The nucleolus contains all of the information needed for DNA replication, flow of proteins and RNA, all while controlling the synthesis of protein and cell replication. The nucleolus has several parts: the nuclear envelope, the chromatin and the actual DNA.
The nuclear envelope is full of pores that the cell uses to regulate the flow of material in and out of the nucleus. This material can be macromolecules, such as protein or RNA. These materials are limited in how much is allowed to come in and out of the nucleus.
There are exceptions to this process, which apply to things like water, ATP and ions. These exceptions are small molecules which do not carry information like the macromolecules. This is just one way of the nucleus controlling the information that flows in and out of it.
Most important in the nucleus is the DNA, which is made up of long chains called chromosomes. These are protected by the nucleus envelope and are required for the process of transcription or protein synthesis. It is also the crucial element of cell division, both the mitosis and meiosis. The cell being able to access the DNA and reproduce itself is key in many aspects of the health and growth of organisms.