The cell nucleus of a eukaryote contains a nucleolus, chromatin, ribosomes, nucleoplasm and pores. However, the nuclear envelope surrounds the nucleus and consists of an inner and outer membrane with pores. These pores are present to regulate the flow of materials like messenger RNA and protein moving through the membrane.
The nuclear envelope also keeps the nucleus contained within it so that cytoplasm does not enter. The nuclear envelope contains the nucleoplasm, the fluid that fills the nucleus. The chromatin consists of protein and DNA, while the nucleolus is the central structure of the nucleus that manufactures ribosomes and RNA.
Plants and animals have eukaryotic cells. In these cells, the nuclei make up approximately 10 percent of their volume.