Translation occurs in the cytoplasm where the ribosomes are located. Ribosomes are made of a small and large subunit which surround the mRNA.
Transfer RNA (tRNA) molecules are 75 - 95 nucleotides long and have four arms and three loops. True to its name, tRNA transfers amino acids to the site of the growing protein chain (polypeptide). Each tRNA molecule (in red below) recognises a specific, three base-pair mRNA code or codon (the DNA form of a codon is called a triplet and the sequence on the tRNA is called an anticodon). Since there are three bases and four possible nucleotides, there can be up to 64 (4x4x4) possible tRNA molecules. Three of these tRNA molecules recognise "stop" or termination codons which have been named amber (UAG), opal (UGA) and ochre (UAA).
The codon indicates which amino acid is to be added and the amino acid is attached to the tRNA molecule at the acceptor arm. As we can see from the table below, most amino acids are represented by more than one codon. This means that the expected protein can still be synthesised, even when a degree of mutation occurs in the DNA or mRNA.
There are 20 essential amino acids, however they can be combined in any order, just like the four nucleotides. This permits the production of the many different proteins which let organisms grow and function.
When the large ribosmal subunit, small ribosomal subunit, mRNA and the tRNA carrying a methionine come together in the cytoplasm, the ribosome becomes active and the synthesis of a polypeptide, or "translation", is initiated. The AUG codon binds at the protein binding site (P) of the ribosome and AUG is always the first codon of an mRNA.
The next complementary tRNA will bind at the attachment binding site (A) of the ribosome. The adjacent amino acids are then joined by a peptide bond via a peptidase enzyme. Thus the polypeptide chain begins to grow and as it does, it is passed to the next tRNA currently occupying the A site.
The ribosome then moves 1 codon down the mRNA in a 5' to 3' direction. This is achieved by a translocase enzyme. As the process of ribosome translocation continues, the "old" tRNA is released to bind another amino acid and go in search of a new codon. The binding of a new aminoacid is mediated by an enzyme called amino-acyl-tRNA synthase
The process continues along the mRNA until a stop codon is reached. While there is no tRNA for a stop codon, there is an enzyme called release factor which cleaves the polypeptide chain resulting in a new protein.
Finally, the entire complex is disrupted, the ribosome separates and the mRNA is released to be used again or degraded. Translation occurs at multiple sites along an mRNA so that many ribosomes can be seen by electron microscopy bound to a single mRNA strand with many polypeptide chains forming from each.
Not only do different sequences make different proteins, but slight sequence changes can radically change the shape of a protein. The shape or structure of the protein is essential for its correct function e.g. as an enzyme or an ion channel embedded in a cell membrane.