= Prophase I
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<Prophase I> is the extended first preparatory stage of <meiosis I>, after <DNA replication> and before homologous <chromosomes> separate. Homologues align, the <synaptonemal complex> can assemble between them, and <crossing over> exchanges material between nonsister <chromatids>. These events connect inherited copies while preparing their later segregation.
Suppose a diploid <cell> has $2n=4$ <chromosomes>. After <DNA replication> it still has four <chromosomes>, each consisting of two <chromatids>. Pairing the homologues produces two pairs, each containing four <chromatids>; it does not create eight independent homologous <chromosomes>. Distinguishing chromosome count from DNA-copy count prevents a common error in following <meiosis>.
The events are experimentally separable. Yeast lacking normal Zip1 can pair homologues while failing to assemble the usual <synaptonemal complex> and to progress normally through <meiosis>.<ZIP1 is a synaptonemal complex protein required for meiotic chromosome synapsis by M. Sym and others>[1993, abstract: pairing, complex-assembly defect and meiotic arrest]{ref}
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