It is the end of September 2026. In Year 2 of the liceo linguistico (the Italian academic high school focused on languages), Natural Sciences (the two weekly hours in which chemistry and biology travel together) is finishing the chapter on the cell and getting ready to open the one on cell division, which typically starts between November and December. The test usually falls in December: from now there are about nine to ten weeks, just the right time to build a method before mitosis and meiosis overlap in your head, which happens to almost everyone. The very same topic, with the very same risk of confusion, also arrives in Year 2 at the liceo scientifico and in technical schools. If you got here from another school track, everything that follows applies anyway.
If you still have gaps on the cell (structure, organelles, membrane), that is a different and earlier chapter: I wrote about it in biomolecules and the cell in Year 2. Here I take the cell as known and go straight to the point that trips up most of the class in the test: how it divides, and why there are two different ways of doing it.
Chromosome, chromatid, chromatin: three words that get confused
Before the stages, we need to fix three terms that get swapped constantly in assignments.
- Chromatin: this is the DNA of the cell when it is not dividing, spread through the nucleus in a loosely packed form that is not visible as a distinct structure under the light microscope.
- Chromosome: when the cell is about to divide, the chromatin condenses and becomes visible as a compact structure. It is the same genetic information, just packaged differently.
- Chromatid: after a chromosome has been duplicated, it is made of two identical copies joined at a point called the centromere. Each of the two copies is called a chromatid (or “sister chromatid”). As long as they are joined, that X-shaped structure counts as a single chromosome with two chromatids, not as two chromosomes.
This third point is the most common source of mistakes in tests, and I come back to it later with a dedicated exercise.
The cell cycle: where division fits in
The life of a cell is divided into two big moments: interphase, in which the cell grows and prepares, and the M phase, in which it really divides. Interphase has three sub-phases:
- G1: the cell grows and carries out its normal functions. The chromosomes are still single (one chromatid each).
- S: the cell duplicates its DNA. Each chromosome becomes a structure with two identical chromatids.
- G2: the cell keeps growing and prepares for division, with the chromosomes already duplicated.
Only after G2 does the M phase begin, which in a somatic cell is mitosis and in the cells that produce gametes is meiosis.
Ploidy: 2n and n
Each species has a characteristic number of chromosomes, organised in pairs of homologous chromosomes (homologues): two chromosomes that carry the same genes (one of maternal origin, one paternal), but not necessarily the same versions of those genes. A cell with the complete pairs is called diploid, written 2n; a cell with only one copy per pair, so half the chromosomes, is called haploid, written n. In humans 2n = 46 (23 pairs), n = 23. The cells of the body (somatic cells) are diploid; only the gametes (egg cells and sperm cells) are haploid.
The stages of mitosis
Mitosis produces two daughter cells identical to each other and to the mother cell, both diploid (2n). It is used for growth, tissue repair and asexual reproduction in organisms that use it.
- Prophase: the chromosomes (already duplicated in S) condense and become visible; the nucleus dissolves; the mitotic spindle forms.
- Metaphase: the chromosomes, each with two chromatids, line up on the equatorial plane of the cell.
- Anaphase: the sister chromatids separate and move towards the two opposite poles. From this moment each chromatid is a chromosome in its own right, because it is no longer attached to a twin copy.
- Telophase and cytokinesis: the nuclei re-form at the two poles, and the cell physically divides into two daughter cells, each with the same diploid set as the mother cell.
Meiosis: two divisions, not one
Meiosis produces the gametes (egg cells and sperm cells) and has a different purpose from mitosis: to halve the number of chromosomes (from 2n to n), so that the union of two gametes at fertilisation restores the correct diploid number. To do this it needs two successive divisions, preceded by a single duplication of the DNA.
Meiosis I: the reductional division
This is the division that really halves the number of chromosomes, and it has a step that mitosis does not:
- Prophase I: the homologous chromosomes (one maternal, one paternal) pair up point by point, forming structures called tetrads (4 chromatids side by side, 2 for each homologue). At this moment crossing over takes place: segments of DNA are exchanged between non-sister chromatids of paired homologues, mixing the maternal and paternal gene combinations on the same chromosome.
- Metaphase I: the pairs of homologues (not the single chromosomes) line up on the equatorial plane.
- Anaphase I: the homologues separate from one another and move to opposite poles, but the two chromatids of each chromosome are still joined. This is where the number of chromosomes is halved.
- Telophase I: two cells form, each haploid (n chromosomes), but each chromosome is still made of two chromatids.
Meiosis II: the equational division
This second division looks like a normal mitosis, except that it starts from a cell that is already haploid:
- Prophase II, Metaphase II: the chromosomes (still with two chromatids) line up.
- Anaphase II: the sister chromatids separate, exactly as in anaphase of mitosis.
- Telophase II and cytokinesis: in total, from the two divisions, you get four haploid daughter cells, genetically different from one another.
