The Nucleus

The nucleus is found in all eukaryotic cells. It is a round structure, surrounded by a membrane that serves as the control center for all activities that take place within the cell. If it is removed, the cell dies. Out of the many organelles in the cell, the nucleus is the largest. Surrounding the nucleus is the nuclear envelope. It is a double membrane that separates the nucleus from the rest of the cell. At some points along the nuclear envelope the inner and outer membrane are joined and they form very small pores. Because of these pores, the nuclear envelope, like the cell membrane, is selectively permeable. It allows the contents of the nucleus, the nucleoplasm, to have a different chemical composition than the rest of the cell. Much of the nucleoplasm consists of chromatin, various proteins bound to DNA. Usually the chromatin appears as long, thin threads called chromosomes. During cell division, the chromosomes shorten by coiling and become thick enough to be clearly visible when they are stained. The most visible structure in the nucleus is the nucleolus, which functions in the production of ribosomes. Sometimes, there are two or more nucleoli; the number depends on the species and stage in the cell's reproductive cycle. The nucleus also controls the protein synthesis in the cytoplasm. It sends molecular messengers in the from of RNA. This messenger RNA (mRNA) is made in the nucleus following the instructions encoded in the DNA. The mRNA carries the genetic messages to the cytoplasm through the pores in the nuclear envelope. Once the mRNA reaches the cytoplasm, it attaches itself to the ribosomes, and the genetic message is translated into the primary structure of a specific protein.

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