Mitosis vs meiosis: the contrast the IMAT tests

Learning these processes as separate lists makes their differences harder to explain. Start with the contrast: which structures separate, and how does that change each daughter cell?

Start with the purpose

Mitosis copies a cell. Growth, repair, replacing dead cells, asexual reproduction — anywhere the body needs another cell exactly like this one.

Meiosis produces haploid cells. In animals, it is part of gamete production; in plants it produces spores. Sperm and egg cells need half the chromosome number, so that fertilisation restores the full set rather than doubling it every generation. Meiosis also generates genetic variation through assortment and recombination.

Two different jobs, two different mechanisms. Once you hold the purpose, the mechanism stops being arbitrary.

The comparison that answers most questions

	Mitosis	Meiosis
Divisions	One	Two (I and II)
Daughter cells	2	4
Chromosome sets	Preserved: for example 2n → 2n, or n → n	Reduced: 2n → n
Genetically	Usually alike, apart from mutation or segregation errors	Variation arises through assortment and recombination
Human context	Growth and replacement in dividing cells	Germline cells during gamete formation
Homologues pair up	No	Yes — prophase I
Crossing over	No	Yes — prophase I

In humans: mitosis turns one 46-chromosome cell into two 46-chromosome cells. Meiosis produces haploid products with 23 chromosomes. In human sperm formation this yields four haploid cells; egg formation divides the cytoplasm unequally, producing one large functional egg and polar bodies.

What separates in each division

Name the structure that separates before identifying the division.

• Mitosis, anaphase: sister chromatids separate.
• Meiosis, anaphase I: homologous chromosomes separate. Sister chromatids stay joined at the centromere.
• Meiosis, anaphase II: sister chromatids separate.

Anaphase I is the odd one out, and it is the reason meiosis needs two divisions at all. The first division halves the chromosome number by separating the partners of each pair; the second division then works like a mitosis on haploid cells, splitting the chromatids.

If a question describes chromatids separating, it is either mitosis or meiosis II — never meiosis I.

Where the variation comes from

Meiosis generates genetic variation through two mechanisms, both in the first division:

Crossing over (prophase I). Homologous chromosomes pair up and exchange segments at the chiasmata. The chromatids that emerge are recombinant — they carry a mix of maternal and paternal alleles.

Independent assortment (metaphase I). Each homologous pair lines up independently of the others, so which member of each pair goes to which pole is random. With 23 pairs, that alone gives 2²³ possible combinations — over eight million, before crossing over adds its own layer.

Fertilisation then combines two such cells, which is why siblings differ.

Chromosome number versus DNA amount

A common trap: these two are not the same thing, and they change at different moments.

After DNA replication in S phase, a human cell still has 46 chromosomes, but each now consists of two sister chromatids — so the DNA content has doubled while the chromosome count has not. When sister chromatids separate in anaphase, each becomes a chromosome in its own right: the count temporarily doubles within the still-undivided cell. Cytokinesis then separates the daughter cells. Count the specified cell or nucleus at the specified stage, not just the DNA strands.

That is why after meiosis I you have 23 chromosomes but 46 chromatids, and only after meiosis II do you reach 23 chromosomes with 23 chromatids.

When it goes wrong

If a pair fails to separate — non-disjunction — a gamete ends up with one chromosome too many or too few. Fertilisation then produces a zygote with an abnormal count, such as trisomy 21. Non-disjunction can happen in anaphase I (homologues fail to separate) or anaphase II (chromatids fail to separate), and questions sometimes ask you to distinguish the two by looking at the resulting gametes.

The five confusions that cost marks

1. “Meiosis I and II are basically the same.” They are not. The first is a reduction division separating homologues; the second separates chromatids.
2. “Sister chromatids separate in anaphase I.” They separate in anaphase II.
3. “Crossing over happens in both.” Only in prophase I of meiosis.
4. “Chromosome number and DNA amount change together.” DNA content per daughter cell halves at each meiotic division; ploidy falls in meiosis I and remains haploid after meiosis II.
5. “Mitosis creates variation.” It creates copies. Any variation from mitosis is a mutation — an error, not a mechanism.

Each of these is a belief rather than a gap, which is why re-reading the chapter rarely fixes them. What fixes them is meeting the contrast directly — which is exactly what the exam does.