Nucleus and nucleolus.
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The nucleus of the eukaryotic cell is sometimes thought of as the command and control center of the cell. Like many other organelles, the nucleus is surrounded by a membrane: actually a double membrane. This double membrane is more properly termed the nuclear envelope. This envelope has numerous large pores that allow RNA and other macromolecules to enter the nucleus.
Inside the nucleus are the chromosomes embedded in a grainy appearing nucleoplasm. During much of cell's life cycle the chromsomes are not visable under the light microscope and are collectively called chromatin. The reason they are not visable during much of the cell's life cycle is because the chromosomes are stretched out to expose the maximum amount of surface area for the various chemical reactions the chromosomes are involved in.
I have colored part of the nucleus a darker purple to indicate a "structure" that is termed the nucleolus. Under the light microscope the nucleoli appear as dark staining bodies, but in reality they are more of an illusion in that what they represent are regions in the nucleus where DNA from many chromosomes come together during the first stages of protein synthesis. Specifically the nucleolus is the region of the nucleus where production of a kind of RNA called ribosomal RNA is taking place. The dense aggregation of mucleic acids, DNA and RNA take up extra stains used by microscopists than does the surrounding chromatin.Thus, most biologists do not class the nucleolus as an organelle.