In prokaryotes, translation occurs while transcription is taking place (strongly indicating that translation occurs in the 5' to 3' direction). In eukaryotes, transcription occurs in the nucleus and translation occurs in the cytoplasm, either on free floating ribosomes or upon the rough endoplasmic reticulum. When several ribosomes are translating the same mRNA simultaneously, the group is called a polyribosome or polysome. The separation of transcription and translation in eukaryotes allows for much more complexity in the regulation of gene expression. Cells which are especially active in protein synthesis contain large numbers of ribosomes.