Figure 19-35 Meiosis. Pre-meiotic cells have two copies of each chromosome (2n), one derived from the paternal parent and one from the maternal parent. For simplicity, the paternal and maternal homologs of only one chromosome are diagrammed. Step 1: All chromosomes are replicated during S phase before the first meiotic division, giving a 4n chromosomal complement. Cohesin complexes (not shown) link the sister chromatids composing each replicated chromosome along their full lengths. Step 2: As chromosomes condense during the first meiotic prophase, the replicated homologs pair and undergo homologous recombination, leading to at least one crossover event. At metaphase I, shown here, both chromatids of one chromosome associate with microtubules emanating from one spindle pole, but each member of a homologous chromosome pair associates with microtubules emanating from opposite poles. Step 3: During anaphase of meiosis I, the homologous chromosomes, each consisting of two chromatids, are pulled to opposite spindle poles. Step 4: Cytokinesis yields two daughter cells (now 2n), which enter meiosis II without undergoing DNA replication. At metaphase of meiosis II, shown here, the sister chromatids associate with spindle microtubules from opposite spindle poles, as they do in mitosis. Steps 5 and 6: Segregation of sister chromatids to opposite spindle poles during anaphase of meiosis II, followed by cytokinesis, generates haploid gametes (1n) containing one copy of each chromosome. Micrographs on the left show meiotic metaphase I and metaphase II in developing gametes from Lilium (lily) ovules. Chromosomes are aligned at the metaphase plate.

[Photos courtesy of Ed Reschke/Peter Arnold, Inc./Photolibrary/Getty Images.]