment of bivalents generates 2ⁿ possible combinations (n = haploid number).
- Nondisjunction outcomes depend on which division is affected — know the gamete ratios (Meiosis I error: all 4 gametes abnormal; Meiosis II error: 2 normal, 2 abnormal).
- Plants: no centrioles but still form spindles. Cell plate, not cleavage furrow, for cytokinesis.
Check yourself
3 review questions from the chapter. Try each one, then open the answer.
A diploid cell with 12 chromosomes (2n=12) undergoes meiosis. How many chromosomes and chromatids are present in each cell during Metaphase I?
Show answer
12 chromosomes (as 6 bivalents/tetrads) and 24 chromatids. DNA has replicated, so each chromosome consists of two sister chromatids. Since homologous pairs are still together, there are 12 distinct centromeres.
If nondisjunction occurs during Meiosis I, how many of the four resulting gametes are abnormal?
Show answer
All four gametes are abnormal. When homologous chromosomes fail to separate in Anaphase I, two gametes receive both homologues (n+1) and two gametes receive neither (n−1). No normal gametes are produced.
A researcher observes cells where sister chromatids are being pulled to opposite poles, but homologous chromosomes are not paired. Which phase and division is this?
Show answer
This is Anaphase of Meiosis II (or Anaphase of Mitosis). The key clue is that sister chromatids — not homologous chromosomes — are separating, and homologous pairs are absent. In Meiosis I, homologues separate while sister chromatids remain attached; that is not what is observed here.
Study tools & related lessonsYou’ll learn to · Related
You’ll learn to
- List and describe the stages of mitosis (prophase, metaphase, anaphase, telophase) and cytokinesis.
- Compare meiosis I and meiosis II, emphasizing the events of prophase I (synapsis, crossing over) and the separation that occurs in each division.
- Distinguish between homologous chromosomes and sister chromatids.
- Predict the consequences of nondisjunction in meiosis I versus meiosis II.
Sources & references
- OpenStax Biology 2e, Chapter 10: "Cell Reproduction" and Chapter 11: "Meiosis and Sexual Reproduction"
- NCBI Bookshelf, Molecular Biology of the Cell, 4th edition, Chapter 17: "The Cell Cycle and Programmed Cell Death"
- NIH Genetics Home Reference: "How do cells divide?"
This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.
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