The nucleus is basically the brain of the cell. It controls most cell processes and contains the hereditary information of DNA, which is the substance that holds coded instructions for making proteins and other important molecules. The nucleus is surrounded by a double-layer membrane called the nuclear envelope, which is dotted with thousands of pores which allow materials to pass into and out of the nucleus. The nucleus contains a small, dense region known as the nucleolus, where the assembly of ribosomes begins. The nucleus also contains chromatin, which is basically DNA bound to protein. Most of the time, it is spread out in the nucleus; however, when a cell divides, the chromatin condenses to form chromosomes which carry the genetic information from one generation to the next.