Meiosis is a two-part cell division that turns one diploid cell (with 46 chromosomes in humans) into four haploid cells (with 23 chromosomes each). Meiosis I separates paired homologous chromosomes, cutting the chromosome number in half. Meiosis II then separates sister chromatids, much like a regular mitotic division. Each stage has distinct events that work together to produce genetically unique gametes.
Before Meiosis Begins: DNA Replication
Before meiosis I starts, the cell copies all of its DNA during S phase. A human cell enters meiosis with 46 chromosomes, but each chromosome now consists of two identical sister chromatids joined at the centromere. That gives the cell 92 chromatids total. This replication step is critical because it provides the raw material for two rounds of division.
Prophase I: The Longest and Most Complex Stage
Prophase I is where most of the action happens, and it’s by far the longest phase of meiosis. It unfolds in five sub-stages, each with a specific role in preparing chromosomes for division.
Leptotene and Zygotene
During leptotene, chromosomes begin to condense and the cell creates deliberate breaks in its own DNA, called double-strand breaks. The chromatin shifts to one side of the nucleus, forming a characteristic half-moon shape. In zygotene, homologous chromosomes (the matching pairs you inherited from each parent) begin finding each other and lining up side by side. A protein scaffold called the synaptonemal complex assembles between them, zipping the homologous pairs tightly together in a process called synapsis.
Pachytene
By pachytene, synapsis is complete. Each pair of homologous chromosomes is fully connected, forming a structure called a tetrad or bivalent, which contains four chromatids total (two from each homolog). This is when crossing over happens: segments of DNA swap between non-sister chromatids within the tetrad. The earlier double-strand breaks are repaired using the homologous chromosome as a template, and in the process, genetic material is physically exchanged between the maternal and paternal copies. This is the first major source of genetic variation in meiosis.
Diplotene and Diakinesis
In diplotene, the synaptonemal complex disassembles and the homologous chromosomes begin to pull apart. They don’t separate completely, though. The sites where crossing over occurred remain physically linked as X-shaped structures called chiasmata. These chiasmata are essential: they hold the homologous pair together so the chromosomes can align properly later. Cells that fail to form chiasmata often produce gametes with abnormal chromosome numbers. During diakinesis, chromosomes condense further until the six bivalents (in organisms like the roundworm C. elegans, or 23 bivalents in humans) are clearly visible as compact, distinct structures. The nuclear envelope breaks down, and the cell is ready for division.
Metaphase I Through Telophase I
In metaphase I, the bivalents line up along the center of the cell. Spindle fibers from opposite poles attach to the kinetochores of each homologous pair. Here’s an important detail: both sister chromatids of one homolog face the same pole, while both sister chromatids of the other homolog face the opposite pole. This monopolar orientation is what allows whole homologs, rather than individual chromatids, to be pulled apart.
The orientation of each bivalent at the cell’s equator is random. Whether your maternal or paternal copy of chromosome 1 goes to the left or right pole is independent of where chromosome 2 ends up. This randomness is called independent assortment, and it’s the second major source of genetic variation. With 23 chromosome pairs in humans, independent assortment alone can produce 2^23 (over 8 million) different gamete combinations from a single person.
During anaphase I, the spindle fibers pull homologous chromosomes apart to opposite poles. The protein glue (cohesin) along the chromosome arms is cut, allowing the homologs to separate. Critically, cohesin at the centromeres is protected from being cut during this division. That protection keeps sister chromatids attached to each other, so they travel together as a unit.
In telophase I, the separated chromosomes arrive at opposite poles and a nuclear envelope reforms around each set. The cell divides through cytokinesis, producing two daughter cells. Each cell now has 23 chromosomes (haploid), but each chromosome still consists of two sister chromatids, so each cell contains 46 chromatids.
Interkinesis: The Brief Pause
Between the two divisions, there is a short period called interkinesis. The nuclear membrane reforms and, in some cell types, the spindle disintegrates and chromosomes briefly relax. The key point is that no DNA replication occurs during this phase. The cell moves into meiosis II with the same amount of DNA it had at the end of meiosis I.
Meiosis II: Splitting Sister Chromatids
Meiosis II closely resembles a normal mitotic division, but it starts with a haploid cell instead of a diploid one. No crossing over or homolog pairing happens here.
In prophase II, the nuclear envelope dissolves again and a new spindle forms. In metaphase II, the 23 chromosomes (each still made of two sister chromatids) line up individually along the cell’s equator. Unlike meiosis I, where both sister chromatids faced the same pole, in meiosis II the sister chromatids orient toward opposite poles in a bipolar fashion.
Anaphase II is where the final separation occurs. The centromeric cohesin that was protected during meiosis I is now deprotected, allowing the enzyme separase to cut it. The sister chromatids are pulled apart to opposite sides of the cell, becoming individual chromosomes. In telophase II, a nuclear envelope forms around each set of chromosomes, and cytokinesis divides the cell. Each of the two cells from meiosis I produces two daughter cells, yielding four haploid cells total.
What Each Cell Ends Up With
Each of the four final cells contains 23 chromosomes, and each chromosome now consists of a single chromatid (23 chromatids total per cell). In males, all four cells develop into sperm. In females, the divisions are unequal: one large egg cell receives most of the cytoplasm, and the other three become small polar bodies that typically degrade.
No two of these four cells are genetically identical. Crossing over during prophase I shuffled genetic material between maternal and paternal chromosomes, and independent assortment during metaphase I randomized which version of each chromosome ended up in which cell. Together, these two mechanisms generate an enormous amount of genetic diversity in every round of gamete production.
How Meiosis I and Meiosis II Differ
- What separates: Meiosis I pulls apart homologous chromosomes. Meiosis II pulls apart sister chromatids.
- Chromosome number: Meiosis I reduces the count from 46 to 23 (reductional division). Meiosis II keeps it at 23 but splits the chromatids (equational division).
- Genetic recombination: Crossing over and independent assortment occur in meiosis I. Neither happens in meiosis II.
- DNA replication: Occurs before meiosis I but not before meiosis II.
- Spindle attachment: In meiosis I, sister chromatids attach to the same pole (monopolar). In meiosis II, they attach to opposite poles (bipolar), just like in mitosis.
- Starting cells: Meiosis I begins with one diploid cell and ends with two haploid cells. Meiosis II begins with two haploid cells and ends with four haploid cells.