The nucleus is the control center of an atom. It contains protons and neutrons, which are collectively known as nucleons. The number of protons in the nucleus determines the element that an atom is. For example, all atoms with six protons in their nucleus are carbon atoms.
The number of neutrons in a nucleus can vary, however, even among atoms of the same element. Atoms with different numbers of neutrons are called isotopes.
Most nuclei contain more than one proton (hydrogen being a notable exception). In addition to holding together most of the mass within an atom, nuclear forces also hold together groups of atoms to form molecules and solids—the basis for everyday matter around us.
A nucleus is a small, round organelle that is found in nearly all eukaryotic cells. It contains the cell’s chromosomes and controls the cell’s genetic activity. The nucleus is surrounded by a double membrane called the nuclear envelope, which separates it from the rest of the cell.
The nucleolus is a small organelle within the nucleus that helps to assemble ribosomes. Ribosomes are responsible for protein synthesis in cells. The nucleolus also contains DNA, which codes for ribosomal RNA (rRNA). rRNA plays a significant role in protein synthesis and cellular metabolism.
The nuclear envelope protects the chromosomes from damage and keeps them organized so they can be properly replicated during cell division. When replication occurs, each chromosome copies itself and then divides into two identical halves called chromatids.