I
Central, positively charged core of an atom .
It consists of positively charged neutrons , known collectively as nucleons, held together by the strong force . The number of nucleons can range from 1 to about 270, depending on the element. Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Some nuclei, especially heavier ones, are unstable, or radioactive (see radioactivity ), emitting energy in the form of alpha rays (see alpha decay ), beta rays (see beta decay ), or gamma rays . The nucleus makes up nearly all the mass but only a minute fraction of the volume of the atom.
II
Specialized structure occurring in most cells (except bacteria ) and separated from the rest of the cell by the nuclear membrane.
This membrane seems to be continuous with the cell's endoplasmic reticulum and has pores that permits the passage of large molecules. The nucleus controls and regulates the cell's activities (e.g., growth and metabolism) and carries the genes . Nucleoli are small bodies often seen within the nucleus that play an important part in the synthesis of {{link=RNA">RNA and protein. A cell normally contains only one nucleus.