Cell Division: Mitosis and Meiosis
Dividing to Grow and Reproduce
Mitosis
Mitosis produces two genetically identical daughter cells (diploid, 2n) from one parent cell. It occurs in growth, repair, and asexual reproduction. Phases remembered by PMAT:
- Prophase: chromosomes condense, spindle forms, nuclear envelope breaks down
- Metaphase: chromosomes align at the metaphase plate (equator)
- Anaphase: sister chromatids separate and move to opposite poles
- Telophase + Cytokinesis: nuclear envelopes reform, cytoplasm divides
Note: interphase (G1, S, G2) precedes mitosis where DNA replication occurs.
Meiosis
Meiosis produces four genetically unique haploid cells (n) — sperm and eggs. Two division rounds:<br>Meiosis I (reductional division): homologous chromosomes separate — this is where independent assortment and crossing over (chiasmata between homologs in prophase I) generate genetic diversity.<br>Meiosis II (like mitosis): sister chromatids separate.
Result: 4 haploid cells with unique combinations of alleles — no two gametes are genetically identical.
Crossing Over and Genetic Diversity
During prophase I of meiosis, homologous chromosomes pair up (synapsis) and exchange segments at points called chiasmata. This crossing over is a major source of genetic diversity in sexually reproducing organisms.
Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Sexual reproduction (gametes) |
| Division rounds | 1 | 2 |
| Daughter cells | 2 | 4 |
| Ploidy of daughters | Diploid (2n) | Haploid (n) |
| Genetic outcome | Identical to parent | Unique (crossing over + assortment) |
| Where it occurs | All somatic cells | Gonads (testes, ovaries) |
A diploid organism (2n=46) undergoes meiosis. How many chromosomes does each resulting cell contain?
Meiosis halves the chromosome number. 46 ÷ 2 = 23 chromosomes (haploid, n=23) in each gamete.
Correct answer: 23
Crossing over during meiosis occurs in which phase?
Crossing over (chiasmata formation between homologous chromosomes) occurs during prophase I of meiosis I.
Correct answer: Prophase I
Explain why meiosis produces genetically unique cells while mitosis does not.
Both mechanisms ensure sexual reproduction generates genetic variation.
Correct answer: Meiosis produces genetic uniqueness through two mechanisms: 1) crossing over in prophase I shuffles alleles between homologs; 2) independent assortment randomly allocates homologs to daughter cells (2²³ combinations for humans). Mitosis simply copies chromosomes and distributes them identically.