The cell's nucleus is usually the largest organelle in a cell. It is substantially larger in animals cells that in prokaryotic cells. The nucleus has three major roles within the cell. The nucleus is the site for DNA duplication to support cell reproduction as well as the site of DNA control of the cells activities. The third important role is that the nucleus contains the nucleolus, which is responsible for the assembly of ribosomes from certain proteins and RNA. The nucleus contains nearly all of the cell's genetic material
The nucleus rests inside the nuclear envelope, which is made up of two membranes. The two membranes are very close together and dotted with nuclear pores. The nuclear pores connect the inside of the nucleus to the cytoplasm. Each of these pores is surrounded by a pore complex, which consists of eight large protein granules in the shape of an octagon. Both RNA and proteins enter and exit through these pores. Proteins that move through the pores require active transport by carrier proteins. This transport is crucial to the cells basic functioning because it is needed for gene expression and chromosomal maintenance. Smaller molecules however are allowed to pass in and out of the nucleus freely. The nuclear envelope also acts as a barrier to prevent DNA and RNA viruses from being able to enter the nucleus. Many viruses require access to the proteins inside the nucleus in order to replicate themselves. In protecting the nucleus from these viruses the nuclear envelope performs a very important function.
Located inside the nucleus is a fibrous complex made when DNA combines with proteins called chromatin. Chromatins are long thread-like objects that just before cell division condense into chromosomes. Nucleoplasm surrounds the chromatin and is made up of water and dissolved substances. Also found in the nucleoplasm is a network of structural proteins called the nuclear matrix. On the edge of the nucleus the chromatin attaches to the nuclear lamina, a protein mesh formed through the polymerization of proteins called lamins in filaments. The function of the nuclear lamina is to help maintain the shape of the nucleus, in much the same way that the cytoskeleton helps to maintain the shape of the cell.
During the majority of a cell's lifecycle the nuclear envelope is a stable structure. However, when the cell begins to divide, the nuclear envelope breaks up into pieces. When the DNA that was duplicated has been put in the daughter cell the nuclear envelope reforms.