Mitosis and Meiosis Explained Simply
Both processes divide cells, but they pursue completely different goals. Mitosis produces two exact copies — it is the division for growth and repair. Meiosis produces four different cells with half the chromosome set — it is the division for reproduction. Remember that one difference and you already have the core of the topic.
Chromosome — packaged DNA. Humans have 46, that is 23 pairs.
Diploid (2n) — every chromosome is present twice, once from the mother and once from the father.
Haploid (n) — only one set. That is how egg and sperm cells are built.
Chromatid — one of the two identical halves of a duplicated chromosome.
Homologous chromosomes — the maternal and paternal copy of the same chromosome.
Crossing over — the exchange of pieces between homologous chromosomes during meiosis.
Interphase — the time between two divisions. This is when the DNA is duplicated.
Mitosis: the same thing twice
Before every division the cell duplicates its DNA. Each chromosome then consists of two identical chromatids that are still joined together. In mitosis exactly these chromatids are separated and shared between two daughter cells. The result: two cells with the same genetic material as the mother cell.
The process runs through four nameable stages. In prophase the chromosomes condense until they become visible under the microscope as X shapes, and the nuclear envelope dissolves. In metaphase all the chromosomes line up in a single plane in the middle — that arrangement is the most reliable way to recognise the stage in an exam picture. In anaphase spindle fibres pull the chromatids towards the poles. In telophase two new nuclei form, and afterwards the cell pinches itself in two.
| Mitosis | Meiosis | |
|---|---|---|
| Number of divisions | one | two in a row |
| Daughter cells | 2 | 4 |
| Chromosome set | 2n, stays the same | n, is halved |
| Genetic material | identical | different every time |
| Crossing over | no | yes, in prophase I |
| Where in the body | almost everywhere | only in testes and ovaries |
| What for | growth, repair, replacement | germ cells, variation |
Meiosis: halve and shuffle
Meiosis has two jobs. First it has to halve the chromosome set, otherwise a child would have twice as many chromosomes as its parents. Second it makes sure that no two germ cells are alike.
In the first meiotic division it is not the chromatids that are separated but the homologous chromosomes — the maternal one goes into one cell, the paternal one into the other. Which copy goes where is decided independently for each of the 23 pairs. That alone gives 223, more than eight million different combinations. Before that, crossing over also takes place: homologous chromosomes lie alongside each other and swap sections. The chromosomes themselves are therefore already reshuffled, and the number of possibilities becomes effectively unlimited.
The second meiotic division then runs like a mitosis: the chromatids are separated. Two cells become four, each with a single chromosome set.
Distribution of the 23 pairs: 223 = 8,388,608 combinations
Egg and sperm cell together: 223 · 223 ≈ 7 · 1013
Crossing over comes on top of that — which is why no two siblings are alike.
💬 Mia asks Grandpa Theo
Why isn't mitosis enough on its own? Then all cells would be the same and everything would be much simpler.
For your own body it is enough. But imagine your egg cell had 46 chromosomes and the sperm cell did too — the child would have 92, the grandchild 184. After a few generations it would be over. So somewhere the number has to be halved.
And the shuffling is just a nice side effect?
On the contrary, that is the second main purpose. If all offspring were identical, a single disease could wipe out an entire species. Variation is an insurance policy.
Mixing up mitosis and meiosis I. In mitosis chromatids are separated, in meiosis I whole chromosomes are.
Believing the DNA is duplicated again before the second division. It is duplicated only once, before meiosis I.
Putting crossing over into mitosis. It belongs exclusively to prophase I.
Treating chromosome and chromatid as the same thing. A duplicated chromosome consists of two chromatids but is still one chromosome.
A human cell with 46 chromosomes goes through meiosis. How many chromosomes and how many chromatids does a cell contain before meiosis I, after meiosis I and after meiosis II?
Show solution
Before meiosis I (after DNA duplication): 46 chromosomes, each
made of 2 chromatids, so 92 chromatids.
After meiosis I: the homologous chromosomes have been separated, so
23 chromosomes per cell — but each still consists of 2 chromatids, so
46 chromatids. The cell is already haploid even though it still holds
twice the DNA of a finished gamete. This intermediate state is a favourite exam
question.
After meiosis II: the chromatids have been separated. 23 chromosomes
of 1 chromatid each, so 23 chromatids. That is the finished germ
cell.
If a pair of chromosomes fails to separate cleanly during meiosis, it is called non-disjunction. The germ cell then has one chromosome too many or too few — the best known consequence is trisomy 21.