Mitosis vs meiosis: the stages side by side, and the three differences that get tested

Mitosis is one division that makes two identical diploid cells; meiosis is two divisions that make four genetically different haploid cells. Both start from a cell that has copied its DNA, so every chromosome is two sister chromatids. The three differences tests ask: how many divisions (one versus two), what comes out (two identical 2n cells versus four different n cells), and what separates in the first anaphase (sister chromatids in mitosis, whole homologous pairs in meiosis I). Below, both processes traced on one human cell with the counts at every stage, the counting error almost everyone makes, and a self-check.

The words first: chromosome vs chromatid, homologue, haploid vs diploid

Five words, and the counting rule the whole page rests on.

A chromosome is one DNA molecule with its proteins. After the DNA has been copied, the chromosome is two identical copies joined at the centromere; the copies are sister chromatids, and the pair of them is still counted as one chromosome. The rule: count centromeres for chromosomes, count DNA molecules for chromatids (each sister is one molecule, so do not count the arms of the X shape). The two counts are equal until the DNA is copied, and they stay different until an anaphase pulls the sisters apart.

Homologous chromosomes are a pair, one from each parent, carrying the same genes in the same order (possibly as different alleles). They are not identical and they are not sisters.

Diploid (2n) means both of each homologous pair are present; haploid (n) means one of each. A human body cell is 2n = 46; a human gamete is n = 23.

Before either division: the S phase doubles the chromatids, not the chromosomes

Interphase has three parts: G₁ (growth), S (synthesis, the DNA copying, which the DNA-replication page traces), and G₂ (more growth and checks). After S, a human cell has 46 chromosomes and 92 chromatids: every chromosome is now two sisters joined at one centromere. The number of centromeres has not changed, so the number of chromosomes has not changed.

Say the rule once more with the numbers: 46 centromeres, so 46 chromosomes; 92 DNA molecules, so 92 chromatids. Both mitosis and meiosis begin from this cell.

Stages of mitosis traced: one division, two identical cells

Mitosis is for growth, repair and asexual reproduction. One division, and the products match the parent.

Stage	What happens	Chromosomes per cell	Chromatids per cell
Prophase	Chromosomes condense, the spindle forms, the nuclear envelope breaks down	46	92
Metaphase	46 chromosomes line up single file at the cell's equator	46	92
Anaphase	Sister chromatids separate and move to opposite poles; each is now a chromosome	92 (46 heading to each pole)	92
Telophase and cytokinesis	Two nuclei re-form; the cell divides	46 in each of two cells	46 in each

The anaphase row is the one to read twice. The moment the sisters separate, each is a chromosome with its own centromere, so the count jumps to 92 for a moment, with 46 going to each pole. After cytokinesis each daughter has 46 chromosomes and 46 chromatids: one copy of everything the parent had, identical to it.

Stages of meiosis traced: two divisions, four different cells, crossing over

Meiosis makes gametes in the germ line. Two divisions, no S phase between them, and the products differ from the parent and from one another.

Meiosis I separates homologous pairs.

Stage	What happens	Chromosomes per cell	Chromatids per cell
Prophase I	Homologues pair up as tetrads; crossing over swaps segments between non-sister chromatids at chiasmata	46	92
Metaphase I	23 pairs line up in two rows at the equator; which way each pair faces is random (independent assortment)	46	92
Anaphase I	Homologues separate; sister chromatids stay joined	46 (23 heading to each pole)	92
Telophase I and cytokinesis	Two cells, each with one of every pair	23 in each of two cells	46 in each

After meiosis I the cells are already haploid, 23 chromosomes each, but every chromosome is still two chromatids, so 46 chromatids per cell.

Meiosis II separates the sisters, in both cells at once.

Stage	What happens	Chromosomes per cell	Chromatids per cell
Prophase II	A spindle forms in each cell	23	46
Metaphase II	23 chromosomes line up single file	23	46
Anaphase II	Sister chromatids separate	46 (23 heading to each pole)	46
Telophase II and cytokinesis	Four cells	23 in each of four cells	23 in each

Four cells, 23 chromosomes each, each chromosome a single chromatid. In males all four become sperm; in females one becomes the egg and the others are small polar bodies that break down.

