through on the mRNA and helps assemble the polypeptide chains specified. Of each ribosome's two subunits, the smaller binds to the mRNA and the larger contains the enzymes that catalyze the peptide bonds between the amino acids being assembled.
The process of assembling the amino acids based on the information supplied by the mRNA is called translation. As a ribosome moves along a strand of mRNA, it reads three nucleotides at a time, the length of a single coding unit, or codon, for a single amino acid. Of the 64 codons, 61 code for amino acids (most amino acids have several
different but synonymous codes) and three signal the end of a gene's sequence. But proteins are not synthesized directly from mRNA. Instead, amino acids are assembled in their proper sequence by the third type of RNA, transfer RNA (tRNA). Free-floating amino acids, after being activated by ATP, are joined to tRNA by enzymes and brought into the ribosome where they are linked together to form polypeptide chains. Each amino acid has at least one specific type of enzyme to bind it to tRNA and at least one specific type of tRNA to carry it to a ribosome. Each tRNA has an unpaired triplet of nucleotide bases, called an anticodon. During the time when the tRNA's amino acid is being linked to the growing polypeptide chain, the tRNA's anticodon forms a temporary bond with the corresponding, complimentary codon of the mRNA. An anticodon that reads AUG would bind with a codon that reads UAC, for example. Once free of its amino acid, the tRNA leaves the ribosome to pick up another amino acid and begin the assembly cycle over again.
