Question
Differentiate between mitosis and meiosis. Include location, number of divisions, chromosome number in daughter cells, crossing over, and the number of daughter cells produced.
This is a NEET 2024 favourite — it appears almost every year in some form, either as a direct table question or embedded in a longer assertion-reason format.
Solution — Step by Step
Mitosis occurs in all somatic (body) cells — skin, liver, bone marrow. Meiosis is restricted to germinal cells only: testes and ovaries. This location difference alone tells you the purpose of each division.
Mitosis = one division, producing two cells. Meiosis = two sequential divisions (Meiosis I + Meiosis II), producing four cells. The first division (Meiosis I) is reductive; the second (Meiosis II) is essentially mitotic.
In mitosis, the parent cell is diploid (2n) and both daughter cells are diploid (2n) — chromosome number is preserved. In meiosis, the parent cell is diploid (2n) but all four daughter cells are haploid (n) — chromosome number is halved.
Crossing over occurs only in Meiosis I (during Prophase I, at the chiasmata). This is why meiosis generates genetic variation. Mitosis has zero crossing over — daughter cells are genetically identical to the parent.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Location | Somatic cells | Germinal cells (gonads) |
| Number of divisions | 1 | 2 (Meiosis I + II) |
| Daughter cells | 2 | 4 |
| Ploidy of daughters | Diploid (2n) | Haploid (n) |
| Crossing over | Absent | Present (Prophase I) |
| Genetic variation | No | Yes |
| Purpose | Growth, repair | Gamete formation |
The key output: mitosis → 2 diploid cells; meiosis → 4 haploid cells.
Why This Works
The logic behind these differences comes down to purpose. Mitosis is for maintenance — your body needs to replace cells with exact copies. Any chromosome number change here would be catastrophic (think cancer, which is essentially uncontrolled mitosis gone wrong).
Meiosis serves reproduction. When two gametes (each haploid, n = 23 in humans) fuse during fertilisation, the resulting zygote is diploid (2n = 46). If gametes were diploid, every generation would double the chromosome count — that’s why the reductive division is essential.
Crossing over in Meiosis I is what gives offspring their unique genetic combinations. Without it, siblings would be genetically identical except for random assortment.
Alternative Method
For a quick NEET MCQ, use the “2 and 4 rule” mentally:
- Mitosis = 2 daughter cells, same ploidy
- Meiosis = 4 daughter cells, half ploidy
Then layer in: meiosis has crossing over, mitosis does not. This gets you through any single-statement true/false or fill-in-the-blank variant in under 15 seconds.
In assertion-reason questions, the reason for meiosis producing genetic variation is crossing over during Prophase I, not random assortment alone. Random assortment is a secondary source. Examiners specifically test whether you can distinguish these two mechanisms.
Common Mistake
Students often say meiosis produces “2 haploid cells.” That describes Meiosis I only. The complete process of meiosis produces 4 haploid cells (in males, 4 functional spermatids; in females, 1 egg + 3 polar bodies — but the division count is still 4). Writing “2 haploid cells” in a NEET answer will cost you the mark.
A second frequent error: confusing synapsis with crossing over. Synapsis is when homologous chromosomes pair up (form bivalents). Crossing over is the actual exchange of segments at the chiasmata — it happens after synapsis. These are two separate events in Prophase I.