The word "nucleus" refers to the central part or core of something. In science, specifically in biology, the nucleus is the tiny structure found in the center of a cell that controls its activities and contains genetic material. Full definition
Experts take the cell nucleus of one human egg cell whose mitochondria have a defect and place it in an egg cell with «healthy» mitochondria. (sciencedaily.com)
Unfortunately, they are caused by the atomic nucleus as well as the electrons, which means that quantum dots containing manganese ions are complex quantum systems. (sciencedaily.com)
It is the DNA located in biological structures called mitochondria, not the DNA contained within the cell nucleus which we normally think of when we refer to «my DNA». (faith.org.uk)