A cell without a nucleus is classified as a prokaryotic cell. The genetic material of prokaryotic cells is free inside the cytoplasm. Prokaryotic cells do not have a need for compartmentalization, unlike more complex cells.
A nucleus is the membrane-enclosed organelle found only in eukaryotic cells. The function of the nucleus is normally to control the activities of the eukaryotic cell by regulating gene expression. A nucleus is often called the "control center" of the cell. Without a nucleus, the liquid interior, also called the cytoplasm, of the prokaryotic cell lacks all sorts of organelles, or "cell parts", found in eukaryotic cells. Organelles of more complex cells help to carry out the work required for the cells to stay alive.
In prokaryotic cells, the genetic material inside is loose and made of a single loop, called a plasmid. Lacking the complex systems existing in eukaryotic cells, prokaryotic cells can only ever reproduce by simple mitosis, or fusion, occurring when cells divide by simple fusion.
The name prokaryote comes from a Greek word meaning "before nuclei." These are known as the primeval cells of earth. The most common type of prokaryote cells are unicellular organisms, such as bacteria, which are the most abundant organism on earth.