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 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.[ref]
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
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