Table of contents · 7 sections
Mitosis and meiosis are the two main types of cell division responsible for organismal growth, regeneration, and reproduction. Understanding their differences is fundamental to genetics, histology, and embryology. This article provides a comprehensive comparison with phases, diagrams, and clinical correlations.
Mitosis vs Meiosis: Key Differences
| Sign | Mitosis | Meiosis |
|---|---|---|
| Biological purpose | Growth, regeneration, asexual reproduction | Gamete formation (sex cells) |
| Location | Somatic cells | Gonads (ovaries, testes) |
| Number of divisions | 1 | 2 (Meiosis I and Meiosis II) |
| Number of daughter cells | 2 | 4 |
| Chromosome set | 2n → 2n (diploid) | 2n → n (haploid) |
| Genetic diversity | Clones (identical) | Unique (crossing over + independent assortment) |
| Crossing over | No (extremely rare) | Yes (Prophase I — obligatory) |
| Homologue conjugation (synapsis) | No | Yes (Prophase I — bivalent formation) |
Phases of Mitosis
Cell cycle: interphase (G1 → S → G2) → mitosis (M). During S phase, DNA is replicated (DNA content changes from 2n to 4n).
- Prophase — chromosomes condense, centrioles migrate to opposite poles, the mitotic spindle forms, and the nuclear envelope breaks down
- Metaphase — chromosomes align along the equatorial plane (metaphase plate), with kinetochores attached to spindle fibers
- Anaphase — centromeres split, and sister chromatids are pulled toward opposite poles
- Telophase — chromosomes decondense, nuclear envelopes reform, and cytokinesis (cytoplasmic division) occurs
Mnemonic: PMAT — Prophase, Metaphase, Anaphase, Telophase.
Phases of Meiosis
Meiosis I (reductional division)
Key difference: homologous chromosomes separate (rather than sister chromatids).
- Prophase I — the longest and most complex phase. Stages: leptotene → zygotene (synapsis → bivalents) → pachytene (crossing over!) → diakinesis (chiasmata visible)
- Metaphase I — bivalents align along the equator. Independent orientation yields 2²³ combinations (>8 million) in humans
- Anaphase I — homologous chromosomes (not chromatids!) separate and move to opposite poles
- Telophase I — 2 haploid cells are formed (n, but each chromosome consists of 2 chromatids)
Meiosis II (equational division)
Mechanism is equivalent to mitosis of a haploid cell. Sister chromatids separate → 4 haploid cells.
Crossing Over: Source of Genetic Diversity
Crossing over occurs during the pachytene stage of Prophase I:
- Homologous chromosomes exchange segments of chromatids
- Exchange points are called chiasmata (visible in diplotene)
- Result: recombinant chromosomes → each gamete is genetically unique
- An average of 1–3 crossovers per chromosome per meiotic event
Combined with independent assortment (2²³) and random fertilization, genetic diversity is virtually limitless.
Clinical Correlations: Meiotic Disorders
| Disorder | Mechanism | Result |
|---|---|---|
| Nondisjunction | Homologues (meiosis I) or chromatids (meiosis II) fail to disjoin | Trisomy: Down (21), Edwards (18), Patau (13), Klinefelter (XXY), monosomy: Turner (X0) |
| Translocations | Exchange of segments between non-homologous chromosomes | Robertsonian translocation 14;21 → familial form of Down syndrome |
| Deletions | Loss of a chromosome fragment | Cri-du-chat syndrome (5p−) |
Cell cycle regulation
- Cyclins and CDKs — cyclin-CDK complexes "permit" phase transitions
- Checkpoints — control points: G1/S (DNA check), G2/M (replication complete?), metaphase (all chromosomes attached?)
- p53 — "guardian of the genome" — halts the cycle in G1 upon DNA damage or triggers apoptosis. p53 mutation → 50% of all malignant tumors
- Rb (retinoblastoma) — blocks E2F → stops G1→S transition. Loss of Rb → retinoblastoma
Frequently asked questions
Why is non-disjunction in meiosis I more dangerous than in meiosis II?
With non-disjunction in meiosis I, all 4 gametes are abnormal (2 with an extra chromosome, 2 with a missing one). With non-disjunction in meiosis II, only 2 of 4 are abnormal, 2 are normal. Therefore, non-disjunction in meiosis I makes twice the contribution to aneuploidy.
How does crossing over differ from translocation?
Crossing over is an exchange between homologous chromosomes (a normal meiotic process). Translocation is an exchange between non-homologous chromosomes (a pathology). Crossing over increases genetic diversity, whereas translocation can disrupt gene function and cause disease.
Study biology through interactive lessons on the platform. Related topics: epithelial tissue, amino acids, necrosis and apoptosis. Use clinical calculators for genetic calculations.