A cell's nucleus is able to control the other activities in a cell by expressing certain segments of its DNA, which creates proteins that perform specific activities. Proteins can vary from enzymes to structural components, but they are needed for almost all processes in a cell and the human body.
In order to create a protein, a segment of the DNA must first be replicated. This process begins in the nucleus, where a copy of the segment called messenger RNA, or mRNA, is created. After being formed, the mRNA exits the nucleus. It is utilized by a ribosome to create the appropriate protein that corresponds to the sequence in the mRNA.