Pachytene is a stage of the first meiotic prophase in which paired homologous chromosomes are closely associated through the synaptonemal complex. Each replicated pair contains four chromatids: two sister chromatids from each homologous chromosome. Pairing therefore does not reduce four chromatids to two.
Pachytene is also an experimentally distinguishable state, rather than merely a point on a clock. In budding yeast, NDT80 gene mutants arrested with full-length synaptonemal complexes even though much genetic recombination proceeded. Comparing chromosome structure, DNA exchange and later division showed that these events can be experimentally separated.[ref]
NDT80, a meiosis-specific gene required for exit from pachytene in Saccharomyces cerevisiae by L. Xu and others. Abstract; pachytene arrest and relatively normal recombination
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