Difficulty: Introductory | Prerequisites: Basic understanding of DNA structure and cell division (mitosis).
Meiosis is the specialised cell division that produces sex cells (gametes). It sits at the heart of sexual reproduction, and you will encounter it repeatedly across genetics, evolution, and developmental biology. Before tackling this material, you should be comfortable with the basic structure of DNA and the difference between a chromosome and a gene. If you understand how mitosis copies cells, meiosis is the next logical step: instead of making identical copies, it halves the chromosome number and shuffles the genetic deck, which is why offspring are never carbon copies of their parents.
Meiosis is the two-stage cell division that turns one diploid cell (46 chromosomes in humans) into four haploid gametes (23 chromosomes each). It halves the chromosome count so that fertilisation restores the full number, and it introduces genetic variation through independent assortment and crossing over.
Meiosis
A type of cell division that reduces the chromosome number by half, producing four genetically unique haploid cells from one diploid parent cell. Think of it as the body's way of making sex cells that carry only half the instruction manual.
Gamete
A reproductive cell (sperm in males, egg in females) that is haploid, meaning it carries one set of chromosomes. In simple terms, gametes are the cells that fuse at fertilisation to start a new organism.
Haploid
A cell that contains only one complete set of chromosomes (23 in humans, written as "n"). Think of it as half the usual genetic toolkit.
Diploid
A cell that contains two complete sets of chromosomes (46 in humans, written as "2n"), one inherited from each parent. In simple terms, this is the normal chromosome count for most of your body's cells.
Homologous chromosomes (homologous pairs)
Paired chromosomes, one from each parent, that carry genes for the same traits at the same locations but may have different alleles. Think of them as two editions of the same textbook: same chapters, but some of the wording differs.
Zygote
The diploid cell formed when a sperm fertilises an egg. In simple terms, this is the very first cell of a new individual.
PMAT
An acronym for the four stages of cell division: Prophase, Metaphase, Anaphase, Telophase. In meiosis, PMAT runs twice (once in Meiosis I, once in Meiosis II).
DNA replication
The process by which a cell copies all of its DNA before division begins. Think of it as photocopying the entire instruction set so each daughter cell can receive a copy.
Sexual reproduction requires two gametes (sperm and egg) to fuse at fertilisation.
If gametes were diploid (46 chromosomes), the zygote would have 92 chromosomes, doubling every generation.
Meiosis solves this by halving the chromosome count to 23 per gamete, so the zygote ends up with the correct 46.
Before meiosis begins, the cell undergoes DNA replication, copying every chromosome.
The cell then divides twice: Meiosis I and Meiosis II.
The result is four haploid daughter cells, each with half the DNA of the original parent cell.
Homologous chromosome pairs line up at the cell's centre during Metaphase I.
Homologous pairs are separated, one going to each daughter cell.
Each daughter cell now has 23 chromosomes (still in their duplicated, X-shaped form).
This is the step that actually reduces the chromosome number from diploid to haploid.
Functions much like mitosis.
Sister chromatids (the two identical halves of each duplicated chromosome) are pulled apart.
Each cell from Meiosis I produces two daughter cells.
Final result: four haploid cells, each with 23 single chromosomes.
In males, all four daughter cells develop into sperm.
In females, typically only one of the four becomes a mature egg (the others become polar bodies).
Each gamete is genetically unique due to independent assortment and crossing over (covered in detail in the companion notes on gene inheritance and genetic variation).
Key relationship to remember:
1 diploid parent cell (2n = 46) → DNA replication → Meiosis I → 2 haploid cells (n = 23, still duplicated) → Meiosis II → 4 haploid daughter cells (n = 23, single chromosomes)
Chromosome number shorthand:
"n" = the haploid number (23 in humans)
"2n" = the diploid number (46 in humans)
After DNA replication but before division, each chromosome consists of two joined sister chromatids.
