mRNA directs synthesis of protein by carrying instructions from DNA to the ribosomes.
Protein synthesis begins when information coding for that particular protein is copied (transcribed) from DNA onto messenger RNA (mRNA) in the cell nucleus. mRNA then leaves the nucleus and enters the cell cytoplasm where it attaches to a ribosome.
Transfer RNA (tRNA) begins to read (translate) the information on the attached mRNA, and corresponding to the instructions, fetches amino acids from the pool of free amino acids in the cytoplasm, and brings them to the ribosome where they are linked together into a polymer, forming the primary structure of the protein.