As prophase I progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. Near the recombination nodule on each chromatid, the double-stranded DNA is cleaved, the cut ends are modified, and a new connection is made between the non-sister chromatids. These assemblies mark the points of later chiasmata and mediate the multistep process of crossover-or genetic recombination-between the non-sister chromatids. Located at intervals along the synaptonemal complex are large protein assemblies called recombination nodules. A partial synaptonemal complex develops only between the regions of homology. In species such as humans, even though the X and Y sex chromosomes are not homologous (most of their genes differ), they have a small region of homology that allows the X and Y chromosomes to pair up during prophase I. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere. Early in prophase I, homologous chromosomes come together to form a synapse. Crossing over can be observed visually after the exchange as chiasmata (singular = chiasma) (Figure 1).įigure 1. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The tight pairing of the homologous chromosomes is called synapsis. In mitosis, homologous chromosomes line up end-to-end so that when they divide, each daughter cell receives a sister chromatid from both members of the homologous pair.) The synaptonemal complex, a lattice of proteins between the homologous chromosomes, first forms at specific locations and then spreads to cover the entire length of the chromosomes.
(Recall that, in mitosis, homologous chromosomes do not pair together. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. Prophase IĮarly in prophase I, before the chromosomes can be seen clearly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. This prepares the cell to enter prophase I, the first meiotic phase. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. Cohesin holds the chromatids together until anaphase II. Finally, the G 2 phase, also called the second gap phase, is the third and final phase of interphase in this phase, the cell undergoes the final preparations for meiosis.ĭuring DNA duplication in the S phase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. The S phase is the second phase of interphase, during which the DNA of the chromosomes is replicated. The G 1 phase, which is also called the first gap phase, is the first phase of the interphase and is focused on cell growth.
Meiosis is preceded by an interphase consisting of the G 1, S, and G 2 phases, which are nearly identical to the phases preceding mitosis.
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