Transcription occurs in the cell nucleus where DNA is unzipped and RNA polymerase adds complementary RNA nucleotides to the DNA template strand, forming mRNA. The mRNA is processed - a cap and tail are added and introns are removed. The completed mRNA contains codons of three nucleotides that code for amino acids. Translation occurs in the cytoplasm where the mRNA binds to ribosomes and tRNA molecules with matching anticodons deliver amino acids specified by mRNA codons, assembling the polypeptide chain specified by the mRNA.