|MadSci Network: Cell Biology|
Sara, The basic structure of the nucleus results from two contributing components, the nuclear envelope and nuclear lamins. The nuclear envelope originates from the endoplasmic reticulum, and in most cells remains connected to it. Nuclear lamins are proteins that form a basket-like structure which gives the nucleus its shape. The nuclear envelope surrounds the nuclear lamins, such that the lamins and the inner nuclear membrane are in contact. During the cell cycle, proteins called cyclin dependent kinases alter the lamins so that they can no longer make the basket structure. When this happens the nuclear membrane disassembles and gets resorbed into the endoplasmic reticulum, thus seeming to disappear. When chromosomes begin to decondense in telophase, extensions of the endoplasmic reticulum associate with and surround each chromosome. These mini-nuclei, called karyomeres, fuse together to form the new nucleus of the daughter cell. Since anaphase led to the separation of the chromosomes at opposite poles of the cell, two nuclei form. During this process, lamins are returned to their original state. They are imported into the newly forming nucleus, where they begin to assemble the basket-like structure to support the nuclear membrane. Reference: Lodish, Berk, Matsudaira, Kaiser, Krieger, Scott, Zipursky, and Darnell, 2003. Chapter 21: Regulating the Eukaryotic Cell Cycle. Molecular Cell Biology, W.H. Freeman and Company. Thank you for your question!
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