Substages of Prophase I
Stages of Meiosis
EUGENE M. MCCARTHY, PHD GENETICS
Prophase I is by far the most complicated phase of meiosis. It is also much longer in meiosis than in mitosis. During this stage, homologs join (synapse) along their lengths and exchange DNA. Prophase I is itself divided into the five substages explained and diagrammed below.
In the diagram of early zygotene at right, the regions where the paternal and maternal homologs have fused is shown in purple. In the next diagram, representing late zygotene, both homolog pairs have fused over their entire lengths (so they are shown entirely in purple). Zygotene is also known as zygonema.
Tetrads: Once the homolog pairs synapse they are called tetrads (each has four chromatids; tetra is Greek for four) or bivalents. Bivalent is the preferred term, but tetrad is, nonetheless, the word more commonly used in most introductory biology classes. Bivalent is the better choice because there are equivalent names for other situations. For example, an unfused homolog is called a univalent. Three fused homologs, a common situation in plants, is called a trivalent, etc.
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During pachytene the two sister chromatids of each chromosome separate from each other. This makes the chromosomes look thicker (pachy- is Greek for thick). Homologs are still paired at this point. Pachytene is also known as pachynema.
Crossing-over: Non-sister chromatids remain in contact throughout pachytene and a kind of localized breakage of the DNA occurs, which is followed by exchanges of DNA between them. This process is called crossing over. Crossing over produces "cross-over chromatids," each composed of distinct blocks of DNA, some blocks derived from the mother, others from the father.
At the beginning of this stage each chromatid of each chromosome is still fused to a chromatid of that chromosome's homolog (recall that sister chromatids are already separate at this point). As diplotene progresses, these initially fused non-sister chromatids begin to separate from each other. However, they cannot separate completely because they are still connected by two strands of DNA at each of the points where exchanges took place. At each such cross-over site, the two strands form an x-shaped structure called a chiasma (pl. chiasmata). The chiasmata then begin moving toward the ends of the chromatids. This process of sliding toward the ends is known as terminalization.
In oocytes, a special, extremely prolonged form of diplotene occurs called dictyotene. The primary oocyte undergoes the first three of the substages of prophase I (leptotene, zygotene, and pachytene) during late fetal life. The process is then suspended during diplotene until puberty or thereafter. Therefore, in dictyotene (and consequently prophase I) can last months or even years, depending on the type of organism in question. Diplotene is also known as diplonema.
During this, the last stage of Prophase I, the nucleolus disappears, terminalization reaches completion, and the chromosomes coil tightly, and so become shorter and thicker. The nuclear envelope begins to disappear. The centrosomes reach the poles.