IMAT Biology · Cell Division and Reproduction

Meiosis, Chromosome Numbers and Variation

Meiosis is two divisions after one round of DNA copying, so one diploid cell makes four haploid cells that are genetically different from each other.

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Why this matters

Meiosis is asked as often as mitosis. The papers want chromosome numbers in the zygote and in cells made by each division, the state of a cell before and during meiosis, which stage a description shows, what crossing over produces, and (in 2026) what tells meiosis apart from mitosis.

Concept 1 of 4: The stages of meiosis I and meiosis II

Meiosis I separates the two members of each homologous pair, so each new cell gets one of every chromosome: that is where the number halves. Meiosis II then splits each chromosome into its two chromatids, exactly like mitosis. The DNA is copied once, before meiosis I, and never between the two divisions.

Definition

Meiosis is a reduction division: one diploid cell gives four haploid cells.

Prophase I: homologous chromosomes pair up (synapsis) to form bivalents (tetrads: two chromosomes, four chromatids). Crossing over happens at points called chiasmata.
Meiosis I separates homologous chromosomes. Meiosis II separates sister chromatids.
Between the two divisions (interkinesis) there is no S phase.
In animals meiosis happens in the gonads and makes gametes; in plants it happens in sporangia and makes spores.

Stage	What happens	Chromosomes in each cell
Prophase I	Chromosomes condense, homologues pair into bivalents, crossing over	2n, each with two chromatids
Metaphase I	Bivalents (pairs) line up on the equator, each pair facing a random pole	2n, as pairs
Anaphase I	Homologous chromosomes go to opposite poles; sister chromatids stay joined	2n in the cell, n moving to each pole
Telophase I	Two cells form	n, each still with two chromatids
Metaphase II	Chromosomes line up singly on the equator	n, each with two chromatids
Anaphase II	Centromeres split; sister chromatids go to opposite poles	n moving to each pole
Telophase II	Four haploid cells form	n, each with one chromatid

Pairs on the equator means meiosis I; single chromosomes on the equator means mitosis or meiosis II.

Practice this conceptself-check · 4 quick reps

Homologous pairs moving apart means anaphase I

If whole chromosomes (each still two chromatids) are moving to the poles in pairs that have split, the cell is in anaphase I. In mitosis and in anaphase II it is single chromatids that move. Look at what is being separated, not just at the movement.

Concept 2 of 4: Chromosome number (n) and DNA content (c) at each stage of meiosis

Track two numbers separately. The chromosome number halves once, in meiosis I. The DNA content doubles once, in S phase, and then halves twice. Fertilisation then joins two haploid gametes and restores the diploid number.

Definition

For a species with diploid number 2n:

Before meiosis (G1): 2n chromosomes, DNA 2c. After S phase: still 2n, but 4c.
After meiosis I: each cell is haploid (n chromosomes), but each chromosome still has two chromatids, so 2c.
After meiosis II: each of the four cells has n chromosomes and 1c.
The four cells together hold 4n chromosomes, double the 2n of the original cell, because DNA was copied once and the cell divided twice.
Fertilisation: $n + n = 2n$ . The zygote, and every body cell made from it by mitosis, is diploid.

Ploidy and DNA through meiosis

2n,\,4c \xrightarrow{\text{meiosis I}} n,\,2c \xrightarrow{\text{meiosis II}} n,\,c

$n$ haploid number: one set of chromosomes
$c$ DNA content of one unreplicated set

Worked example

An animal has

2n = 14

. For one cell that goes through meiosis, give the chromosomes, chromatids and DNA content (in c) after S phase, after meiosis I and after meiosis II.

Practice this conceptself-check · 4 quick reps

A cell after meiosis I is haploid, even though its DNA is 2c

Ploidy counts chromosome sets, not DNA. After meiosis I each cell has one chromosome from every pair, so it is haploid. Its chromosomes still have two chromatids, so its DNA content equals that of a diploid G1 cell. Calling it diploid because of the DNA is wrong.

Daughter cells of mitosis are not haploid

Mitosis keeps the parent's number: a 2n cell gives 2n daughters. Only meiosis halves it. A zygote is 2n, every cell mitosis makes from it is 2n, and only the cells meiosis makes are n.

Concept 3 of 4: Sources of variation in meiosis: crossing over, independent assortment, random fertilisation

Meiosis shuffles the genes three ways. Crossing over swaps pieces between the maternal and paternal chromosome of a pair. Independent assortment lets each pair face either pole at random. Then any sperm can meet any egg. Together they make almost every gamete and every zygote unique.

Definition

Three sources of genetic variation in sexual reproduction:

Crossing over (prophase I): non-sister chromatids of a homologous pair exchange segments. One crossover gives 2 recombinant and 2 parental chromatids out of the 4, and the four cells made at the end each get a different chromatid.
Independent assortment (metaphase I): each pair lines up facing either pole at random, so a cell with haploid number n can make $2^n$ chromosome combinations.
Random fertilisation: any one of the male's gametes can fuse with any one of the female's.
New alleles themselves come only from mutation; meiosis reshuffles alleles that already exist.

