Question
Differentiate between mitosis and meiosis. Compare them on the basis of number of divisions, daughter cells produced, chromosome number, crossing over, and biological significance.
(NCERT Class 11, very frequently asked in NEET)
Solution — Step by Step
Mitosis produces two genetically identical daughter cells with the same chromosome number as the parent. It is used for growth, repair, and asexual reproduction.
Meiosis produces four genetically different daughter cells with half the chromosome number. It is used to form gametes (eggs and sperm) for sexual reproduction.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 (meiosis I + meiosis II) |
| Daughter cells | 2 | 4 |
| Chromosome number | Same as parent (2n → 2n) | Halved (2n → n) |
| DNA replication | Before division | Before meiosis I only (not before meiosis II) |
| Synapsis | Does not occur | Occurs in Prophase I (homologous pairing) |
| Crossing over | Does not occur | Occurs in Prophase I (pachytene) |
| Genetic identity | Daughter cells are identical to parent | Daughter cells are genetically unique |
| Chiasmata | Absent | Present |
| Metaphase alignment | Individual chromosomes at equator | Bivalents at equator (Meiosis I); individual chromosomes (Meiosis II) |
| Centromere splitting | Anaphase | Anaphase II (not in Anaphase I) |
| Where it occurs | Somatic cells | Reproductive cells (germ cells) |
Significance of mitosis:
- Growth of multicellular organisms (from a single zygote to trillions of cells)
- Repair and regeneration of damaged tissues
- Asexual reproduction (e.g., vegetative propagation in plants)
- Maintains genetic stability — daughter cells are exact copies
Significance of meiosis:
- Produces haploid gametes, maintaining the chromosome number across generations (without meiosis, the chromosome number would double every generation)
- Generates genetic variation through crossing over and independent assortment
- Provides raw material for natural selection and evolution
Why This Works
The fundamental difference is purpose. Mitosis maintains the genome — it is a conservative, copying process. Meiosis shuffles the genome — it is a diversifying process. Both are essential: mitosis for building and maintaining the body, meiosis for creating genetically diverse offspring.
The reduction in chromosome number during meiosis is achieved by separating homologous chromosomes in Meiosis I (reductional division). Meiosis II then separates sister chromatids (equational division, similar to mitosis). This two-step process ensures gametes have exactly half the chromosomes.
Quick recall for NEET: Meiosis I is the reductional division (2n → n, homologous chromosomes separate). Meiosis II is the equational division (n → n, sister chromatids separate). This means the actual chromosome number reduction happens in Meiosis I, not Meiosis II.
Common Mistake
The most common error: writing that “crossing over occurs in mitosis.” Crossing over is exclusive to meiosis (Prophase I). In mitosis, there is no synapsis (homologous pairing), so crossing over cannot occur. If a NEET question asks “which process does NOT occur in mitosis?” — crossing over and synapsis are the top answers.
Another frequent mistake: stating that meiosis produces 2 cells. Meiosis produces 4 haploid cells. Two rounds of division (meiosis I + meiosis II) starting from one diploid cell give: 1 → 2 (after meiosis I) → 4 (after meiosis II). In oogenesis, only one functional egg is produced (the other three become polar bodies), but four cells are still formed.