95 3.3 Mitosis and meiosis Meiosis then proceeds through the following stages (Figures 3-24 and 3-29): 1. Prophase I: As in mitosis, the sister chromatids become visible, closely adhered side by side. However, in contrast with mitosis, the sister chromatids (although fully replicated at the DNA level) show an apparently undivided centromere. The sister chromatid pairs at this stage are called dyads, from the Greek word for “two.” 2. Metaphase I: The homologous dyads now pair to form structures called bivalents. Thus, any one bivalent contains a total of four chromatids, sometimes referred to as a tetrad (Greek; four). This stage represents the most obvious difference from mitosis. Pairing of the dyads to form a bivalent is accomplished by molecular assemblages called synaptonemal complexes along the middle of the tetrads (Figure 3-30). Although the existence of synaptonemal complexes has been known for some time, the precise working of these structures is still a topic of research. Nonsister chromatids of the tetrad engage in a
This is the end of the preview. Sign up
access the rest of the document.