At the start of meiosis, a reproductive cell contains enough genetic material to create four sex cells, or gametes.

The first stage of meiosis, not surprisingly called Meiosis I, is dedicated to separating the homologous pairs of chromosomes into separate cells.

Each of the new cells only contains one copy of each chromosome instead of two. The number of chromosomes has been reduced by one half, so the daughter cells are haploid, or 1n.

Prophase I
During Prophase I, the chromatin coils and thickens, the nucleolus disappears, and the nuclear membrane is disassembled. Spindle fibers begin to form and the centrosomes start to move to the poles.

Homologous chromosomes recognize each other by the placement of their centromeres, and move together and intertwine in a process called synapsis. Where the pairs are in physical contact, they may exchange sections of themselves with one another, in a process called crossing over.

Prometaphase/Metaphase I
Centromeres attach to spindle fibers in early metaphase I (sometimes called prometaphase I). Each chromosome in a pair is attached to spindle fibers radiating from a different centrosome, on opposite sides of the cell. The pull of opposing fibers begins to slide the chromosomes towards the center of the cell.

Metaphase I
By the end of metaphase I, the chromosomes are tugged into alignment along a plane at the midpoint of the cell called the metaphase plate. Homologous chromosomes are lined up side-by-side, one on each side of the metaphase plate. There is no coordination between maternal and paternal chromosomes—each pair of homologous chromosomes moves independently of every other pair.

Anaphase I
In anaphase I, motor proteins on the centromeres reel the chromosomes down the attached spindle fibers, and the fibers themselves start to shorten. Both these actions pull the homologous pairs away from one another and towards opposite poles of the cell.

This action segregates one half of each chromosome pair in each end of the cell, and sets up production of haploid daughter cells.

Telophase I
Telophase I returns the cell to a somewhat interphase-like appearance. The chromosomes unwind and become once again long and thread like, the nuclear membrane reforms around them, and the nucleolus reappears.

Cytokinesis splits the cell into two separate cells, with both chromosomes and cytoplasm divided evenly in half. Each daughter cell has one member of each homologous pair of chromosomes and is haploid. Each chromosome consists of two sister chromatids. Because of crossing-over in Prophase I, the sister chromatids are not identical.