The Nucleus varies in size from species to species of cells and bacteria and also can appear differently depending on the kinds of cells they are present in. The nucleus is on such vital importance to the cell because it contains the DNA that the cell needs to synthesize proteins and regulate mostly every other aspect of its function. During cell reproduction, the nucleus breaks apart and the genetic material it contains is replicated to form exact copies that are transferred to the daughter cell. It also contains the Nucleolus, an organelle responsible for the production ribosomes for the cell. It is protected by the nuclear membrane, which regulates what can enter and exit the nucleus. It lets some small water-soluble molecules pass directly through the membrane itself while keeping larger molecules such as the DNA contained in the nucleus inside. However, when nucleic acids need to enter or exit the nucleus, they can pass through one of the nuclear pores, which are openings in the membrane that allow substances to pass into or out of the nucleus. The nucleus is also known as the "brain of the cell" due to its use in controlling the cell's actions by the DNA it holds.