The cell nucleus is the part of a cell that contains the cells chromosomes.The area is separated from the rest of the cell by a membrane. The nucleus controls how the function of DNA is expressed in the cell, largely by controlling the location of the cell's genes. When stained, the nucleus can usually be seen under a microscope within the cell.
Although the nucleus is typically only about six micrometers in diameter, it is the largest single structure within the cell.
Although not all cells have a nucleus, those without a nucleus, such as red blood cells, are unable to replicate themselves.