Structure of the Nucleus

The nucleus is the single largest organelle found in animal cells often taking up as much as ten percent of the total volume of the cell. These dense, somewhat spherical organelles are enclosed in a double membrane composed of a lipid bi-layer. This membrane regulates the passage of material in and out of the nucleus.

Small pores in the nuclear membrane are made up of numerous proteins which serve as passageways through the envelope. These pores can number as few as fifty per nucleus in an organism like yeast, to hundreds of proteins in some vertebrates and as many as four thousand pores in a mammalian cell. The size and number of nuclear pores selectively permits the movement of smaller molecules through the membrane while keeping from moving in or out. Large molecules must be actively transported into the nucleus instead via structures that mediate the binding of these molecules to nuclear transport proteins.

In many animal cells, two systems of intermediate filaments provide mechanical support for the nucleus. The first is the nuclear lamina, which forms a framework on the inside of the envelope, while the second and less organized support structure is found on the cytoplasm-side of the envelope. These two systems not only provide structural support for the nuclear membrane, they also serve as an anchor site for chromosomes and the nuclear pores.[6]

The nucleus houses most of the cell's genetic material, found as linear DNA molecules organized into chromosomes. Interestingly, every human cell contains approximately two meters of DNA. A small amount of the cell's genes are found in the mitochondria.

The nucleolus is a large, dense structure found in the nucleus. It does not have its own membrane, and is made up of three different, unique regions. Its main job is to synthesize RNA and collect ribosomes. The structure of the nucleolus depends on its purpose in the nucleus.

There are a number of other structures found inside the nucleus, none of which are enclosed in their own membranes. Some of these structures have been identified by name, such as Cajal bodies and Gemini of coiled bodies, while others are simply referred to asparaspecklesor splicing speckles. Not much is known about most of these structures other than they demonstrate that the nucleoplasm contains organized and useful components.

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