Mitosis and meiosis are two ways cells divide. Mitosis produces two cells with the same chromosome set as the parent cell: it allows the organism to grow and repair tissues. Meiosis consists of two consecutive divisions, resulting in four cells with half the chromosome set: this is how gametes are formed. This page covers the phases of mitosis and meiosis in order, the chromosome and DNA sets in each phase, crossing-over and conjugation, a comparison table, and a self-check quiz.
Cell life cycle and interphase
A cell does not divide continuously. Its life cycle is the time from the cell's formation to its own division, consisting of two unequal parts: a long interphase (preparation) and a short division phase.
Interphase is divided into three periods:
- G1, presynthetic — the cell grows, synthesizes proteins, and increases its volume of cytoplasm and organelles;
- S, synthetic — DNA replication occurs: each chromosome builds a second chromatid;
- G2, postsynthetic — the cell stores energy, duplicates centrioles, and prepares spindle proteins.
Remember the main point: DNA doubling occurs in interphase, before division begins. Both mitosis and meiosis start with already doubled material — a 2n4c set. Interphase itself is not considered a phase of mitosis or meiosis.
During interphase, chromosomes are not visible even under a microscope: they are uncoiled and exist in the nucleus as chromatin. The structure of the nucleus, chromosomes, and the cell center, from which spindle fibers grow, is detailed in our material on plant and animal cell structure.
What 2n2c means: how to count chromosomes and DNA
In notation like 2n4c, the letter n denotes the number of chromosomes, and the letter c denotes the amount of DNA (specifically, the number of chromatids). A haploid set is n, a diploid set is 2n. The notation 2n2c is read as: "diploid set of chromosomes, each chromosome consisting of one chromatid," and 2n4c as: "diploid set, each chromosome consisting of two chromatids."
The most common mistake is counting chromosomes by chromatids. After doubling, a chromosome looks like two sticks, but it is one chromosome: both chromatids are held together by a single centromere. The number of chromosomes equals the number of centromeres.
In humans, 2n = 46. This means that in the G1 phase, the cell has 46 chromosomes and 46 DNA molecules (2n2c). After the S phase, there are still 46 chromosomes, but 92 DNA molecules (2n4c). Only in the anaphase of mitosis, when centromeres divide, does the number of chromosomes become 92 — 46 at each pole.
Phases of mitosis: prophase, metaphase, anaphase, telophase
The four phases of mitosis, step-by-step, with chromosome and DNA sets in each, are shown in the diagram below. Here, we address three common points of confusion.
Why is it 4n4c in anaphase? While the cell is intact, it has twice as many chromosomes as each daughter cell will have: the centromeres have divided, and each chromatid has become an independent chromosome. However, the set is counted both for the whole cell (4n4c) and per pole (2n2c) — always check what the question is asking for.
Why are chromosomes counted in metaphase? This is when they are maximally coiled and aligned in a single row — they are visible and can be counted. This is why karyotypes are studied using metaphase plates.
How division ends. The division of the cytoplasm is called cytokinesis, and it occurs differently in plant cells than in animal cells: the cell plate grows from the inside out, from the center to the edges, because the rigid cell wall prevents the cell from pinching in half.
Meiosis I: conjugation, crossing-over, and homologous separation
Meiosis is two consecutive divisions after a single DNA replication. The first division is called reductional: this is where the number of chromosomes is halved. The order of phases and sets are in the diagram below.
Why the set decreases here. Chromatids do not separate in anaphase I — whole chromosomes move apart, so the pole receives a haploid set of two-chromatid chromosomes: n2c, not nc. They become single-chromatid chromosomes only after the second division.
Why conjugation and crossing-over are needed. A pair of closely associated homologs (a bivalent) consists of two chromosomes and four chromatids. The exchange of segments between non-sister chromatids mixes maternal and paternal genes within a single chromosome — this is the source of hereditary variation.
The randomness that decides everything. Each pair of homologs turns toward the poles independently of the others. In humans, this provides 2²³ ≈ 8.4 million set combinations before any crossing-over even occurs.
Between divisions, there is a short interkinesis — and it lacks a synthetic period: DNA does not replicate a second time.
Meiosis II: chromatid separation
The second division of meiosis is called equational — it follows the rules of mitosis, but in a haploid cell. The phases and sets are in the diagram below.
What is important to understand here. Individual chromosomes align at the equator, not bivalents: the homologs have no one to conjugate with, as they separated in the previous division. This time, centromeres divide — sister chromatids separate.
Why all four cells are different. First, crossing-over exchanged segments between non-sister chromatids of two homologs, and then the homologs themselves moved into cells in random combinations. Therefore, meiosis does not produce copies, but a new set of genes each time — this is the basis of offspring diversity.
