细胞核

The nucleus, a prominent feature of eukaryotic cells, is a membrane-bound organelle that houses genetic information, the DNA, which is organized differentially depending on the lifecycle phase of the cell.

For instance, DNA protein complexes, known as chromatin, are suspended in nucleoplasm, a gel-like substance, and are particularly concentrated in a region called the nucleolus. When the cell's not dividing, chromatin fibers are loosely arranged and resemble tangled strings, allowing for easier access to the DNA during transcription. In contrast, when the cell's about to divide, chromatin fibers are compressed into chromosomes.

The nucleolus is also where the sub-units of ribosomes, structures necessary for translation, are made. Surrounding the nucleus is a double membrane nuclear envelope that contains protein lined pores to control the flow of ions, molecules, ribosomal parts and messenger RNA between the inner nucleoplasm and the cytoplasm.

While most cells in the body have one nucleus, some, like red blood cells, are anucleated. They lack a nucleus. Whereas others, such as skeletal muscle cells, are multinucleated with more than one nucleus due to structural and functional needs.