Meiosis begins with replicated chromosomes, but its first division treats homologous chromosomes differently from mitosis. During meiotic prophase I, homologs recognize one another, align along their lengths and can exchange corresponding DNA segments.

Synapsis pairs homologous chromosomes

The close alignment of homologous chromosomes is called synapsis. In many eukaryotes, a protein structure called the synaptonemal complex helps stabilize this side-by-side alignment during prophase I.

Because each homolog has already replicated, a synapsed homolog pair contains four chromatids:

homolog 1:  sister A1 — sister A2
homolog 2:  sister a1 — sister a2

The four-chromatid structure is often called a tetrad or bivalent.

Meiotic recombination occurs between nonsister chromatids

Homologous recombination can be initiated by programmed DNA double-strand breaks. A broken chromatid can use a nonsister chromatid of the homologous chromosome as a homologous template.

Strand invasion, DNA synthesis and resolution of recombination intermediates can produce a crossover, in which chromosome segments on opposite sides of the recombination site are exchanged between the participating nonsister chromatids.

A crossover therefore creates recombinant chromatids that contain DNA derived from both homologous chromosome copies.

Chiasmata provide a physical connection

After the synaptonemal complex disassembles, crossover sites can remain visible as chiasmata (singular: chiasma), physical connections between homologous chromosomes.

Chiasmata, together with sister-chromatid cohesion, help keep homologous chromosomes mechanically linked until they are attached to the meiosis-I spindle.

This matters because homologs must behave as a coordinated pair before they segregate to opposite poles.

Not every recombination event must become a crossover

Homologous recombination can also resolve without exchanging flanking chromosome arms. Such outcomes are noncrossovers.

The key meiotic specialization is therefore not simply 'DNA repair happens'. Meiosis deliberately uses homolog pairing and homologous-recombination machinery so that replicated maternal- and paternal-derived homologs can become physically connected and, in some cases, exchange corresponding DNA segments.

This prepares homologous chromosome pairs for the distinctive segregation of meiosis I.