Chromosome and DNA sets by mitosis and meiosis phase
Problems asking "how many chromosomes and DNA molecules are in such-and-such phase" are solved mechanically if you keep two rules in mind: the amount of DNA increases only in the S phase, and the number of chromosomes changes only where chromatids separate. Below is the set for each phase.
| Phase | Chromosomes and DNA |
|---|---|
| Interphase | |
| Presynthetic period (G1) | 2n2csingle-chromatid chromosomes |
| Synthetic period (S) | 2n4cDNA doubled |
| Postsynthetic period (G2) | 2n4cpreparation for division |
| Mitosis | |
| Prophase | 2n4c |
| Metaphase | 2n4cchromosomes at equator |
| Anaphase | 4n4cchromatids separated, 2n2c at pole |
| Telophase | 2n2cin each of the two cells |
| Meiosis I | |
| Prophase I | 2n4cconjugation and crossing-over |
| Metaphase I | 2n4cbivalents at equator |
| Anaphase I | 2n4chomologs separated, n2c at pole |
| Telophase I | n2cin each of the two cells |
| Meiosis II | |
| Prophase II | n2c |
| Metaphase II | n2cchromosomes at equator individually |
| Anaphase II | 2n2cchromatids separated, nc at pole |
| Telophase II | ncin each of the four cells |
Differences between mitosis and meiosis: comparison table
The two divisions have much in common: both occur after DNA replication in interphase, both have the same phase names, both use a spindle, and both end with cytoplasmic division. Therefore, in an answer, it is convenient to first name the similarities, and then the differences point by point in the table.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | Mitosis: one | Meiosis: two consecutive — meiosis I and meiosis II |
| Number of cells produced | Mitosis: two | Meiosis: four |
| Daughter cell set | Mitosis: 2n — same as parent cell | Meiosis: n — half as much, haploid |
| Conjugation and crossing-over | Mitosis: none | Meiosis: present, in prophase I |
| What is at the equator in metaphase | Mitosis: individual chromosomes in a single row | Meiosis: in metaphase I — bivalents in two rows, in metaphase II — individual chromosomes |
| What separates in anaphase | Mitosis: sister chromatids | Meiosis: in anaphase I — whole homologous chromosomes, in anaphase II — chromatids |
| Prophase | Mitosis: short and simple | Meiosis: prophase I is long and complex |
| Genetic result | Mitosis: cells with the same gene set as the parent | Meiosis: cells with new gene combinations, all different |
| Where it occurs | Mitosis: in somatic cells: growth, renewal and tissue repair, asexual reproduction | Meiosis: during formation of gametes in animals and spores in plants |
Biological significance of mitosis and meiosis
The significance of mitosis lies in precision. Daughter cells receive exactly the same set of chromosomes and genes as the parent, so mitosis ensures:
- organism growth and an increase in cell number;
- tissue renewal and healing of damage (regeneration);
- cleavage of the zygote after fertilization;
- asexual and vegetative reproduction — from the division of an amoeba to plant propagation by cuttings.
The significance of meiosis lies in two things at once.
First: constancy of the species' chromosome number. If gametes were diploid, the chromosome number would double with each fertilization. Meiosis halves the set, and upon gamete fusion (n + n), the zygote again receives a diploid 2n set.
Second: combinatorial variation. New gene combinations arise in three ways: during crossing-over in prophase I, during random and independent separation of homologs in anaphase I, and during the random meeting of gametes during fertilization. Just through independent assortment, a human can produce 2 to the power of 23 — more than eight million — different gametes by chromosome set. This diversity provides the material for natural selection.
In animals, meiosis occurs during gamete formation in the gonads; in plants, during spore formation.
Task analysis
Example 1. How many chromosomes and DNA in different phases of mitosis
Task: in Drosophila, 2n = 8. Determine the number of chromosomes and DNA molecules in prophase, anaphase, and telophase of mitosis.
Step 1. Starting point. Before division, in the G1 phase, the set is 2n2c: 8 chromosomes and 8 DNA molecules. In the S phase, DNA doubled, becoming 2n4c — still 8 chromosomes, but 16 DNA molecules.
Step 2. Prophase. Nothing separates, the set is the same: 8 chromosomes, 16 DNA molecules (2n4c).
Step 3. Anaphase. Centromeres divided, each chromatid became a separate chromosome: there are now 16 chromosomes, 16 DNA molecules (4n4c). At each pole, there are 8 chromosomes.
Step 4. Telophase. The cytoplasm divides, and each of the two young cells ends up with 8 chromosomes and 8 DNA molecules (2n2c).
Logic check: the number of DNA molecules never increased during mitosis itself — it only halved between the cells.
Example 2. Set in meiosis phases
Task: in corn, 2n = 20. How many chromosomes and DNA molecules are in the cell at the end of telophase I and at the end of all meiosis?
Step 1. Before meiosis, DNA doubled: 20 chromosomes and 40 DNA molecules (2n4c).
Step 2. Anaphase I. Whole homologs move to the poles, centromeres do not divide. At each pole, 10 chromosomes gather, and each has two chromatids.
Step 3. End of telophase I. In each of the two cells, there are 10 chromosomes and 20 DNA molecules (n2c).
