In a diploid cell, chromosomes occur as pairs. Chromosomes duplicate just before nuclear division. Duplication produces two identical parts called sister chromatids, held together at centromere. Nonsister chromatids do not share the same centromere. Meiosis keeps chromosome number constant across chromosome number.
Gametes have only one member of each homologous pair.
Meiosis I is the nuclear division at the first meiotic division. Prior to meiosis I, DNA replication occurs and each chromosome has two sister chromatids. During synapsis, the two sets of paired chromosomes lay alongside each other as bivalents or a tetrad. After crossing-over occurs, sister chromatids of a chromosome are no longer identical. No replication of DNA needed between meiosis I and II because chromosomes were already doubled.