Difficulty: Introductory | Prerequisites: Cell Division Foundations notes and Mitosis notes.
Sexual reproduction requires cells with half the normal chromosome number, so that when sperm and egg fuse, the full number is restored. Meiosis is the process that produces those half-chromosome cells (gametes). It is also the engine behind genetic diversity: crossing over and independent assortment shuffle alleles every generation, which is why siblings from the same parents are never identical (unless they are identical twins).
This topic assumes you are comfortable with the mitosis stages (PMAT), because meiosis II mirrors them closely. If you have not reviewed the Mitosis notes, do that first.
Meiosis takes one diploid cell and produces four genetically unique haploid cells (gametes). It involves two rounds of division: meiosis I (the reduction division, where homologous chromosomes separate) and meiosis II (where sister chromatids separate, just like mitosis). Crossing over and independent assortment during meiosis I are the main sources of genetic variation.
Synapsis
The process by which homologous chromosomes pair up tightly during prophase I of meiosis. Think of it as two matching chromosomes zipping together side by side so they can exchange material.
Crossing over (genetic recombination)
The exchange of segments of DNA between homologous chromosomes during prophase I, while they are paired in a tetrad. This shuffles alleles between the maternal and paternal copies, creating new combinations of genes.
Chiasma (plural: chiasmata)
The physical point on the chromosome where crossing over occurs. In simple terms, the chiasma is the spot where two homologous chromatids are visibly crossed and swapping DNA.
Independent assortment
The principle that homologous chromosome pairs line up randomly at the metaphase plate during meiosis I, so the maternal and paternal chromosomes are distributed to daughter cells in any combination. Together with crossing over, this is the main source of genetic variety.
Reduction division
Another name for meiosis I, because it reduces the chromosome number from diploid (2n) to haploid (n). The "reduction" refers to the fact that homologous pairs are separated, so each daughter cell ends up with one set of chromosomes rather than two.
Meiosis consists of two consecutive divisions. Only the first is preceded by interphase (and therefore by DNA replication). The second follows immediately.
During prophase I, homologous chromosomes pair up (synapsis) and form tetrads. While paired, non-sister chromatids can exchange segments of DNA at points called chiasmata. The result is that each chromatid in the tetrad may carry a new combination of alleles. Together with independent assortment, crossing over is the reason meiosis produces genetically unique cells.
Interphase
Identical to the interphase before mitosis: G1 (growth), S (DNA replication, sister chromatids form), G2 (further growth, microtubule synthesis).
Prophase I
Homologous chromosomes pair up through synapsis, forming tetrads (four chromatids together).
Crossing over occurs at chiasmata, exchanging genetic material between homologues.
The nuclear envelope breaks down, and spindle fibres begin to form.
This is where genetic diversity is generated.
Metaphase I
Tetrads (paired homologues) line up along the metaphase plate.
They line up in pairs, not single file. The orientation of each pair is random (independent assortment).
Anaphase I
Homologous chromosomes separate and move to opposite poles.
This is the critical difference from mitosis: whole chromosomes (each still consisting of two sister chromatids) go to opposite poles, not individual chromatids.
Telophase I
Chromosomes arrive at opposite poles and de-condense.
The cell splits into two cells, each containing one set of chromosomes (now haploid in chromosome number, though each chromosome still has two chromatids).
There is no interphase between meiosis I and meiosis II, and no further DNA replication.
Prophase II
Similar to mitosis: chromosomes are in the middle of the cell, spindle fibres start to form.
Metaphase II
Chromosomes line up along the metaphase plate in single file (not in pairs, unlike metaphase I).
Anaphase II
Sister chromatids are pulled apart by spindle fibres and move to opposite poles, exactly as in mitosis.
Telophase II
Nuclei re-form. Cytokinesis follows, splitting each cell into two.
The final result: four haploid cells, each with 23 chromosomes (in humans), each genetically unique.
