The nucleus is the largest organelle in an animal cell and can usually be seen using a light microscope. The nucleus contains the DNA of the cell-the genetic code which allows the cell to reproduce and which allows it to make all the proteins it needs to carry out the normal business of living. The nucleus is a large round darkly stained organelle usually found in the center of the cell. The nucleus is the "control center" of the cell. It is made up of chromosomes. When the cell is not reproducing the chromosomes are very long threadlike structures which wind back and forth throughout the nucleus. When a cell starts to reproduce the chromosomes coil up into distinct rod-like bodies. The chromosomes control heredity and are passed on to new cells when the parent cell reproduces.