At the center of the cell, the nucleus is found. Surrounding the nucleus and its contents is a double-membrane structure called the nucleus envelope, or nuclear envelope. The inner portion of the membrane, facing the nucleus' contents, is made up of the nuclear lamina. The lamina is organized in a mesh-like pattern that can bind to components of the nucleus for support. The outer portion of the membrane, facing the rest of the cell, is also composed of a filament network.
The nuclear envelope also contains many small openings called pores. These openings allow for cellular components to be transported between the nucleus and the rest of the cell throughout the cell's life cycle. The pores allow only for specific components to pass through. Some are designed for protein transport, while others are for RNA.
The nucleus contains DNA, the component of the cell necessary for cellular division. The DNA contains the "instructions" to assemble a new cell. Chromatin and chromosomes are also contained within the nucleus at various parts of the cell cycle. The nucleus envelope separates these contents from the rest of the cell.
Cellular division, mitosis or meiosis, occurs in the nucleus. The chromatin assembles into chromosomes and align in the proper orientation. Through a series of actions, the chromosomes split in two and move to opposite sides of the nucleus. Next, the nuclear envelope begins to split into two. A new cell, with its own nucleus and envelope, is then created.