Méiose I

During prophase I, a diploid cell's chromatin condenses into typical X-shaped chromosomes that anchor to the nuclear envelope. Homologous chromosomes—one paternal and one maternal copy—pair together as protein threads align equivalent genes between them. These chromosomes then undergo crossing over and the synaptonemal complex, exchanging genetic material while remaining attached at exchange points.

The meiotic spindle apparatus, composed of microtubules from centrosomes, attaches to kinetochores on chromosome centromeres during prometaphase I. Microtubules elongate and fasten each homologous chromosome pair to opposite poles. During anaphase I, microtubule retraction pulls homologous chromosomes apart, segregating them to opposite sides of the cell.

Meiosis I produces genetically distinct haploid cells because homologous chromosomes are randomly oriented along the cell's midline during metaphase I before separating. Each resulting cell receives one chromosome from every homologous pair, but which maternal or paternal copy enters each cell varies randomly, creating genetic diversity.

The synaptonemal complex is a protein framework connecting paired homologous chromosomes, consisting of lateral elements along each chromosome, a central element between them, and transverse filaments linking these components. This ladder-like structure precisely aligns homologous chromosomes, enabling crossing over between equivalent genetic segments. The complex dissolves after recombination occurs.

During telophase I, separated chromosomes settle at opposite sides of the cell, relax from their condensed state, and become encircled by new nuclear envelopes. Concurrently, the cytoplasm divides through cytokinesis, forming two distinct cells. Meiosis I concludes with two genetically unique haploid cells, each containing half the original chromosome number.

Nondisjunction occurs when homologous chromosomes fail to separate evenly, causing both copies to enter one cell while the other receives none. When the cell with both homologs completes meiosis II, both resulting daughter cells possess two chromosome copies instead of one. This can produce trisomy conditions, such as trisomy 21, resulting from nondisjunction chromosomes and aneuploidy.

Meiosis I progresses through prophase I, where chromosomes condense and pair; prometaphase I, when the nuclear envelope disperses and microtubules attach to kinetochores; metaphase I, with random chromosome pair positioning; anaphase I, involving microtubule-driven separation; and telophase I, where chromosomes decondense and nuclear envelopes reform before cytokinesis.