Fertilisation doubles the chromosome number every generation. Something has to halve it again, or the number would climb forever. That job belongs to meiosis — and while it is halving, it also shuffles the genetic deck.
📘 What you need to know
Meiosis produces four haploid (n) nuclei that are genetically different from each other and from the parent. It makes gametes.
There are two rounds of division: meiosis I and meiosis II. The chromosomes replicate before meiosis I and not in between.
The chromosome number halves in meiosis I, which is why it is called reduction division.
In prophase I, homologous chromosomes pair up as a bivalent and crossing over occurs at points called chiasmata.
Variation comes from crossing over and from the random orientation of bivalents at the equator.
Non-disjunction is a failure to separate correctly, giving gametes with the wrong number of chromosomes — as in Down syndrome (trisomy 21).
Two divisions, one replication
The nucleus of the original parent cell is diploid (2n): it holds two sets of chromosomes, one from each parent. Before meiosis starts, those chromosomes replicate, so each is a pair of sister chromatids.
Then the nucleus divides twice.
Two divisions but only one round of replication — that is the whole reason the chromosome number ends up halved.
Meiosis I
The homologous pairs of chromosomes are split up, giving two haploid (n) nuclei.
At this point each chromosome still consists of two chromatids.
Because the pairs are separated here, the chromosome number goes from 2n to n in the first division. This is the reduction division.
Spindle fibres attach to the centromeres of the bivalents and pull the homologous chromosomes to opposite poles. The chromatids of each chromosome stay joined.
Meiosis II
There is no replication between meiosis I and meiosis II.
The chromatids that make up each chromosome now separate, giving four haploid nuclei.
Each chromosome now consists of a single chromatid.
Exam favourite: “In which division does the chromosome number halve?” The answer is meiosis I, every time. Meiosis II is much more like mitosis — it separates chromatids and does not change the number.
Crossing over
During prophase I, homologous chromosomes pair up very closely. A pair like this is called a bivalent, and the pairing process is synapsis. Since replication has already happened, each chromosome is two sister chromatids — so a bivalent contains four DNA molecules.
Two non-sister chromatids (one from each homologous chromosome) form a junction.
At that junction, called a chiasma, the two chromatids break and rejoin with each other.
The break happens at exactly the same position on both, so whole genes are exchanged cleanly.
Non-sister chromatids are homologous but not genetically identical, so some of the alleles swapped over are different versions.
Only the two chromatids involved in the chiasma are changed. Their sisters carry on unaltered, which is why a bivalent can produce four different chromatids.
The result is chromatids carrying completely new combinations of alleles that were not present in the parent cell — these are called recombinant chromosomes. Since those chromatids end up in different gametes, crossing over is a major source of genetic variation.
Random orientation of bivalents
At metaphase of meiosis I, the bivalents line up at the equator. Spindle microtubules grow out from the poles and attach to the centromeres, and the two homologous chromosomes in each bivalent are attached to different poles.
Which chromosome ends up facing which pole depends on how the bivalent happened to land. That orientation is completely random, and each bivalent orients independently of every other one.
Two bivalents give four combinations. Add a third and it doubles again — the numbers grow very fast.
The number of possible chromosome combinations from random orientation is 2n, where n is the number of homologous pairs.
Combinations from random orientation
possible combinations = 2n (n = number of homologous pairs)
Humans have 46 chromosomes, so 23 homologous pairs. That gives 223 = 8 388 608 possible combinations from random orientation alone — before crossing over adds any more.
Why a sexual life cycle needs meiosis
In sexual reproduction, the nuclei of two gametes fuse to form a zygote. That fusion is fertilisation, and it doubles the chromosome number each time it happens.
So the number has to be halved at some stage, or it would double every generation without limit. In animals, that halving happens during meiosis, when the gametes are made.
The balance that keeps the number stable
meiosis: 2n → n then fertilisation: n + n → 2n
In an asexual life cycle there is no fusion of gametes, so the offspring are genetically identical to the parent and no halving is needed. In a sexual life cycle the offspring are genetically distinct from each other and from both parents.