Feature | Mitosis | Meiosis |
|---|---|---|
Cell type produced | Somatic (body) cells | Gametes (sex cells) |
Purpose | Growth, repair, asexual reproduction | Formation of gametes for sexual reproduction |
Number of divisions | 1 round (PMAT) | 2 rounds (PMAT twice) |
Preceded by interphase? | Always | Only before meiosis I |
Daughter cells produced | 2 diploid cells | 4 haploid cells |
Genetic identity | Identical to parent cell | Unique (crossing over + independent assortment) |
Crossing over? | No | Yes (prophase I) |
What separates in anaphase? | Sister chromatids | Homologous chromosomes (anaphase I), then sister chromatids (anaphase II) |
Chromosome number maintained? | Yes (2n to 2n) | Halved (2n to n) |
Students frequently mix up what separates in anaphase I versus anaphase II. In anaphase I, homologous chromosomes separate (whole chromosomes move apart). In anaphase II, sister chromatids separate (identical copies pull apart). This is one of the most heavily tested distinctions in introductory biology.
A common belief is that meiosis II includes its own round of DNA replication. It does not. There is no interphase (and no S phase) between meiosis I and meiosis II. The DNA was already replicated before meiosis I.
Students sometimes think crossing over happens between sister chromatids. It does not. Crossing over exchanges segments between non-sister chromatids of homologous chromosomes. Sister chromatids are identical, so swapping between them would produce no new combinations.
Some students assume that all four daughter cells from meiosis are identical to one another. They are not. Crossing over and independent assortment ensure that each of the four haploid cells is genetically unique.
The mitosis vs. meiosis comparison table is exam gold. Know every row.
Be able to explain why meiosis I is called the "reduction division" (it reduces chromosome count from 2n to n).
Understand the two sources of genetic variation in meiosis: crossing over (prophase I) and independent assortment (metaphase I).
Know that meiosis II is mechanically identical to mitosis (sister chromatids separate), and be prepared for questions that test this parallel.
Be ready to explain how crossing over works: homologous chromosomes pair up (synapsis), non-sister chromatids exchange segments at chiasmata, producing recombinant chromosomes.
If a question gives you a diploid number and asks for the result of meiosis, the answer is four haploid cells with half that number.
True or false: Meiosis produces two identical diploid cells.
Fill in the blank: The exchange of DNA between homologous chromosomes is called ____.
True or false: There is an interphase between meiosis I and meiosis II.
Fill in the blank: In anaphase I, ____ separate, whereas in anaphase II, ____ separate.
True or false: Meiosis II is mechanically similar to mitosis.
Answers: 1. False (four unique haploid cells). 2. Crossing over. 3. False (no interphase between the two divisions). 4. Homologous chromosomes; sister chromatids. 5. True.
Q: A cell with a diploid number of 16 undergoes meiosis. How many cells are produced, and how many chromosomes does each contain?
A: Four haploid cells, each with 8 chromosomes.
Q: Explain the difference between what happens at the metaphase plate in metaphase I and metaphase II.
A: In metaphase I, tetrads (paired homologous chromosomes) line up in pairs along the metaphase plate, and their orientation is random (independent assortment). In metaphase II, individual chromosomes (each still consisting of two sister chromatids) line up in single file, as in mitosis.
Q: What are the two main mechanisms that generate genetic diversity during meiosis?
A: Crossing over (exchange of DNA segments between homologous chromosomes during prophase I) and independent assortment (random orientation of homologous pairs at metaphase I).
Q: Why is meiosis I called the reduction division?
A: Because it reduces the chromosome number from diploid (2n) to haploid (n) by separating homologous chromosome pairs, so each daughter cell receives only one chromosome from each pair.
Q: Explain why crossing over between sister chromatids would not produce genetic variation.
A: Sister chromatids are identical copies of the same chromosome. Exchanging segments between identical sequences produces no new allele combinations. Variation comes from exchange between non-sister chromatids of homologous chromosomes, which may carry different alleles.
Meiosis is the biological basis of Mendelian genetics. The way homologous chromosomes segregate during anaphase I is the physical explanation for Mendel's Law of Segregation, and independent assortment explains his Law of Independent Assortment.
It also connects to population genetics and evolution. Genetic variation produced by meiosis is the raw material on which natural selection acts.
Errors in meiosis (non-disjunction) lead to conditions such as Down syndrome (trisomy 21), which you will encounter in genetics and human biology units.
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