A nucleus (plural nuclei) was a component of a biological cell.
The nucleus of the space amoeba contained more than forty chromosomes. While attempting to free the USS Enterprise from the creature's clutches in 2268, a probe filled with antimatter was used to destroy the organelle in question. (TOS: "The Immunity Syndrome")
In 2366, the USS Enterprise-D's sickbay supplied nanites – tiny machines built from the atom up – which were designed for exposure only to the inside of nuclei during cellular surgeries. (TNG: "Evolution")