Prophase I of meiosis
In the leptotene stage, the cell's genetic material, which is normally in a loosely arranged pile known as chromatin, condenses into visible threadlike structures. Along the thread, centromeres are visible as small beads of tightly coiled chromatin. Recall that centromeres are connection sites between sister chromatids, which are not yet distinguishable. As the chromatin becomes progressively ordered and visible, homologous chromosomes find each other and bind together. In this process, called synapsis, a protein structure called the synaptonemal complex attaches the homologous chromosomes tightly together all along their lengths.
The zygotene stage sees the completion of synapsis. The paired homologous chromosomes are said to be bivalent. They may also be referred to as a tetrad, a reference to the four sister chromatids. During this stage, one percent of DNA that wasn't replicated during S phase is replicated. The significance of this cleanup act is unclear.
The pachytene stage heralds crossing over. Nonsister chromatids of homologous chromosomes exchange segments of genetic information. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
During the diplotene stage, the synaptonemal complex degrades. Homologous chromosomes fall apart and begin to repel each other. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. They are held together by virtue of recombination nodules, betraying the sites of previous crossing over, the chiasmata.
Chromosomes recondense during the diakinesis stage. Sites of crossing over entangle together, effectively overlapping, making chiasmata clearly visible. In general, every chromosome will have crossed over at least once. The nucleoli disappears and the nuclear membrane disintegrates into vesicles.
During these stages, centrioles are migrating to the two poles of the cell. These centrioles, which were duplicated during interphase, function as microtubule coordinating centers. Centrioles sprout microtubules, essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it disintegrates, attaching to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the attached microtubule toward the originating centriole, like a train on a track. There are two kinetochores on each tetrad, one for each centrosome. Prophase I is the longest phase in meiosis.
Microtubules that attach to the kinetochores are known as kinetochore microtubules. Other microtubules will interact with microtubules from the opposite centriole.