The nucleus is generally the most conspicuous organelle within a eukaryotic cell, averaging about 5 microns in diameter. It contains most of the genetic information for the cell. (Some genes are located in the mitochondria and chloroplasts.) The nucleus is contained within a nuclear envelope. This envelope separates the contents of the nucleus from the rest of the cell.
The nuclear envelope is a double membrane. It is composed of two lipid bilayers with associated proteins. The two layers are separated by about 20 to 40 nano-meters. The envelope is perforated by pores, which are about 100 nano-meters in diameter. At the lip of each pore, the two layers of the double membrane are fused. This pore-complex regulates the entance and exit of particles, as well as macromolecules.
The nuclear lacrima is located on the nuclear side of the envelope. It is composed of a net-like array of proteins, which serve the purpose of maintaining the shape of the nucleus. It is also possible that a framework of fibers distributed throughout the nucleus is in existence. This theoretical structural component is called the nuclear matrix.
The two main components which make up the content of the nucleus are chromatin, and the nucleolus. Chromatin contains the genetic information of the cell. It is a large mass, which is apparent on even a light microscope, when propperly stained. As the cell begins to divide, this chromatin, which is stringy and entangled, condenses to form multiple separate structures called chromosomes. Each eukaryotic species has a characteristic number of chromosomes. For example, a human cell has 46 chromosomes in its nucleus, save the sex-cells which contain only 23.
The nucleuolus is the largest non-dividing member of the nucleus. It is responsible for the synthesis of ribosomal components, which pass through the nuclear pores and into the cytoplasm, where they are formed into ribosomes.
This is the Basic Structure of the Nucleus