Source: Mastering Bio, University of Texas at Austin
Difficulty: Introductory to Intermediate | Prerequisites: Cell cycle phases (G1, S, G2, M), basic chromosome structure, difference between DNA and chromatin.
Meiosis is the specialised form of cell division that produces gametes (sex cells) with half the chromosome number of the parent cell. Where mitosis makes identical copies for growth and repair, meiosis introduces genetic variation, which is the raw material for evolution. This topic builds directly on your understanding of the cell cycle and chromosome structure. It feeds forward into Mendelian genetics, inheritance patterns, and population genetics. If you are comfortable with what happens during interphase and mitosis, you are ready for this.
Meiosis is two rounds of division (meiosis I and meiosis II) that turn one diploid cell into four haploid cells. Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids. Three mechanisms generate genetic variation along the way: crossing over, independent assortment of homologues, and random fertilisation.
Diploid (2n)
A cell containing two complete sets of chromosomes, one inherited from each parent. In humans, 2n = 46.
Haploid (n)
A cell containing a single set of chromosomes. Gametes (eggs and sperm) are haploid. In humans, n = 23.
Homologous chromosomes (homologues)
A pair of chromosomes (one maternal, one paternal) that carry the same gene loci but may have different alleles of each gene. Think of them as "matching" chromosomes that are similar but not identical.
Sister chromatids
Two identical copies of a single chromosome, produced during DNA replication and joined at the centromere. In simple terms, they are exact duplicates held together until they are pulled apart during division.
Non-sister chromatids
Chromatids belonging to different members of a homologous pair. These are the chromatids involved in crossing over.
Centromere
The region of a chromosome where sister chromatids are joined and where spindle fibres attach.
Synapsis
The pairing of homologous chromosomes during prophase I, in which homologues line up gene by gene along their length.
Tetrad (bivalent)
The four-chromatid structure formed when two homologous chromosomes (each consisting of two sister chromatids) are synapsed during prophase I. Think of it as four chromatids bundled together: two from one homologue, two from the other.
Crossing over (recombination)
The exchange of segments between non-sister chromatids of homologous chromosomes during prophase I. This produces recombinant chromosomes with new combinations of alleles.
Recombinant chromosome
A chromosome that carries a mix of alleles from both the maternal and paternal homologues, produced by crossing over.
Independent assortment
The random orientation of homologous pairs at metaphase I, which determines how maternal and paternal chromosomes are sorted into daughter cells. For a cell with n chromosomes, there are 2^n possible combinations.
Random fertilisation
The chance union of any one egg with any one sperm, further multiplying genetic variation beyond what meiosis alone produces.
Gamete
A haploid reproductive cell (egg or sperm) produced by meiosis. When two gametes fuse at fertilisation, the diploid chromosome number is restored.
Meiosis I separates homologous chromosomes. The cell enters meiosis I as a diploid cell (2n) with replicated chromosomes (each chromosome = two sister chromatids). It ends as two haploid cells, each still containing sister chromatids.
Prophase I: Chromosomes condense. Homologous chromosomes undergo synapsis, forming tetrads. Crossing over occurs between non-sister chromatids. A spindle forms. This is the longest and most complex phase of meiosis.
Metaphase I: Homologous pairs (tetrads) line up at the metaphase plate in pairs (not single file). The orientation of each pair is random, which is the basis of independent assortment.
Anaphase I: Homologous chromosomes separate and migrate to opposite poles. Sister chromatids remain joined. This is the step that reduces the chromosome number.
Telophase I and cytokinesis: Two haploid cells form. Each cell has one member of each homologous pair, still as two sister chromatids joined at the centromere.
Meiosis II resembles mitosis. It separates sister chromatids. No further DNA replication occurs between meiosis I and meiosis II.
Prophase II: A spindle forms in each haploid cell. Chromosomes condense again if they had decondensed.
Metaphase II: Individual chromosomes (each consisting of two sister chromatids) align single file along the equator of the cell.
Anaphase II: Sister chromatids separate at the centromere and move to opposite poles.
Telophase II and cytokinesis: Four haploid daughter cells form, each with a single set of unreplicated chromosomes.
DNA content is halved in both meiosis I and meiosis II. Ploidy level changes from diploid to haploid during meiosis I and remains haploid in meiosis II.
A cell in G0 prior to meiosis has a DNA content of 2n (the diploid amount, unreplicated). After S phase, DNA content doubles (to 4n worth of DNA in a 2n cell). After meiosis I, each cell has 2n worth of DNA in a haploid (n) cell. After meiosis II, each cell has n worth of DNA.
Different organisms alternate between haploid and diploid states differently:
Animals: Diploid-dominant. Meiosis produces gametes directly. Fertilisation restores 2n.
Plants and some algae: Alternation of generations. Both haploid and diploid stages undergo mitosis. Meiosis produces spores (not gametes), which divide by mitosis to produce a haploid organism that later produces gametes.
Most fungi and some protists: Haploid-dominant. The diploid stage is brief; meiosis occurs soon after fertilisation.
The diploid number is always restored by fertilisation.
During prophase I, non-sister chromatids of homologous chromosomes exchange segments. This produces recombinant chromosomes that carry a new mix of maternal and paternal alleles. Without crossing over, each chromosome would be entirely maternal or entirely paternal.
If crossing over did not occur, there would be less genetic variation among gametes. Meiosis would still produce haploid cells, and independent assortment would still operate, but the diversity of allele combinations on any single chromosome would be lost.
