Meiosis vs mitosis: what's the difference?
Mitosis divides a cell once to make two genetically identical diploid cells, used for growth and repair. Meiosis divides twice to make four genetically different haploid cells, the gametes used in sexual reproduction.
Watch it explained
How is meiosis different from mitosis?
Follows one cell down two paths: mitosis makes two identical diploid copies (2n to 2n), while in meiosis homologous chromosomes pair up and cross over, the pairs split in meiosis I, and the cell ends as four genetically unique haploid gametes (2n to n).
Scenes, in order:
- Same cell, two fates
- Mitosis: perfect copies
- Partners pair up
- Pairs split, not copies
- Four unique gametes
Side by side
- Purpose: mitosis makes body (somatic) cells for growth, repair and asexual reproduction; meiosis makes eggs and sperm (or spores in plants and fungi).
- Divisions: mitosis has one; meiosis has two (meiosis I and meiosis II), after a single round of DNA replication.
- Chromosome number: in humans, mitosis goes from 46 to 46; meiosis goes from 46 to 23.
- Pairing: homologous chromosomes never pair in mitosis. In prophase I of meiosis they pair up (synapsis) and swap segments (crossing over).
- Result: two identical cells from mitosis; four cells from meiosis, each with a different mix of alleles.
Where the variety comes from
Meiosis makes genetically unique cells in two ways. Crossing over in prophase I exchanges pieces between a maternal and a paternal chromosome, so each chromatid becomes a new combination. Then in metaphase I the homologous pairs line up at random, each pair independently of the others (independent assortment). With 23 pairs in humans, that alone allows 2^23 combinations, about 8.4 million, before crossing over adds more. Fertilization, which joins two such gametes, multiplies the variety again.
The key line to remember: meiosis I separates homologous chromosomes, so it is the division that halves the chromosome number. Meiosis II separates sister chromatids, much like mitosis does.
Mistakes that cost points
- Saying DNA replicates again before meiosis II. It does not; replication happens once, before meiosis I.
- Calling the cells after meiosis I diploid. They are already haploid, even though each chromosome still has two sister chromatids.
- Counting chromatids as chromosomes. Count centromeres: one centromere, one chromosome.
- Mixing up anaphase I (homologs move apart) with anaphase II and mitotic anaphase (sister chromatids move apart).
For how this connects to inheritance and to copying DNA in the S phase, see DNA replication and the AP Biology unit on heredity.
Watch the two paths
The film on this page follows one cell down both routes: mitosis making two identical diploid copies, then homologs pairing, crossing over and splitting in meiosis I to end as four unique gametes. If a step still feels slippery, ask Isotoon about that step in your own words, such as why crossing over happens before the pairs separate. More on cells and inheritance is on the biology page.
Questions
- Is meiosis I or meiosis II more like mitosis?
- Meiosis II. Like mitosis, it pulls sister chromatids apart. Meiosis I is the unusual division, because it separates homologous chromosomes instead.
- Do haploid cells ever divide by mitosis?
- Yes. In plants, fungi and many algae, haploid cells divide by mitosis as part of the life cycle. Mitosis keeps whatever chromosome number the cell already has.
- Why do we need meiosis at all?
- Without halving the chromosome number, each generation's fertilization would double it. Meiosis keeps the number constant across generations and shuffles alleles so offspring differ.