Answer:
The homologous chromosomes pair together in prophase 1 of meiosis, but they do not during prophase 1 of mitosis. This is achieved by a process known as synapsis, where the similar chromosomes pair according to sequence similarity. The homologous chromosomes are held together by a protein structure known as the synaptonemal complex in a chromosome body known as a tetrad (because it contains 4 replicated chromosomes known as chromatids) or bivalent (if the organism is diploid). This pairing during prophase 1 of meiosis allows recombination to take place between the homologous chromosomes. This occurs early during prophase but the manifestation of recombination only becomes visible during the later stages of prophase 1 and in metaphase 1. Because the chromosomes adopt different structures during prophase 1 of meiosis, this stage is sub-divided into 5 stages: leptotene, zygotene, packytene, diplotene and diakinesis. It is during diplotene and diakinesis that the physical manifestation of recombination can be seen. This is the presence of chiasmata (chiasma, singular). These are the sites where recombination, or exchanges between homologous chromosomes, has taken place. By the end of prophase 1, it is only the chiasmata that holds the homologous chromosomes together. This constriction make the tetrads adopt a variety of structures, the shape of which depends upon the number of chiasmata formed. The tetrads stay in this conformation until metaphase 1. Synapsis, the formation of the synaptonemal complex, the formation of chiasmata does not take place during prophase 1 of mitosis and these processes represent the major differences between prophase of the two nuclear divisions.
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