Question
List the differences between mitosis and meiosis in a tabular form. Where does each occur, and what is the biological significance of each?
Solution — Step by Step
Mitosis is a type of cell division where one parent cell divides to produce two genetically identical daughter cells, each with the same chromosome number as the parent.
Meiosis is a specialised type of cell division where one parent cell produces four genetically diverse daughter cells, each with half the chromosome number of the parent.
These two processes serve completely different biological purposes — understanding WHY they differ makes the differences easier to remember.
Mitosis occurs in:
- All somatic (body) cells
- During growth and development
- During repair and regeneration of damaged tissue
- During asexual reproduction in some organisms
Meiosis occurs in:
- Gonads (testes and ovaries) for gamete production
- Specifically in cells called gametocytes (spermatocytes and oocytes)
- Also in microspore mother cells and megaspore mother cells in plants
The key: meiosis only happens where gametes (sex cells) are made.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 (Meiosis I + Meiosis II) |
| Number of daughter cells | 2 | 4 |
| Chromosome number in daughter cells | Same as parent (diploid, 2n) | Half of parent (haploid, n) |
| Genetic identity of daughters | Genetically identical (clones) | Genetically different |
| Pairing of homologous chromosomes | Does NOT occur | Occurs in Meiosis I (synapsis) |
| Crossing over | Does NOT occur | Occurs in Prophase I (chiasmata formed) |
| Occurs in | Somatic cells | Germ cells (gonads) |
| Purpose | Growth, repair, asexual reproduction | Sexual reproduction (gamete formation) |
| Duration | Shorter | Longer |
| DNA replication | Once, before division | Once, before Meiosis I only |
| Prophase duration | Short (minutes to hours) | Long (Prophase I can last days to years) |
| End result in humans | 2 cells with 46 chromosomes | 4 cells with 23 chromosomes |
Meiosis I is the reductive division — chromosome number is halved here. Key events:
Prophase I — the most complex and important stage:
- Synapsis: Homologous chromosomes pair up, forming bivalents (tetrads)
- Crossing over: Exchange of segments between non-sister chromatids of homologs at chiasmata
- This is the source of genetic recombination and variation
Metaphase I: Bivalents align at the cell plate
Anaphase I: Homologous chromosomes separate (NOT sister chromatids — that happens in Anaphase II)
Telophase I: Two haploid cells formed (but chromatids still attached)
Meiosis II resembles mitosis — separates sister chromatids.
Significance of Mitosis:
- Maintains constant chromosome number across generations of somatic cells
- Enables growth: from a single fertilised egg to ~37 trillion cells in an adult human
- Enables repair: skin cells, gut lining cells, blood cells are constantly replaced via mitosis
Significance of Meiosis:
- Reduces chromosome number by half → gametes are haploid → when two gametes fuse (fertilisation), the diploid number is restored in every generation
- Crossing over during Prophase I creates new combinations of alleles → genetic variation → raw material for evolution
- Random assortment of chromosomes during Meiosis I also generates variation
Why This Works
The fundamental reason for two types of division:
Mitosis serves continuity within an individual. Every cell in your body needs the full genome, so chromosome number must be maintained.
Meiosis serves continuity across generations. Sexual reproduction requires combining genomes from two parents. If gametes had the full 2n chromosome number and then fused, offspring would have 4n, then 8n — chromosomes would double every generation. Meiosis cuts the number to so fertilisation restores 2n.
Crossing over is a bonus: it shuffles alleles within chromosomes, creating combinations that never existed before. This is why full siblings look different (except identical twins, who arise from one fertilised egg splitting by mitosis).
Alternative Method — Mnemonic for Differences
“Meiosis Makes More Variation”:
- More divisions (2 vs 1)
- More cells (4 vs 2)
- More variation (crossing over, random assortment)
- Value/purpose: for sexual reproduction
NEET asks both direct questions (“How many cells does meiosis produce?”) and application questions (“A primary spermatocyte undergoes meiosis — what are the products?”). The answer: 4 spermatids (haploid), which mature into spermatozoa. A primary oocyte undergoes meiosis to produce 1 egg cell + 2–3 polar bodies (polar bodies disintegrate). This asymmetric result in females is frequently tested.
Common Mistake
Students often say “mitosis produces haploid cells and meiosis produces diploid cells” — this is the exact reverse. Mitosis: diploid parent → 2 diploid daughters (2n → 2n). Meiosis: diploid parent → 4 haploid daughters (2n → n). The confusion arises because both start from a diploid cell; the difference is in the product. Remember: meiosis = reduction division = halving of chromosome number.