The job of the nucleus is to essentially act as the brain of a cell. The nucleus contains the DNA material for a cell. Due to harboring the DNA, the nucleus is responsible for regular functions of the cell, such as administration and the processing of information.
The nucleus is the central organelle of a cell. In most cells, it is the largest organelle. The nucleus contains proteins and acids that control the majority of the cell's functions. The DNA within the nucleus contains the necessary information that tells the cell how to perform its function, such as making a muscle cell work as a muscle cell instead of as a skin cell.
The nucleus also controls reproduction and growth of the cell. Most of the reproductive cycle of a cell takes place in the nucleus where DNA is replicated to form the new cell. Similarly, the DNA within the nucleus controls protein production and metabolism so that the cell can continue working as needed.
The nucleus is protected from the environment of the cell by a membrane called the nuclear envelope. This membrane is not solid, however, and contains pores and holes so that molecules can be exchanged between the internal structure of the nucleus and the rest of the cell components.