A nucleus is a small, dense region consisting of protons and neutrons at the center of an atom. It is the defining characteristic feature of eukaryotic cells and is exclusively found in them. The nucleus is also one of the largest organelles in eukaryotic cells. The term "nucleus" comes from the Latin word nucleus, meaning "the kernel" or "small nut" inside a watery type of fruit.
There are two main types of nuclei: atomic nuclei and cell nuclei. Atomic nuclei consist of electrically positive protons and electrically neutral neutrons, which are held together by the strongest known fundamental force. The atomic nucleus was discovered in 1911 by Ernest Rutherford based on the 1909 Geiger-Marsden gold foil experiment.
Cell nuclei, on the other hand, are membrane-enclosed organelles within a cell that contain the chromosomes. The nucleus serves as the repository of genetic information and as the cells control center. DNA replication, transcription, and RNA processing all take place within the nucleus, with only the final stage of gene expression (translation) localized to the cytoplasm. The nuclear envelope allows gene expression to be regulated by mechanisms that are unique to eukaryotes.
In summary, a nucleus is a small, dense region consisting of protons and neutrons at the center of an atom or a membrane-enclosed organelle within a cell that contains the chromosomes.
