The nucleus is a spherical organelle found in eukaryotic cells that was first discovered by Leeuwenhoek and described in more detail by Robert Brown. It accounts for about 10% of the cell's volume and ranges from 2-5 micrometers in diameter. The nucleus contains genetic material, is surrounded by a double membrane with nuclear pores, and acts as the control center of the cell by storing hereditary material and producing ribosomes and RNA.