Meiosis is the biological basis of genetic diversity in sexually reproducing organisms. This is why siblings from the same parents look different from one another, and it is the same mechanism that plant and animal breeders rely on when they cross varieties to produce new traits. Errors in meiosis (such as nondisjunction, where chromosomes fail to separate properly) can result in conditions like Down syndrome (trisomy 21), making this process directly relevant to clinical genetics and genetic counselling.
Students often confuse meiosis with mitosis. Mitosis produces two identical diploid cells; meiosis produces four genetically unique haploid cells. They are different processes with different purposes.
Students sometimes think that meiosis halves the DNA content in one step. It does not. Meiosis I separates homologous pairs, and Meiosis II separates sister chromatids. Both steps are required.
Students frequently believe gametes are diploid. Gametes are haploid (n = 23 in humans). If they were diploid, the chromosome number would double every generation.
Students occasionally assume that DNA replication occurs between Meiosis I and Meiosis II. It does not. Replication happens only once, before Meiosis I begins.
⚠️ Know the chromosome numbers: 23 per gamete, 46 per somatic cell. Questions that ask you to fill in blanks about sperm, egg, and zygote chromosome counts appear regularly.
⚠️ Be able to distinguish Meiosis I (homologous pairs separate, reduction division) from Meiosis II (sister chromatids separate, similar to mitosis).
⚠️ Understand why meiosis is necessary for fertilisation: if gametes were diploid, chromosome numbers would spiral out of control.
⚠️ Be prepared to compare the DNA content in parent cells versus daughter cells. The answer: daughter cells have half the DNA of the parent cell.
True or False: Meiosis produces two diploid daughter cells. (False, it produces four haploid daughter cells.)
Fill in the blank: Human gametes contain ______ chromosomes. (23)
True or False: DNA replication occurs between Meiosis I and Meiosis II. (False, it occurs only before Meiosis I.)
Fill in the blank: The fusion of two gametes during fertilisation produces a ______. (zygote)
True or False: Gametes are diploid cells. (False, gametes are haploid.)
Q: Why is it necessary for meiosis to halve the chromosome number before fertilisation?
A: Because fertilisation combines genetic material from two parents. If gametes were diploid (46 chromosomes each), the resulting zygote would have 92 chromosomes. Meiosis ensures each gamete is haploid (23 chromosomes), so the zygote has the correct diploid number (46).
Q: How does the amount of DNA in the daughter cells of meiosis compare to the amount in the parent cell?
A: The daughter cells have half the DNA of the parent cell. The parent cell is diploid (2n), and each of the four resulting daughter cells is haploid (n).
Q: What are the two types of gametes and what ploidy do they have?
A: Sperm (male) and egg (female). Both are haploid, containing 23 chromosomes in humans.
Q: Describe the key difference between Meiosis I and Meiosis II.
A: Meiosis I is the reduction division: homologous pairs are separated, reducing the cell from diploid to haploid. Meiosis II is more like mitosis: sister chromatids are separated, and the chromosome number stays the same (haploid).
Q: What must happen to DNA before meiosis begins, and why?
A: The cell must replicate its DNA so that each chromosome consists of two joined sister chromatids. This ensures there is enough genetic material to be divided across two rounds of division and still produce four complete haploid cells.
This material connects directly to gene inheritance and genetic variation. Independent assortment and crossing over (both covered in the companion notes) are mechanisms within meiosis that generate the diversity you see in offspring. Understanding meiosis is also essential for grasping Mendelian genetics, Punnett squares, and inheritance patterns such as autosomal recessive disorders. In evolution, meiosis is the engine that provides the raw variation on which natural selection acts.
Meiosis, meiotic division, reduction division, cell division, gamete formation, gametogenesis, haploid, diploid, homologous chromosomes, homologous pairs, sex cells, sperm, egg, oocyte, spermatocyte, zygote, fertilisation, chromosome number, 2n, n, PMAT, Prophase I, Metaphase I, Anaphase I, Telophase I, Meiosis I, Meiosis II, DNA replication, sister chromatids, daughter cells, sexual reproduction, genetic diversity