Biology
Mitosis vs Meiosis: Differences, Phases & 7 Solved Exercises
Mitosis makes 2 identical cells (2n); meiosis makes 4 different cells (n). Get comparison tables, the phases in order and 7 exercises solved step by step.
Recommended for: Grade 9 · Grade 10
Mitosis and meiosis are the two ways a eukaryotic cell divides. In short: mitosis makes 2 daughter cells identical to the parent (2n → 2n), while meiosis makes 4 haploid cells that differ from each other (2n → n): the gametes. The first is for growth and repair, the second for sexual reproduction.
Mitosis vs meiosis at a glance
| Feature | Mitosis | Meiosis |
|---|---|---|
| Where it happens | Body (somatic) cells | Germ cells (gonads) |
| Divisions | 1 | 2 (meiosis I and II) |
| Daughter cells | 2 | 4 |
| Chromosome number | 2n (unchanged) | n (halved) |
| Same as the parent cell? | Yes, identical | No, genetically different |
| Homologue pairing and crossing over | No | Yes, in prophase I |
| Purpose | Growth, repair, asexual reproduction | Making gametes, variation |
The phases of mitosis
Before dividing, the cell goes through interphase (G1, S, G2) and copies its DNA: each chromosome now has 2 sister chromatids. Mitosis then runs in four phases.
| Phase | What happens |
|---|---|
| Prophase | Chromosomes condense, the nuclear envelope breaks down, the spindle forms |
| Metaphase | Chromosomes line up at the cell’s equator |
| Anaphase | Sister chromatids separate and move to opposite poles |
| Telophase | Two nuclei re-form and cytokinesis divides the cell |
The phases of meiosis
Meiosis I is the reductional division. In prophase I homologues pair up and crossing over occurs; in metaphase I they line up in pairs (bivalents); in anaphase I the homologues separate, not the chromatids. The 2 resulting cells are haploid, but their chromosomes still have 2 chromatids.
Meiosis II resembles mitosis, with no new DNA copying: in anaphase II the sister chromatids separate, giving 4 haploid cells whose chromosomes have 1 chromatid.
How many chromosomes and chromatids?
| Stage (cell with 2n = 8) | Chromosomes | Chromatids |
|---|---|---|
| G1 | 8 | 8 |
| G2 / prophase | 8 | 16 |
| End of mitosis (each daughter) | 8 | 8 |
| End of meiosis I (each cell) | 4 | 8 |
| End of meiosis II (each cell) | 4 | 4 |
The most common mistakes
- Saying meiosis I separates chromatids: it separates homologous chromosomes; chromatids separate in meiosis II.
- Thinking the cell is still diploid after meiosis I: it is haploid, but its DNA is still copied.
- Mixing up chromosomes and chromatids: after S phase the chromosome number is the same, only the chromatid number changes.
- Believing mitosis gives different cells: barring mutations, they are identical to the parent.
- Forgetting that crossing over happens in prophase I and only in meiosis.
To review organelles and the nucleus, read the guide to cell structure and functions; to see how alleles are shared out into gametes, continue with the Mendelian genetics exercises. Below, test yourself on mitosis and meiosis with 7 exercises from easy to advanced, each solved step by step.
Solved exercises
1. A human body cell (2n = 46) enters prophase of mitosis. How many chromosomes and how many chromatids does it have? How many chromosomes will each daughter cell have at the end? base
Show solution
- Before mitosis the cell has already copied its DNA in S phase: the number of chromosomes does not change, but each one now has 2 sister chromatids.
- Chromosomes in prophase: 46.
- Chromatids in prophase: 46 × 2 = 92.
- In anaphase the sister chromatids separate and each becomes its own chromosome, so each pole receives 46 chromosomes.
Answer: In prophase: 46 chromosomes and 92 chromatids. Each daughter cell has 46 chromosomes (2n = 46), identical to the parent cell.
2. Put these four microscope observations in order and name each phase: (A) chromosomes are lined up at the cell's equator; (B) the nuclear envelope breaks down and the spindle forms; (C) sister chromatids move to opposite poles; (D) two new nuclei form and the cell pinches in the middle. base
Show solution
- Mitosis always follows the sequence prophase, metaphase, anaphase, telophase (PMAT).
- (B) is prophase: the nuclear envelope disappears and the spindle is built.
- (A) is metaphase: chromosomes line up on the metaphase plate at the equator.
- (C) is anaphase: sister chromatids separate and are pulled to the poles.
- (D) is telophase with cytokinesis: nuclei re-form and the cell splits in two.
Answer: Order B → A → C → D: prophase, metaphase, anaphase, telophase (with cytokinesis).
3. A cell has 2n = 8 chromosomes. For each stage give the number of chromosomes and chromatids: (a) G1; (b) G2; (c) each daughter cell at the end of mitosis; (d) each cell at the end of meiosis I; (e) each cell at the end of meiosis II. intermedio
Show solution
- (a) In G1 the DNA is not yet copied: 8 chromosomes, each with 1 chromatid, so 8 chromatids.
- (b) In G2 the DNA is copied: still 8 chromosomes, but each has 2 chromatids, so 16 chromatids.
- (c) Mitosis separates sister chromatids: each daughter has 8 single-chromatid chromosomes, so 8 chromatids.
- (d) Meiosis I separates homologous chromosomes (not chromatids): each cell has 4 chromosomes, still with 2 chromatids each, so 8 chromatids.
