Every gamete you made carries just one chromosome from each of your 23 pairs, and no two gametes carry quite the same set. Meiosis does both jobs: it halves the chromosome number and it mixes up the alleles you inherited from each parent.
A diploid (2n) cell has two complete sets of chromosomes, one set from each parent; a haploid (n) cell has one set. The two chromosomes of a homologous pair have the same genes at the same loci (but possibly different alleles), the same length and the same centromere position. Gametes must be haploid so that fertilisation restores the diploid number instead of doubling it each generation, so they are made by meiosis, a reduction division. In meiosis I homologous chromosomes pair up and are separated; in meiosis II sister chromatids are separated, giving four haploid cells. Gametes differ genetically because of crossing over between non-sister chromatids in prophase I, independent assortment (random orientation) of homologous pairs at metaphase I and of sister chromatids at metaphase II, and then random fertilisation.
Press Next stage and watch the two homologous pairs (red from one parent, blue from the other). Check the CELLS readout: one diploid cell with 2n = 4 at the start, two haploid cells after telophase I, four haploid cells with n = 2 at the end. Switch to the plant cell and compare.
| Stage | What the chromosomes, envelope, membrane and spindle do |
|---|---|
| prophase I | chromosomes condense; homologous chromosomes pair to form bivalents; crossing over at chiasmata; nuclear envelope breaks down; spindle forms |
| metaphase I | bivalents line up on the equator, attached to spindle fibres by their centromeres; orientation of each pair is random |
| anaphase I | homologous chromosomes are pulled to opposite poles; sister chromatids stay joined at the centromere |
| telophase I | nuclear envelopes may re-form; the cell divides into two haploid cells |
| prophase II | nuclear envelopes break down again; new spindles form, at right angles to the first |
| metaphase II | individual chromosomes line up on the equator of each cell |
| anaphase II | centromeres divide; sister chromatids are pulled to opposite poles |
| telophase II | nuclear envelopes re-form; cells divide, giving four haploid cells |
Do not mix up the two anaphases. In anaphase I whole homologous chromosomes separate (each still has two chromatids); in anaphase II the sister chromatids separate, just as in mitosis. The chromosome number halves in meiosis I, not meiosis II.
Choose crossing over and press Run it to see the chiasma and the four chromatids AB, Ab, aB and ab. Then choose independent assortment, slide the number of homologous pairs from 1 to 23 and read the GAMETE COMBINATIONS (2n) and ZYGOTE COMBINATIONS readouts.
Crossing over happens between non-sister chromatids of a homologous pair, in prophase I. Sister chromatids are identical copies, so swapping between them would change nothing.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
In the meiosis simulator, what does the CELLS readout show after telophase I?
At anaphase I in the simulator, which structures are pulled to opposite poles?
In the plant cell version, how does the cell divide at the end of telophase?
With the slider at 3 homologous pairs, how many different combinations of chromosomes can the gametes have by independent assortment alone?
In exam photomicrographs and diagrams you identify the stage from where the chromosomes are and how many groups there are.
| What you see | Stage |
|---|---|
| paired homologous chromosomes (bivalents) in one nucleus, chiasmata visible, envelope disappearing | prophase I |
| bivalents (pairs) on the equator of one cell | metaphase I |
| chromosomes, each still with two chromatids, moving to two poles of one cell | anaphase I |
| two cells (or a dividing cell), each with half the number of chromosomes | telophase I |
| single chromosomes on the equator in each of two cells | metaphase II |
| chromatids moving to poles in two cells at once | anaphase II |
| four groups of chromosomes / four nuclei (a tetrad of cells in plants) | telophase II |
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
A cell from an organism with 2n = 18 undergoes meiosis. How many chromosomes and chromatids are present in each cell at metaphase II?
Which statement explains why meiosis must be a reduction division in sexually reproducing organisms?
A photomicrograph shows two cells side by side, each with chromatids moving towards two poles. Which stage is shown?
How does crossing over increase genetic variation in gametes?
This skill is now recorded on your profile. Keep the chain going.