Why do you think the nuclear membrane disappear during mitosis

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Before mitosis begins, cells are in the interphase stage, which is the rest period between mitotic divisions. At this time, the chromosomes are not identifiable but are an undifferentiated loose mass of stretched out DNA molecules within the nucleus. Chromosome replication and other cell processes are occurring in preparation for mitosis. In addition, cells carry out normal metabolic activities at this time. Most mammal somatic cells, including those of humans, spend at least 90% of their time in interphase.

Interphase

At the beginning of the first mitotic stage, prophase, the thread-like doubled chromosomes contract and become visible. The two centrioles move to opposite sides of the nucleus. At the same time, the nuclear membrane begins to break down.

Prophase

During metaphase, the nuclear membrane disappears and the chromosomes become aligned half way between the centrioles. The centromere of each doubled chromosome becomes attached by thread-like spindle fibers to the centrioles which are at polar opposite sides of the cell.

Metaphase

During the following anaphase stage, the chromatids of each chromosome separate at their centromeres and then migrate to the opposite poles of the cell.

Anaphase

During telophase, two distinct nuclear membranes develop encompassing the two identical sets of chromosomes. At the same time, the cytoplasm divides between the two new cells, and the cell membrane begins to pinch off the cell contents into two daughter cells.

Telophase

Finally, the new identical cells separate and once again enter interphase in preparation for a later mitotic division. While the chromosomes are no longer visible, they will be replicated just before mitosis begins.

Interphase

By David H. Nguyen, Ph.D.

The nuclear envelope does not disappear in metaphase of mitosis, because it already did in prophase. The nuclear envelope is a large and complex structure and not just a floppy membrane pouch. The inner surface of the nucleus has a protein skeleton that helps give the nucleus its shape. The nuclear envelope needs to be broken apart so that the chromosomes can be found, aligned in the middle of the cell, and then pulled apart.

The nucleus is a large pouch that contains the cell’s DNA. The skin of the pouch is made of two layers. Each of these two layers is a phospholipid bilayer, meaning each layer is an oily sandwich that prevents water from freely passing through. But the nucleus is more than just a pouch made of two membranes. The inner side of the nuclear membrane contains a scaffold of proteins, like the wooden frame inside the walls of a house. The protein scaffold is attached to and gives shape to the double membrane. When the nuclear envelope degrades during mitosis, both the protein scaffold and the double membranes need to be broken into small pieces.

Mitosis, or cell division, has four general phases: prophase, metaphase, anaphase and telophase. The nuclear envelope breaks apart during prophase. Prophase is the time during which the DNA in the nucleus condenses, meaning it gets packaged by many proteins into thick, finger-like structures. These thick chromosomes make DNA more compact and easily moved within the cell. Prophase is also the time when the nuclear envelope is degraded, broken into small pieces to all the condensed chromosomes to freely float inside the cell.

Metaphase is when the thick, finger-like chromosomes are pulled back and forth until they align in the middle of the cell. In mitosis, which produces two daughter cells that have the same amount of DNA as the original mother cell, the chromosomes align as one column during metaphase. In meiosis, which produces four gametes that have only half the amount of DNA as the mother cell, the chromosomes align as two parallel columns during metaphase. Aligning the chromosomes in the middle of the cell allows them to be easily pulled apart in a way that equally divides the chromosomes to two sides.

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.

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