The gradual juxtaposition of homologs occurs during a prolonged period called meiotic prophase (or prophase I), which can take hours in yeasts, days in mice, and weeks in higher plants. Like their mitotic counterparts, duplicated meiotic prophase chromosomes first appear as long threadlike structures, in which the sister chromatids are so tightly glued together that they appear as one. It is during early prophase I that the homologs begin to associate along their length in a process called pairing, which, in some organisms at least, begins with interactions between complementary DNA sequences (called pairing sites) in the two homologs. As prophase progresses, the homologs become more closely juxtaposed, forming a four-chromatid structure called a bivalent (Figure 17–54A). In most species, homolog pairs are then locked together by homologous recombination: DNA double-strand breaks are formed at several locations in each sister chromatid, resulting in large numbers of DNA recombination events between the homologs (as described in Chapter 5). Some of these events lead to reciprocal DNA exchanges called crossovers, where the DNA of a chromatid crosses over to become continuous with the DNA of a homologous chromatid (Figure 17–54B; also see Figure 5–54).
Homolog pairing Culminates in the Formation of a synaptonemal Complex
The paired homologs are brought into close juxtaposition, with their structural axes (axial cores) about 400 nm apart, by a mechanism that depends in most species on the double-strand DNA breaks that occur in sister chromatids. What pulls the axes together? One possibility is that the large protein machine, called a recombination complex, which assembles on a double-strand break in a chromatid, binds the matching DNA sequence in the nearby homolog and helps reel in this partner. This so-called presynaptic alignment of the homologs is followed by synapsis, in which the axial core of a homolog becomes tightly linked to the axial core of its partner by a closely packed array of transverse filaments to create a synaptonemal complex, which bridges the gap, now only 100 nm, between the homologs (Figure 17–55). Although crossing-over begins before the synaptonemal complex assembles, the final steps occur while the DNA is held in the complex.
The morphological changes that occur during homolog pairing are the basis for dividing meiotic prophase into five sequential stages—leptotene, zygotene, pachytene, diplotene, and diakinesis (Figure 17–56). Prophase starts with leptotene, when homologs condense and pair and genetic recombination begins. At zygotene, the synaptonemal complex begins to assemble at sites where the homologs are closely associated and recombination events are occurring. At pachytene, the assembly process is complete, and the homologs are synapsed
Figure 17–54 Homolog pairing and crossing-over. (a) The structure formed by two closely aligned duplicated homologs is called a bivalent. as in mitosis, the sister chromatids in each homolog are tightly connected along their entire lengths, as well as at their centromeres. at this stage, the homologs are usually joined by a protein complex called the synaptonemal complex (not shown; see Figure 17–55). (B) a later-stage bivalent in which a single crossover has occurred between nonsister chromatids. It is only when the synaptonemal complex disassembles and the paired homologs separate a little at the end of prophase I, as shown, that the crossover is seen microscopically as a thin connection between the homologs called a chiasma.
Figure 17–55 Simplified schematic drawing of a synaptonemal complex.