Non-disjunction and Down syndrome
Non-disjunction is when chromosomes fail to separate correctly during meiosis. It can happen in anaphase I (homologous chromosomes fail to separate) or in anaphase II (chromatids fail to separate).
The gametes produced have an abnormal number of chromosomes — one with an extra copy of a chromosome, and one with no copy at all.
If such a gamete is fertilised, the zygote has the wrong number of chromosomes, and so does every cell that grows from it.
Both faulty gametes are a problem, but they cause different conditions — an extra chromosome and a missing one are not the same thing.
Down syndrome
Down syndrome is also called trisomy 21. Non-disjunction of the 21st pair of homologous chromosomes in anaphase I leaves a gamete with an extra chromosome 21.
The individual has three copies of chromosome 21, so 47 chromosomes in total instead of 46.
The impact varies between individuals. Common features include physical growth delays and reduced intellectual ability, and there can be issues with sight or hearing.
Other trisomies exist: Patau syndrome (trisomy 13) and Edwards syndrome (trisomy 18) are much more serious.
The risk of a chromosomal abnormality rises with maternal age, since non-disjunction is more likely in older ova.
Karyotyping of foetal cells can identify these abnormalities. The cells are obtained by amniocentesis or by chorionic villus sampling.
Worked examples
WORKED EXAMPLE
Count the chromosomes through meiosis
A cell contains 12 chromosomes. (a) How many chromosomes are in each nucleus after meiosis I? (b) How many chromatids does each of those chromosomes have? (c) How many chromosomes are in each nucleus after meiosis II?
(a) After meiosis I
Homologous pairs are separated, so the number halves.
12 ÷ 2 = 6 chromosomes(b) Chromatids at that point
The chromatids have not separated yet.
2 chromatids each(c) After meiosis II
Chromatids separate, but the chromosome number does not change again.
6 chromosomes, each one chromatid6, then 2 chromatids each, then still 6Meiosis II does not halve anything. That is the trap in this question.
WORKED EXAMPLE
Combinations from random orientation
A species has 8 chromosomes in its body cells. Calculate the number of different chromosome combinations possible in its gametes as a result of random orientation.
Step 1: Find the number of homologous pairs8 ÷ 2 = 4 pairs, so n = 4Step 2: Apply the formula2n = 24 = 1616 possible combinationsn is the number of pairs, not the number of chromosomes. Halve first, then raise 2 to that power.
WORKED EXAMPLE
Explain an abnormal karyotype
A karyogram shows three copies of chromosome 21 and 47 chromosomes in total. Explain how this arose.
Step 1: Name the errorNon-disjunction during meiosis, in this case of the 21st pair in anaphase I.
Step 2: The gamete produced
One gamete received both copies of chromosome 21 instead of one.
Step 3: After fertilisation
That gamete fused with a normal gamete carrying one copy, giving three copies in the zygote and in every cell that grew from it.
Non-disjunction giving trisomy 21Show the arithmetic if you can: 2 + 1 = 3 copies, and 47 chromosomes overall.
💡 Exam tip
State clearly that the chromosome number halves in meiosis I. It is the single most tested fact on this page.
Use the proper terms: bivalent, synapsis, chiasma, non-sister chromatids, recombinant.
For variation questions give both mechanisms — crossing over and random orientation — unless told otherwise.
For 2n calculations, halve the chromosome number first to get n.
Say non-sister chromatids for crossing over. Sister chromatids are identical, so swapping between them would change nothing.
Link meiosis to fertilisation: halving exists because fusion doubles.
⚠ Common mix-up
Saying the number halves in meiosis II. It halves in meiosis I.
Thinking DNA replicates again between the two divisions. It does not — one replication, two divisions.
Confusing homologous chromosomes with sister chromatids. Homologous chromosomes are a matching pair from two parents; sister chromatids are identical copies.
Writing that crossing over happens between sister chromatids. It is between non-sister chromatids of a bivalent.
Describing meiosis as producing two cells. Two divisions give four haploid nuclei.
Saying non-disjunction only happens in anaphase I. It can happen in anaphase II as well.
That completes Cell & Nuclear Division. Mitosis copies, meiosis shuffles and halves — and between them they explain how a body is built and how the next generation is different from the last.
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