Gamete combinations from independent assortment

\text{combinations} = 2^{n}

$n$ haploid number (number of homologous pairs)

Worked example

An insect has 4 pairs of chromosomes. Ignoring crossing over, how many chromosome combinations can its gametes have? How many combinations can a zygote of two such insects have?

Practice this conceptself-check · 4 quick reps

Use n, not 2n, in 2 to the power n

Each homologous PAIR has two ways to face the poles, so the power is the number of pairs, which is the haploid number. Using the diploid number squares the true answer.

Crossing over is between non-sister chromatids

Sister chromatids are identical, so swapping between them would change nothing. Crossing over happens between a chromatid of the maternal chromosome and one of the paternal chromosome, in prophase I only. It does not happen in mitosis.

Concept 4 of 4: Mitosis compared with meiosis

Both divisions copy DNA once and use a spindle of microtubules. The difference is the goal: mitosis makes copies for growth and repair, meiosis makes varied haploid cells for sexual reproduction. Every difference in the table follows from that.

Definition

Mitosis keeps the chromosome number and gives identical cells; meiosis halves it and gives genetically varied cells.

Both are preceded by one S phase, and both use spindle microtubules.
Both happen in single-celled eukaryotes too: yeast and many algae undergo meiosis.
Prokaryotes do neither: bacteria divide by binary fission.

Feature	Mitosis	Meiosis
Divisions after one S phase	One	Two
Cells made from one cell	Two	Four
Chromosome number	Same as parent (2n gives 2n)	Halved (2n gives n)
Genetic make-up	Identical to parent and to each other	Different from parent and from each other
Homologues pair, crossing over	Never	In prophase I
Where in humans	Somatic (body) cells everywhere	Germ cells in the testes and ovaries
Role	Growth, repair, asexual reproduction	Making gametes (animals) or spores (plants)

Practice this conceptself-check · 4 quick reps

Meiosis is not limited to multicellular organisms or to special machinery

Single-celled eukaryotes such as yeast reproduce sexually using meiosis, and both mitosis and meiosis build a spindle from microtubules. The real differences are the number of divisions, pairing and crossing over, and haploid, varied products.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Chromosome number (n) and DNA content (c) at each stage of meiosis
Ploidy and DNA through meiosis
$2n,\,4c \xrightarrow{\text{meiosis I}} n,\,2c \xrightarrow{\text{meiosis II}} n,\,c$
Sources of variation in meiosis: crossing over, independent assortment, random fertilisation
Gamete combinations from independent assortment
$\text{combinations} = 2^{n}$

Reference tables (2)

The stages of meiosis I and meiosis II7 rows

Stage	What happens	Chromosomes in each cell
Prophase I	Chromosomes condense, homologues pair into bivalents, crossing over	2n, each with two chromatids
Metaphase I	Bivalents (pairs) line up on the equator, each pair facing a random pole	2n, as pairs
Anaphase I	Homologous chromosomes go to opposite poles; sister chromatids stay joined	2n in the cell, n moving to each pole
Telophase I	Two cells form	n, each still with two chromatids
Metaphase II	Chromosomes line up singly on the equator	n, each with two chromatids
Anaphase II	Centromeres split; sister chromatids go to opposite poles	n moving to each pole
Telophase II	Four haploid cells form	n, each with one chromatid

Pairs on the equator means meiosis I; single chromosomes on the equator means mitosis or meiosis II.

Mitosis compared with meiosis7 rows

Feature	Mitosis	Meiosis
Divisions after one S phase	One	Two
Cells made from one cell	Two	Four
Chromosome number	Same as parent (2n gives 2n)	Halved (2n gives n)
Genetic make-up	Identical to parent and to each other	Different from parent and from each other
Homologues pair, crossing over	Never	In prophase I
Where in humans	Somatic (body) cells everywhere	Germ cells in the testes and ovaries
Role	Growth, repair, asexual reproduction	Making gametes (animals) or spores (plants)

Watch out for (6)

Homologous pairs moving apart means anaphase I
→ The stages of meiosis I and meiosis II
A cell after meiosis I is haploid, even though its DNA is 2c
→ Chromosome number (n) and DNA content (c) at each stage of meiosis
Daughter cells of mitosis are not haploid
→ Chromosome number (n) and DNA content (c) at each stage of meiosis
Use n, not 2n, in 2 to the power n
→ Sources of variation in meiosis: crossing over, independent assortment, random fertilisation
Crossing over is between non-sister chromatids
→ Sources of variation in meiosis: crossing over, independent assortment, random fertilisation
Meiosis is not limited to multicellular organisms or to special machinery
→ Mitosis compared with meiosis

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