why does the nuclear membrane disappear during prophase
What does the pulling of chromosomes during meiosis? Protein tubes called microtubules connect the chromosomes that align in the middle during metaphase to opposite ends of the dividing cell. Microtubules are like ropes that will pull the chromosomes apart. In fact, the microtubules already connected to the chromosomes during prophase, long before metaphase. The connection of microtubules to chromosomes is why the nuclear envelope needed to be broken down during prophase. The nuclear envelope would have gotten in the way.
The microtubules from opposite ends of a dividing cell connect to the chromosomes during prophase. They push and pull on the chromosomes until the chromosomes align in the middle during metaphase.
Chromosomes become visible, crossing-over occurs, the nucleolus disappears, the meiotic spindle forms, and the nuclear envelope disappears. To see prophase I animated, click the Play button. At the start of prophase I, the chromosomes have already duplicated.
During prophase I, they coil and become shorter and thicker and visible under the light microscope. The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. The sites of crossing-over are seen as crisscrossed nonsister chromatids and are called chiasmata (singular: chiasma).
The nucleolus disappears during prophase I. In the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell. The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus. Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.
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