Meiosis I and II: Stages of Gamete Production

Meiosis is a type of cell division that halves the chromosome number, producing four genetically different haploid cells (gametes) from one diploid cell, in two successive divisions called meiosis I and meiosis II. In brief: in humans, a diploid cell with 46 chromosomes undergoes meiosis to produce four haploid cells with 23 chromosomes each. Meiosis I separates the homologous chromosomes, and meiosis II separates the sister chromatids. When two gametes fuse at fertilization, the diploid number is restored.

Meiosis is the basis of sexual reproduction, and it explains why offspring differ from their parents. This guide explains diploid and haploid cells, describes each stage of meiosis I and II, shows how gametes form in males and females, compares meiosis with mitosis, works through a chromosome-counting example and corrects common mistakes.

What are haploid and diploid cells?

Most body cells are diploid (2n), with two sets of chromosomes, one from each parent. In humans, 2n = 46, arranged as 23 pairs of homologous chromosomes. Gametes (sperm and eggs) are haploid (n), with a single set of 23 chromosomes. If gametes were diploid, the chromosome number would double in every generation. Meiosis prevents this by halving the number before fertilization.

Meiosis happens only in the reproductive organs: the testes and the ovaries in animals, and the anthers and ovules in flowering plants. It is preceded by interphase, in which the DNA is replicated, so that each chromosome consists of two sister chromatids.

What happens in meiosis I?

Meiosis I is called the reduction division, because the cell changes from diploid to haploid.

Prophase I

Chromosomes condense. Homologous chromosomes come together and pair up in a process called synapsis, forming a structure called a bivalent (or tetrad, with four chromatids). The chromatids of homologous chromosomes exchange segments of DNA by crossing over, at points called chiasmata. The nuclear envelope breaks down, and the spindle forms. This is the longest stage, and it contributes to genetic variation.

Metaphase I

The bivalents line up on the equator. The orientation of each pair is random, with either the maternal or the paternal chromosome facing a given pole. This is independent assortment.

Anaphase I

The homologous chromosomes are pulled to opposite poles. The centromeres do not split, so each chromosome still consists of two chromatids.

Telophase I and cytokinesis

The chromosomes arrive at the poles, and the cell divides into two cells. Each cell is now haploid, with 23 chromosomes (each with two chromatids), but each has the amount of DNA equal to a diploid cell that has not replicated. A short interphase may follow, but there is no DNA replication.

What happens in meiosis II?

Meiosis II is similar to mitosis, but in haploid cells.

1. Prophase II: chromosomes condense again, and a new spindle forms.
2. Metaphase II: chromosomes line up on the equator, individually (not in pairs).
3. Anaphase II: the centromeres split, and the sister chromatids separate and move to opposite poles.
4. Telophase II and cytokinesis: nuclear envelopes form, and the cells divide.

The result is four haploid cells. Each has 23 single chromosomes. Because of crossing over, the sister chromatids are no longer identical, so the four cells are genetically different from one another.

How are gametes formed?

In males, spermatogenesis produces four functional sperm cells from each cell that undergoes meiosis. The cells then change shape (differentiate) into sperm with a tail. In females, oogenesis produces only one large egg cell from each meiosis. The cytoplasm divides unequally, and the smaller cells, called polar bodies, degenerate. This gives the egg a large store of nutrients. In human females meiosis starts before birth, pauses in prophase I for years, resumes at ovulation and is finished only if the egg is fertilized.

Meiosis and mitosis compared

Worked example: tracking chromosomes through meiosis

A species has 2n = 8 chromosomes. Follow one cell through meiosis.

1. Number of possible gametes by independent assortment: with n = 4 chromosome pairs, each gamete gets one of each pair, so there are 2⁴ = 16 combinations.
2. For humans (n = 23): 2²³ = 8 388 608 combinations (about 8.4 million) from independent assortment alone.
3. Add fertilization: two parents each give 8.4 million possible gametes, so the number of chromosome combinations in a child is 8.4 million × 8.4 million ≈ 7 × 10¹³. This does not yet include crossing over, which increases the number much more.

In the table, the cell after meiosis I has 4 chromosomes, because the homologs have separated, but each still consists of two chromatids. In anaphase II, the number of chromosomes in each cell is momentarily 8 when the chromatids separate, and the cell then divides into cells with 4 each. The numbers show that the chromosome number is reduced at meiosis I, not meiosis II.

How was meiosis discovered?

Oscar Hertwig observed meiosis in sea urchin eggs in 1876. August Weismann predicted in 1887 that a reduction division must exist to keep the chromosome number constant across generations. The behavior of chromosomes in meiosis was linked to Mendel’s laws by Walter Sutton and Theodor Boveri around 1902. Barbara McClintock and Harriet Creighton showed in 1931 that crossing over involves a physical exchange of chromosome segments.

Which mistakes are common?

Mistake 1: Saying that meiosis II halves the chromosome number

The reduction occurs in meiosis I, when homologs separate. Meiosis II separates sister chromatids.

Mistake 2: Mixing up homologous chromosomes and sister chromatids

Homologs are a pair of chromosomes, one from each parent. Sister chromatids are the two copies of the same chromosome.

Mistake 3: Believing that the four cells are identical

Crossing over and independent assortment make them genetically different.

Mistake 4: Thinking that DNA is replicated between meiosis I and II

There is no DNA replication between the two divisions.