Eukaryotic Cell

The nucleus contains the chromosomes. Chromosomes contain the genes, which are DNA sequences used to create RNA. The nucleus is bounded by a double membrane, called the nuclear envelope. Numerous large pores provide channels through which materials enter and exit. One of the chief exports of the nucleus is messenger RNA, which is used in the cytoplasm for protein construction.

Translation occurs in the cytoplasm at ribosomes, large complexes made of protein and RNA. Ribosomes are assembled in the nucleus, in the region called the nucleolus. RNA is synthesized by the enzyme RNA polymerase, which unwinds DNA and transcribes short portions, known as genes. These RNA molecules are processed further before being exported as messenger RNA. Other RNAs made in the nucleus include the RNA used in ribosomes (ribosomal RNA), RNAs that carry amino acids to the ribosome (transfer RNA), and a host of small RNAs that mostly function in the nucleus to modify other RNAs.