1. The center of the atom which contains the protons and neutrons; in cells, structure that contains the cells genetic material (DNA) and controls the cells activities.
2a. The major organelle of eukaryotic cells, in which the chromosomes are separated from the cytoplasm by the nuclear envelope. The nucleus has 3 parts the nucleolus, the chromatin, and the nuclear envelope.
2b. A specialized, usually spherical mass of protoplasm encased in a double membrane, and found in most living eukaryotic cells, directing their growth, metabolism, and reproduction, and functioning in the transmission of genic characters.