meiosis
Meaning of meiosis: The division that halves the chromosome count and shuffles the deck, so no two eggs or sperm carry the same combination.
Definition of meiosis
Meiosis is the specialized cell division that produces eggs and sperm. One round of DNA copying is followed by two divisions instead of one, so each of the four cells produced carries half the usual number of chromosomes. Fertilization restores the full set. The process also rearranges genetic material along the way, so every cell it makes is genetically distinct.
What happens in the two divisions?
In the first division, the matching chromosomes inherited from each parent find each other and pair up. While paired, they swap segments, an exchange called crossing over. The pairs then separate to opposite ends of the cell, and it is this step, not the second, that halves the count. The second division resembles ordinary cell division: the two copies of each remaining chromosome are pulled apart. The result in men is four sperm cells. In women only one of the four becomes an egg, and the other three are discarded as small polar bodies that carry away the extra chromosomes.
How does it differ from mitosis?
| Feature | Mitosis | Meiosis |
|---|---|---|
| Divisions | One | Two |
| Cells produced | Two | Four |
| Chromosome number | Unchanged | Halved |
| Genetic result | Identical copies | Every cell different |
| Where it happens | Body tissues throughout life | Testis and ovary only |
Why does variety arise?
Two mechanisms work together. Crossing over swaps stretches of DNA between matching chromosomes, producing chromosomes that are mosaics of both parents. Independent assortment means each pair lines up without regard to how the others are arranged, so the parental chromosomes are dealt out in a fresh combination each time. Between them the number of possible outcomes from a single person is astronomically large, which is why siblings resemble one another without being alike.
What goes wrong?
The commonest error is nondisjunction, in which a pair fails to separate and one cell ends up with an extra chromosome while the other lacks one. If such a cell is fertilized the embryo carries three copies or one copy of that chromosome, the basis of Down syndrome and other chromosome disorders. Errors of this kind become more frequent in eggs with advancing age, because eggs begin the process before birth and remain paused partway through it for decades before completing it.
Also known as
Used in a sentence
Nondisjunction during meiosis produces a gamete carrying an extra copy of a chromosome.
Good to know
Meiosis is easily confused in writing with miosis, the constriction of the pupil; the two words differ by a single letter and share no meaning at all.
Word origin
Where “meiosis” comes from
From the Greek meiosis, meaning a lessening, from meioun, to make smaller, describing the halving of the chromosome number.