Step 4. Meiosis II. There was no DNA doubling, so in anaphase II, chromatids simply separate. At the end of meiosis, four cells are produced, each with 10 chromosomes and 10 DNA molecules (nc).
Answer: after the first division, 10 chromosomes and 20 DNA molecules; after the second — 10 chromosomes and 10 DNA molecules.
Example 3. Identify division by description
Task: "In the cell, 12 structures are visible, each consisting of four chromatids. They have aligned at the equator in two rows." What division and phase is this? What is the initial set of the cell?
Step 1. A structure of four chromatids is a bivalent, i.e., a pair of associated homologous chromosomes. Bivalents exist only in meiosis: conjugation occurred in prophase I.
Step 2. Bivalents stand at the equator in two rows — this means metaphase I.
Step 3. One bivalent is two chromosomes. There are 12 bivalents, so there are 24 chromosomes, and the initial set of the cell is 2n = 24. DNA molecules in this phase are 48 (2n4c).
Answer: meiosis I metaphase, 2n = 24, cell set 2n4c.
How not to make a mistake: if individual two-chromatid chromosomes stood at the equator in a single row, it would be mitosis metaphase (at 2n) or metaphase II (at a haploid set).
Common mistakes
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Thinking that DNA doubles in prophase, right before division.
DNA doubling (replication) occurs in the S phase of interphase, long before prophase. Both mitosis and meiosis start with a 2n4c set.
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Counting chromosomes by chromatids: seeing 46 two-chromatid chromosomes and writing "92 chromosomes."
The number of chromosomes equals the number of centromeres. As long as chromatids are connected by one centromere, it is one chromosome: 46 chromosomes and 92 DNA molecules. There will be 92 chromosomes only in anaphase, when centromeres divide.
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Writing that homologous chromosomes separate in mitosis anaphase.
In mitosis anaphase, sister chromatids of the same chromosome separate. Homologous chromosomes separate only once during the entire division — in meiosis anaphase I.
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Thinking that DNA doubles again before the second division of meiosis.
There is no synthetic period in interkinesis. One DNA doubling — two divisions: this is exactly why the set ends up halved.
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Attributing crossing-over to mitosis or metaphase I.
Crossing-over occurs in meiosis prophase I, when homologs are connected in a bivalent (conjugation). In mitosis, homologs do not conjugate, so there is no crossing-over in it.
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Writing that meiosis produces four identical cells, and the chromosome number halves in each of the two divisions.
The set halves only in the first division — it is called reductional. The second division does not halve the chromosome number in the cell; it separates chromatids. And the resulting four cells are genetically different: crossing-over and random separation of homologs give them different gene combinations.
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Calling interphase the first phase of mitosis.
Interphase is part of the cell's life cycle, not a division phase. Mitosis has four phases: prophase, metaphase, anaphase, telophase. But problems on chromosome counting cannot be solved without interphase: it is in its S phase that the set becomes 2n4c.
Questions and answers
How does mitosis differ from meiosis, in short?
Mitosis is one division, producing two cells with the original 2n set and the same gene set. Meiosis is two consecutive divisions after one DNA doubling, producing four cells with a haploid n set, and they all differ in gene combination. Furthermore, only in meiosis is there conjugation and crossing-over, and in anaphase I, whole homologous chromosomes separate, not chromatids.
How many cells are formed during mitosis and meiosis?
In mitosis — two cells with a 2n2c set. In meiosis — four cells with an nc set. Hence the difference in purpose: mitosis increases the number of identical cells, meiosis prepares gametes.
In which phase does crossing-over occur?
In prophase of the first division of meiosis. Crossing-over is the exchange of identical segments between non-sister chromatids of homologous chromosomes. It is possible only when homologs are close together in a bivalent, i.e., after conjugation.
What is chromosome conjugation and a bivalent?
Conjugation is the pairing of homologous chromosomes along their entire length in meiosis prophase I. The resulting pair is called a bivalent: it contains two chromosomes and four chromatids (which is why it is sometimes called a tetrad). In metaphase I, it is bivalents that align at the equator, not individual chromosomes.
Why is the set in mitosis anaphase written as 4n4c?
Because centromeres have divided, and each former chromatid has become an independent chromosome. The number of DNA molecules remains the same (4c), but the number of chromosomes has temporarily doubled — 4n. As soon as the cytoplasm divides, each of the two cells will have the normal 2n2c set.
Where in the body does mitosis occur, and where does meiosis occur?
Mitosis occurs in somatic cells: in growth zones, in skin, in the intestinal lining, in bone marrow, during wound healing. In animals, meiosis occurs in the gonads during gamete formation, and in plants — during spore formation.
Is interphase part of mitosis?
No. Interphase is preparation for division and part of the cell's life cycle. Mitosis has four phases: prophase, metaphase, anaphase, and telophase. But problems on chromosome counting cannot be solved without interphase: it is in its S phase that the set becomes 2n4c.
