Cell Cycle and Cell Division
Mitosis, meiosis, significance.
Mitosis stages
Prophase, metaphase, anaphase, telophase, cytokinesis.
Meiosis I and II
Crossing over, reduction division, significance.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Where | All somatic cells | Germ cells only (testis, ovary) |
| Cell divisions | One | Two (Meiosis I + II) |
| Chromosome number | 2n → 2n | 2n → n (reduction division) |
| Daughter cells | 2 | 4 |
| Genetic content | Identical to parent | Genetically varied (crossing over) |
| Crossing over | NO | YES (Prophase I) |
| Synapsis (homologous pairing) | NO | YES (Prophase I) |
| Function | Growth, repair, asexual reproduction | Gamete formation, genetic diversity |
Stages of mitosis: Prophase → Metaphase → Anaphase → Telophase + Cytokinesis.
Prophase: chromatin condenses into chromosomes; nuclear envelope breaks down; spindle forms.
Metaphase: chromosomes line up at the metaphase plate (equator). Each chromosome has 2 sister chromatids joined at centromere.
Anaphase: sister chromatids separate; chromosomes move to opposite poles. This is when 2n stays 2n (each pole gets one of each pair).
Telophase: nuclear envelopes reform; chromosomes decondense.
Meiosis I (reduction division): separates HOMOLOGOUS chromosomes (different pairs). Chromosome number halves.
Prophase I has 5 sub-stages: Leptotene → Zygotene → Pachytene → Diplotene → Diakinesis.
- Zygotene: homologous chromosomes pair (synapsis), forming bivalents.
- Pachytene: crossing over at chiasmata — recombination of genetic material between non-sister chromatids.
- Diplotene: homologs start to separate but stay attached at chiasmata.
Metaphase I: bivalents (NOT individual chromosomes) line up.
Anaphase I: homologs separate. Sister chromatids stay together. This is the reduction step.
Telophase I + Cytokinesis: two haploid cells, each chromosome still has 2 chromatids.
Meiosis II: like mitosis but on haploid cells. Sister chromatids separate.
End result: 4 haploid cells, all genetically different from the original diploid.
Memory hook:
- Mitosis: 1 cell → 2 identical cells (think "twins").
- Meiosis: 1 cell → 4 unique cells (think "kids in a family — same parents, different traits").
JEE/NEET trap. "Crossing over occurs during ___?" Answer: pachytene of prophase I. NOT metaphase I.
Chromosome behavior summary:
- Mitosis Anaphase: sister chromatids of each chromosome separate.
- Meiosis I Anaphase: homologous chromosomes separate (sister chromatids STAY together).
- Meiosis II Anaphase: sister chromatids separate (like mitosis).