The nucleus of a cell is a double membrane that surrounding nuclear DNA, making it the control center of the cell. Nuclear pores regulate the transport of molecules into and out of the nucleus. These channels regulate the transport of large molecules while smaller molecules and ions can cross freely. RNA is an excelent example of a macromolecule that can leave the nucleus. Ions, such as sodium and chlorine ions, can cross freely, but only along a gradient. Ion concentration can also be regulated by ion channels.