Gene expression consists of two primary steps: transcription and translation. During transcription, the DNA sequence of a gene is copied into a complementary RNA molecule by an enzyme called RNA polymerase. This process occurs in the nucleus of eukaryotic cells or in the cytoplasm of prokaryotic cells. Translation occurs in the cytoplasm and involves the interaction between mRNA and ribosomes. The ribosome reads the mRNA sequence in groups of three nucleotides called codons, each of which typically corresponds to a specific amino acid. Transfer RNA (tRNA) molecules transport amino acids to the ribosome, where they are joined together to form a protein. Protein synthesis proceeds until the ribosome encounters a "stop" codon, signaling the end of translation.