Mitosis and meiosis compared, and why meiosis matters

Two divisions side by side

Mitosis and meiosis both start with copied chromosomes. They end in very different ways.

• Where. Mitosis: body (somatic) cells, and haploid cells of some plants and insects. Meiosis: special diploid cells that make gametes or spores.
• Divisions. Mitosis: one. Meiosis: two.
• DNA copying. Mitosis: once for each division. Meiosis: once for two divisions.
• Daughter cells. Mitosis: two. Meiosis: four.
• Chromosome number. Mitosis: stays the same (equational). Meiosis: halved (reduction).
• Pairing of homologues. Mitosis: none. Meiosis: synapsis in prophase I.
• Crossing over. Mitosis: none. Meiosis: in pachytene.
• Metaphase. Mitosis: single chromosomes on the plate. Meiosis: bivalents on the plate in metaphase I.
• Anaphase. Mitosis: the centromeres split. Meiosis: the homologues separate in anaphase I, and the centromeres split in anaphase II.
• The daughter cells. Mitosis: identical to the parent cell. Meiosis: different from the parent cell and from each other.
• Job. Mitosis: growth and repair. Meiosis: gametes or spores, with variation.

Why meiosis matters

It keeps the chromosome number steady. Meiosis halves the number, and fertilisation brings it back. So each species keeps its own number from one generation to the next, even though the process itself paradoxically reduces the number by half.

It makes variation. There are three sources.

• Crossing over in pachytene makes chromatids with genes from both parents.
• Independent assortment. In metaphase I, every bivalent lines up in one of two ways, and the bivalents choose independently. A cell with $n$ pairs can make $2^n$ kinds of gamete. In a human, $n = 23$, which gives over 8 million kinds, before crossing over adds more.
• Random fertilisation. Any sperm can meet any egg. Two parents who each make about 8 million kinds of gamete can make about 70 trillion different zygotes.

Variations are important for the process of evolution. Mitosis cannot make them, because its daughter cells are copies.

Where does meiosis take place?

The place depends on the life cycle.

• Gametic meiosis. In animals, meiosis in the gonads makes the gametes directly.
• Sporic meiosis. In plants, meiosis in the spore mother cells of the sporophyte makes haploid spores. The spores grow by mitosis into the gametophyte, and the gametophyte makes gametes by mitosis.
• Zygotic meiosis. In many fungi and some algae, the zygote divides by meiosis soon after fertilisation, and the body is haploid.

When the separation fails

Sometimes a pair of homologues fails to separate in anaphase I, or sister chromatids fail to separate in anaphase II. This is non-disjunction. It makes gametes with one chromosome too many or too few. After fertilisation, the zygote may have 47 chromosomes, as in Down syndrome (an extra chromosome 21), or 45 chromosomes, as in Turner syndrome.

You should see that the number of kinds of gametes climbs very fast. Seven pairs already give 128 kinds. Twenty three pairs give more than 8 million.

Try it in Python

The program counts the kinds of gamete and zygote for four organisms.

Practise with new numbers

Common mistakes

• Saying that mitosis makes variation. Its daughter cells are copies of the parent cell.
• Saying that the daughter cells of meiosis are identical. They differ, because of crossing over and independent assortment.
• Saying that meiosis takes place in all body cells. Only the special diploid cells that make gametes or spores divide by meiosis.
• Saying that meiosis keeps the number of chromosomes the same. It halves the number. Fertilisation restores it.
• Adding instead of multiplying. The kinds of gametes grow as $2^n$, not as $2n$.

Remember these

• Mitosis: one division, two identical diploid cells. Meiosis: two divisions, four haploid cells that differ.
• Meiosis keeps the chromosome number constant across generations and makes variation.
• Variation comes from crossing over, independent assortment ($2^n$ kinds of gamete) and random fertilisation.
• Non-disjunction gives gametes with $n + 1$ or $n - 1$ chromosomes.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Give three differences between mitosis and meiosis.

2. Name three sources of variation in sexual reproduction.

3. An organism has a haploid number of 4. How many kinds of gamete can it make by independent assortment alone?

4. What is non-disjunction? What can its result be in humans?

5. In a fern, in which cells does meiosis take place, and what does it make?

References

• NCERT (2023). Biology, Textbook for Class XI, chapter 10.
• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 11, Meiosis and Sexual Reproduction.