If gametes contained same number of chromosomes as body cells, doubling would soon fill cells. Life cycle refers to all reproductive events between one generation and next. Meiosis occurs at different points during life cycle of various organisms. 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.