At metaphase I, each homologous pair orients randomly at the metaphase plate. The maternal homologue can face either pole, independently of how every other pair orients. For a cell with n chromosome pairs, this produces 2^n possible combinations of maternal and paternal chromosomes in the gametes.
For example, a diploid cell with 2n = 6 (three pairs) can produce 2^3 = 8 different chromosome combinations. For humans (n = 23), there are 2^23 = 8.4 million possible combinations from independent assortment alone.
Any one of the millions of genetically distinct sperm can fuse with any one of the genetically distinct eggs. This multiplies the variation produced by meiosis enormously.
DNA replication (heredity: faithful copying; variation: occasional mutation)
Crossing over (new allele combinations on chromosomes)
Fertilisation (combining two independent haploid genomes)
Independent assortment at metaphase I to anaphase I (random distribution of homologues)
Sister chromatid separation at metaphase II to anaphase II (distributing recombinant and non-recombinant chromatids)
Students often confuse homologous chromosomes with sister chromatids. Homologous chromosomes are two separate chromosomes (one from each parent) that carry the same genes at the same loci but may carry different alleles. Sister chromatids are identical copies of one chromosome, joined at the centromere, produced by DNA replication.
Students sometimes think meiosis I and meiosis II are the same process repeated. They are not. Meiosis I separates homologues (reductive); meiosis II separates sister chromatids (equational, similar to mitosis).
Students often believe crossing over can happen during mitosis. Crossing over is specific to prophase I of meiosis, where homologues are synapsed. In mitosis, homologues do not pair up.
Students sometimes think a species with 2n = 16 has 16 pairs. It does not. It has 8 pairs (16 total chromosomes, arranged as 8 homologous pairs).
Students frequently confuse DNA content with ploidy. After meiosis I, a cell is haploid (n) but still has two sister chromatids per chromosome, so its DNA content is still elevated relative to a fully divided haploid cell.
⚠️ Know which phase each event occurs in. This is heavily tested: synapsis = prophase I; homologues separate = anaphase I; sister chromatids separate = anaphase II; chromosomes line up as pairs = metaphase I; chromosomes line up single file = metaphase II.
⚠️ Be able to identify meiotic phases from diagrams. Look at whether chromosomes are in pairs (meiosis I) or singles (meiosis II), and whether they are at the plate, separating, or at the poles.
⚠️ Understand the difference between processes unique to meiosis (synapsis, crossing over, reduction of ploidy) and those shared with mitosis (sister chromatid separation, spindle formation).
⚠️ Crossing over produces recombinant chromosomes. Know how to identify recombinant gametes in diagrams (they show mixed colours or mixed allele patterns on a single chromosome).
⚠️ Throughout meiosis I, each chromosome consists of two sister chromatids joined by a centromere. This persists from the start of prophase I through the end of telophase I.
⚠️ Questions about "what would happen without crossing over" test whether you understand its specific contribution: less variation, but meiosis still works, and independent assortment still operates.
True or false: Meiosis I produces two haploid cells.
True.
Fill in the blank: Synapsis occurs during ______.
Prophase I.
True or false: Sister chromatids separate during anaphase I.
False. Sister chromatids separate during anaphase II. Homologous chromosomes separate during anaphase I.
Fill in the blank: A species with 2n = 16 has ______ homologous pairs.
True or false: If crossing over did not occur, meiosis would not produce haploid cells.
False. Meiosis would still produce haploid cells. There would just be less genetic variation among them.
Q: Meiosis I produces how many cells, and what is their ploidy?
A: Two cells, each haploid (n). Each still contains sister chromatids.
Q: Meiosis II typically produces how many cells total, and what is their ploidy?
A: Four cells total (from the two cells that entered meiosis II), each haploid (n) with unreplicated chromosomes.
Q: During which phase do sister chromatids separate?
A: Anaphase II.
Q: At the end of which phase and cytokinesis do haploid cells contain chromosomes that each consist of two sister chromatids?
A: Telophase I. After meiosis I, each haploid cell still has chromosomes made of two joined sister chromatids.
Q: During which phase do chromosomes align single file along the equator of a haploid cell?
A: Metaphase II.
Q: How are sister chromatids and homologous chromosomes different?
A: Homologous chromosomes contain the same gene loci but may have different alleles. Sister chromatids are identical copies of each other produced during DNA replication.
Q: Which processes lead to the most genetic variation in sexually reproducing organisms?
A: Crossing over, independent assortment of chromosomes in meiosis, and random fertilisation.
Q: Which sample of DNA, with a relative content of 2n, might be from a cell that stopped dividing in G0 prior to meiosis?
A: The sample showing a DNA content equal to the basic diploid (2n) amount (unreplicated). This corresponds to a cell that entered G0 before S phase.
Q: If crossing over did not occur, which statement about meiosis would be true?
A: There would be less genetic variation among gametes. Independent assortment would still occur, and meiosis would still produce haploid cells, but recombinant chromosomes would not form.
Meiosis connects directly to Mendelian genetics: independent assortment is the physical basis of Mendel's Law of Independent Assortment, and crossing over explains why linked genes sometimes separate. Understanding ploidy changes is essential for grasping chromosomal abnormalities (nondisjunction, aneuploidy) covered in later units. The life cycle material ties into plant biology (alternation of generations) and evolutionary biology (why sexual reproduction persists despite its costs).
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