- (e) Meiosis II separates sister chromatids: each final cell has 4 single-chromatid chromosomes, so 4 chromatids (n = 4).
Answer: (a) 8 chromosomes / 8 chromatids; (b) 8 / 16; (c) 8 / 8; (d) 4 / 8; (e) 4 / 4.
4. A fertilized egg undergoes 5 rounds of mitosis in which every cell divides each time. (a) How many cells are there at the end? (b) How many sperm form from 50 primary spermatocytes? (c) How many mature egg cells form from 50 primary oocytes? intermedio
Show solution
- (a) Each round of mitosis doubles the number of cells: after 5 rounds there are 2⁵ = 32 cells.
- (b) Each primary spermatocyte completes meiosis and gives 4 sperm: 50 × 4 = 200.
- (c) Each primary oocyte gives only 1 functional egg: the cytoplasm divides very unequally and the other cells, the polar bodies, degenerate.
- So 50 primary oocytes produce 50 mature eggs.
Answer: (a) 32 cells; (b) 200 sperm; (c) 50 egg cells (plus polar bodies that degenerate).
5. In a cell with 2n = 4 you see three metaphases: (A) 4 chromosomes in a single line at the equator; (B) 2 pairs of homologous chromosomes paired up at the equator; (C) 2 chromosomes in a single line at the equator, each with two chromatids. Identify the type of division in each. intermedio
Show solution
- First count chromosomes: 2n = 4, so a diploid cell has 4 chromosomes and a haploid cell has 2.
- (A) 4 chromosomes all in a single line: the cell is diploid and the chromosomes are not paired, so this is metaphase of mitosis.
- (B) homologous chromosomes are paired up (bivalents): this happens only in metaphase I of meiosis.
- (C) only 2 chromosomes in a single line: the cell is already haploid (n = 2), so this is metaphase II of meiosis.
Answer: (A) mitosis metaphase; (B) metaphase I (meiosis I); (C) metaphase II (meiosis II).
6. How many different chromosome combinations can a human gamete (n = 23) have from independent assortment of homologues alone, without crossing over? How many different zygotes could two parents produce? avanzato
Show solution
- Each of the 23 homologous pairs lines up in metaphase I in two equally likely ways: either homologue can go to either pole.
- The choices are independent, so they multiply: 2 × 2 × ... (23 times) = 2²³.
- 2²³ = 8,388,608 different gametes per parent.
- A zygote comes from two gametes: 2²³ × 2²³ = 2⁴⁶, about 7.04 × 10¹³ combinations (over 70 trillion).
- Crossing over swaps segments between homologues and increases the variety even more.
Answer: About 8.4 million gametes per parent (2²³ = 8,388,608) and about 7.04 × 10¹³ possible zygotes (2⁴⁶), not counting crossing over.
7. In human meiosis, nondisjunction of chromosome 21 occurs. How many chromosomes do the four gametes have if the error happens (a) in meiosis I? (b) in meiosis II, in only one of the two cells? And what results if a 24-chromosome gamete is fertilized by a normal one? avanzato
Show solution
- Nondisjunction means a pair fails to separate. A normal gamete has 23 chromosomes.
- (a) Error in meiosis I: both homologues of 21 go to the same cell. That cell gives 2 gametes with 24 chromosomes; the other, with no chromosome 21, gives 2 gametes with 22. All 4 gametes are abnormal.
- (b) Error in meiosis II in one cell only: the normal cell gives 2 gametes of 23; the other gives 1 gamete of 24 (two copies of 21) and 1 gamete of 22. So 2 normal, 1 with 24 and 1 with 22.
- A 24-chromosome gamete plus a normal 23-chromosome gamete gives a zygote with 24 + 23 = 47 chromosomes and three copies of chromosome 21.
Answer: (a) 24, 24, 22, 22; (b) 23, 23, 24, 22. A 24-chromosome gamete + a normal gamete = 47 chromosomes: trisomy 21 (Down syndrome).
FAQ
What is the difference between mitosis and meiosis?
Mitosis is a single division that turns one diploid cell (2n) into 2 identical diploid daughter cells: it is used for growth and repair. Meiosis consists of 2 consecutive divisions and turns one diploid cell into 4 haploid (n) cells that are genetically different from each other: the gametes, meaning sperm and egg cells. Only meiosis has homologue pairing and crossing over.
Why does meiosis halve the number of chromosomes?
Because one round of DNA copying is followed by two divisions. In meiosis I the homologous chromosomes separate, so 2n becomes n; in meiosis II the sister chromatids separate with no new copying. At fertilization the two gametes (n + n) restore the species' 2n number.
When does crossing over happen?
In prophase I of meiosis. Homologous chromosomes pair up (synapsis) and swap segments of DNA at points called chiasmata. The result is recombinant chromosomes that mix the alleles from the mother and the father.
Is the cell haploid or diploid after meiosis I?
It is haploid, because it has n chromosomes. However, each chromosome still consists of 2 sister chromatids, since chromatids only separate in meiosis II. That is why its DNA content is still double that of a gamete.
How many cells are produced by mitosis and by meiosis?
Mitosis produces 2 cells and meiosis produces 4. In spermatogenesis all 4 become sperm; in oogenesis only one functional egg forms, along with polar bodies that degenerate.