Mitosis and meiosis compared
| Mitosis | Meiosis | |
|---|---|---|
| Number of divisions | 1 | 2 (meiosis I and II) |
| Daughter cells | 2 | 4 |
| Chromosome set | Diploid (2n → 2n) | Haploid (2n → n) |
| Genetic similarity to the mother cell | Identical | Different (crossing over + independent assortment) |
| Pairing of homologues | Does not happen | Happens in prophase I |
| Crossing over | Does not happen (normally) | Happens in prophase I |
| Function | Growth, repair, asexual reproduction | Formation of gametes |
Why meiosis creates genetic variability
Two mechanisms, both in meiosis I, explain why the gametes produced by a single individual are never identical to one another.
The first is crossing over, already described: it swaps pieces of DNA between homologous chromatids, so every chromosome that ends up in a gamete is a mosaic of the original maternal and paternal chromosomes, not an intact copy of one or the other.
The second is independent assortment: in metaphase I, for each pair of homologues, which of the two ends up at one pole or the other is random and independent of the other pairs. With pairs of homologues, the number of different chromosome combinations obtainable in the gametes is
In humans, with pairs of homologues, this gives possible combinations from independent assortment alone, before even counting crossing over, which multiplies the variability further. This is why, apart from identical twins, two siblings never have the same set of chromosomes.
Typical mistakes to avoid in the test
| Typical mistake | Why it is wrong |
|---|---|
| Counting chromatids as if they were separate chromosomes | As long as they are joined by the centromere, two sister chromatids form a single chromosome |
| Saying that meiosis II halves the number of chromosomes again | False: the halving happens only in meiosis I (anaphase I). Meiosis II separates sister chromatids and does not change the number of chromosomes (n → n) |
| Confusing “homologues” with “sister chromatids” | Homologues are two different chromosomes (one maternal, one paternal) that pair up only in meiosis; sister chromatids are two identical copies of the same chromosome, produced by duplication in the S phase |
| Thinking that crossing over also happens in mitosis | In mitosis there is no pairing of homologues, so there is no crossing over: the daughter cells are identical to the mother |
| Forgetting that meiosis produces four cells, not two | There are two divisions in sequence: meiosis I gives 2 cells, each of which divides again in meiosis II, for a total of 4 |
Three worked exercises
1. Counting chromosomes and chromatids in mitosis (2n = 8)
A cell has 2n = 8. How many chromosomes and how many chromatids does it have in G1, in G2 and in anaphase of mitosis?
- G1: the DNA is not yet duplicated. 8 chromosomes, each with 1 chromatid → 8 chromatids in total.
- G2 (after the S phase): the DNA has been duplicated. There are still 8 chromosomes (the number of chromosomes does not change when DNA is duplicated), but each now has 2 chromatids → 16 chromatids in total.
- Anaphase of mitosis: the sister chromatids separate and each becomes an independent chromosome. In the cell (before it even divides in two) there are now 16 chromosomes, each with 1 chromatid → 16 chromatids in total. After cytokinesis, each of the two daughter cells again has 8 chromosomes with 1 chromatid, the same starting state (G1).
2. Gamete combinations by independent assortment (2n = 6)
An organism has 2n = 6, so n = 3 pairs of homologues. How many different chromosome combinations can it produce in its gametes, considering only independent assortment?
Each pair of homologues separates independently of the others, and for each pair the gamete receives one of the two homologues: 2 possibilities for each of the 3 pairs, so
8 different chromosome combinations are possible in the gametes of this organism, without even counting crossing over.
3. Recognising the stage from a description
“In the cell the homologous chromosomes are seen paired up, with exchange of segments between non-sister chromatids.” Which stage is it?
It is prophase I of meiosis. Two clues identify it unambiguously: the pairing of homologues (which happens only in meiosis, never in mitosis) and the exchange of segments between chromatids, that is, crossing over, which is specific to this moment of the cycle.
“The sister chromatids separate and move towards opposite poles, without the homologues being paired.” Which stage is it, and how can you tell?
It can be anaphase of mitosis or anaphase II of meiosis: in both, sister chromatids (not homologues) separate and each chromatid becomes a chromosome in its own right, so the count of chromosomes in the cell briefly doubles, as in exercise 1. The real halving happens only in anaphase I, where the homologues separate. To tell the two apart you need to know whether the starting cell was diploid (then it is mitosis) or already haploid, as the product of an earlier meiosis I (then it is meiosis II).
What you will not find (yet) on the site, and where to look
We do not have a dedicated practice test on mitosis and meiosis, but for the chapter that comes right after there is the Mendelian genetics quiz: fifteen questions on crosses, the Punnett square and blood groups, with a guide in the Mendelian genetics exercises. For the rest of the science programme of the linguistico, and to set up this chapter well before December, you will find the overview on the page for science tutoring; if you want a hand, the introductory call is free and lasts half an hour. If your goal is the medicine admission test, the same topic at that level is in Cell and molecular biology for the medicine admission test and in Biology in the Medicine filter semester.
