The mechanisms behind mitotic recombination are similar to those behind meiotic recombination. Meiosis begins with DNA replication and meiotic crossing over. There is evidence that CO recombination facilitates meiotic chromosome segregation. Immediately following DNA replication, meiotic cells enter a prolonged G-like stage known as meiotic prophase. The budding yeast "Saccharomyces cerevisiae" is a model organism used for studying meiotic recombination. The oocyte completes its second meiotic division. It occurs at sexual maturity when hormones stimulate the oocyte to complete meiotic division I. Proper segregation is essential for producing haploid meiotic products with a normal complement of chromosomes. This process appears to involve genetic recombination as well as a meiotic mechanism. Dmc1 function appears to be limited to meiotic recombination. MLH1 protein appears to localize to sites of crossing over in meiotic chromosomes. Such Down syndrome trisomies are indistinguishable from Down syndrome trisomy created through meiotic nondisjunction. Molecular models of meiotic recombination have evolved over the years as relevant evidence accumulated. Trisomy 18 (47,XX,+18) is caused by a meiotic nondisjunction event. Thus, such findings suggest that meiotic sex arose early in eukaryotic evolution.