Schematic of typical animal cell, showing subcellular
5 rough endoplasmic reticulum (ER)
6 Golgi apparatus
8 smooth ER
The nucleus is spheroidal in shape and separated from the cytoplasm by a double-membrane structure called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or synthesized, into a special RNA, called mRNA. This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. In prokaryotes, DNA processing takes place in the cytoplasm. Inside the nucleus of eukaryotes is the nucleolus.