Three things make the four cells genetically different: crossing over in prophase I, which makes new combinations of alleles along a chromosome; independent assortment in metaphase I, where each of the 23 pairs faces one way or the other at random, giving 2²³ = 8,388,608 possible combinations of whole chromosomes in one gamete; and random fertilisation, since any of a parent's gametes may meet any of the other's.

Mitosis vs meiosis: the three differences that get tested, side by side

	Mitosis	Meiosis
Number of divisions	1	2
Products	2 cells, genetically identical, diploid (46)	4 cells, genetically different, haploid (23)
What separates in the first anaphase	Sister chromatids	Homologous pairs (sisters separate in anaphase II)
Pairing of homologues	No	Yes, in prophase I
Crossing over	No	Yes, in prophase I
Where	Body (somatic) cells	Germ-line cells
Chromosome number	Kept	Halved

The first three rows are the three differences. The question that separates a level 3 answer from a level 4 is "which anaphase?" Meiosis II looks like mitosis, single file and sisters separating, and a marker expects you to say so and then say what makes it different: the cell it starts from is haploid and has had no S phase, so the products are 23 single chromatids, not 46.

The counting error almost everyone makes

Treating a replicated chromosome as two chromosomes. Do that and the metaphase count doubles to 92, meiosis I appears to halve nothing (92 to 46), and the whole table is off by a factor of two in the wrong places.

The rule once more, with the four numbers that a human cell gives:

• After the S phase: 46 chromosomes, 92 chromatids.
• After mitosis: 46 and 46, in each of two cells.
• After meiosis I: 23 and 46, in each of two cells.
• After meiosis II: 23 and 23, in each of four cells.

A second swap: anaphase I against anaphase II. Homologues separate in I; sisters in II. A third: thinking the cells after meiosis I are still diploid. They are haploid, 23 chromosomes, with the sisters still attached.

Why SBI4U assumes this, and where it comes back

Punnett squares work because of meiosis. Segregation, one allele of each gene per gamete, is anaphase I pulling the homologues apart. Independent assortment, the genes on different chromosomes sorting independently, is metaphase I's random orientation of each pair. The Punnett-square page multiplies those gametes; this page is why the multiplication is legal. And when anaphase I or II goes wrong and both copies travel to one pole, the gamete has the wrong count; that is nondisjunction, the mechanism behind the chromosome-number disorders the course names.

Check: the tables cold, then one sibling

With the page closed: write the four-row mitosis table and the eight-row meiosis table from memory, both count columns; write the three differences as three sentences; answer "in which anaphase do homologues separate?" Then open the page and mark it. Roediger and Karpicke (2006) found that students who tested themselves remembered more a week later than students who re-read the material for the same time, even though the re-readers felt more confident; redrawing the diagram from memory is the biology version, and the honest-marking rules stop "I would have got it" from counting.

Then a sibling, cold. An animal has 2n = 8. Give its chromosome and chromatid counts after the S phase, after mitosis, after meiosis I and after meiosis II, and the number of whole-chromosome combinations independent assortment alone can put in one gamete.

Answer. After S: 8 chromosomes, 16 chromatids. After mitosis: 8 and 8 in each of two cells. After meiosis I: 4 and 8 in each of two cells. After meiosis II: 4 and 4 in each of four cells. Four pairs, each facing one of two ways: 2⁴ = 16 combinations. The human numbers are the same table with 46, 92, 23 and 2²³.

Where this sits in the course

This page is the Grade 11 biology prerequisite that the SBI4U course map assumes; the course has no mitosis or meiosis idea of its own. Under the map it sits beneath idea 15, DNA replication, which is the S phase that doubles the chromatids before either division, and idea 21, Patterns of inheritance, where meiosis is why each gamete carries one allele. The other biology and chemistry explainers are in the science guides.

Mitosis vs meiosis on one screen

• The counting rule: centromeres for chromosomes, DNA molecules for chromatids.
• The four human counts: 46/92 after S; 46/46 after mitosis; 23/46 after meiosis I; 23/23 after meiosis II.
• The three differences: one division or two; two identical diploid cells or four different haploid cells; sisters or homologues in the first anaphase.
• The anaphase question: homologues in anaphase I, sisters in anaphase II (and in mitosis).
• Variation: crossing over (prophase I), independent assortment (metaphase I, 2²³ = 8,388,608), random fertilisation.