In addition, the nuclear membrane has broken down entirely. The homologous chromosomes are still held together at chiasmata. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to create recombinant chromosomes.Ī second event in Prophase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from one parent of the individual and some DNA from the other parent. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. The crossover events are the first source of genetic variation in the nuclei produced by meiosis. The result is an exchange of genetic material between homologous chromosomes. Crossover occurs between non-sister chromatids of homologous chromosomes. Crossing over occurs at chaiasmata (singular = chiasma), the point of contact between non-sister chromosomes of a homologous pair (Figure 2).Īt the end of prophase I, the pairs are held together only at the chiasmata and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.įigure 2. 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 (Figure 1). 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 tight pairing of the homologous chromosomes is called synapsis. (Recall that, in mitosis, homologous chromosomes do not pair together. The chromosomes are bound tightly together and in perfect alignment by a protein lattice at the centromere.Īs the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. Early in prophase I, homologous chromosomes come together to form a synapse. "Shaping meiotic prophase chromosomes: cohesins and synaptonemal complex proteins". "Sex chromosomes, synapsis, and cohesins: a complex affair". "Homologous pairing and chromosome dynamics in meiosis and mitosis". "DNA damage response protein TOPBP1 regulates X chromosome silencing in the mammalian germ line". elegans initiates by a conserved mechanism and is dispensable for homologous chromosome synapsis". When crossing-over is complete, the homologous chromosomes separate into chromosomes with recombinant chromatids. The homologous chromosomes and synaptonemal complex form a structure called a bivalent. The complex holds a synapsis in a fixed state and provides the framework for chiasma formation and the exchange of genetic material in crossing-over. The synaptonemal complex appears as a central line flanked by two lateral lines, which are attached to homologous chromosomes. A ribbon-like protein framework called the synaptonemal complex forms. In prophase I, the two different versions of each chromosome (homologues) find each other and connect so they can line up parallel to each other on the metaphase plate and ultimately be separated to form two daughter cells. When meiosis starts, each cell contains two copies of each chromosome-one from each parent. The DNA breaks at the chiasma and the genetic material from one homologue swaps with that from the other chromosome. An x-shaped structure called a chiasma forms where the arms of chromosomes overlap. In addition to stabilizing the homologous chromosomes so they separate correctly, synapsis facilitates the exchange of genetic material between the chromosomes.
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