Nucleus
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There are two different types of cells, eukaryotic and prokaryotic. Not all cells have a nucleus. Only eukaryotic cells have a nucleus. In an eukaryotic cell the nucleus is the command center. To have DNA you don't need to have a nucleus. If you don't have a nucleus your DNA is most likely floating around the cell in the nucleoid region. The nucleus contains the hereditary information, controls growth and controls reproduction of a cell. Whatever happens in the cell the nucleus will know about it. the nucleus is not always the center of the cell.You most likely won't find the nucleus near the edge of the cell. The edge of the cell is a dangerous place for the nucleus. It is a membrane bound organelle. There are multiple hypothesis that explain the existence of the nucleus but there are four major ones. The first one is the syntrophic model. It states that the relationship between the archaea and bacteria created the eukaryotic cell. A second one proposes that eukaryotic cells evolved from bacteria without the endosymbiotic stage. A third theory which is the most controversial one is known as viral eukaryogenesis. It says that they originated from an infection of a prokaryote by a virus. A fourth proposal is the exo membrane hypothesis. Its says that the nucleus originated from a single ancestral cell that evolved a second exterior cell membrane. The interior membrane enclosed around the original cell and then became the nuclear membrane.