The nucleus contains a nuclear envelope or membrane surrounding this organelle of the cell. It contains pores that allow transport between the nucleus and the cytoplasm. During the process of transcription, for example, RNA must be able to copy the genetic code in the DNA and carry out to the cytoplasm to be translated. This is accomplished by passing out of the nuclear pores. The DNA remains inside the nucleus and its membrane is the barrier between the nucleus and the cytoplasm. The membrane consists of ribosomes, and its construction is that of a lipid bilayer. The outer layer joins to the rough endoplasmic reticulum tubules which are studded with ribosomes and the inner layer contains proteins.