13.12: Translation

Lesson: Translation

Translation is the process of synthesizing proteins from the genetic information carried by messenger RNA (mRNA). Following transcription, it constitutes the final step in the expression of genes. This process is carried out by ribosomes, complexes of protein and specialized RNA molecules. Ribosomes, transfer RNA (tRNA), and other proteins produce a chain of amino acids—the polypeptide—as the end product of translation.

Translation Produces the Building Blocks of Life

Proteins are called the building blocks of life because they make up the vast majority of all organisms—from muscle fibers to hairs on your head to components of your immune system—and the blueprint for each of these proteins is encoded by the genes found in the DNA of every cell. The central dogma in biology dictates that genetic information is converted into functional proteins by the processes of transcription and translation.

Translation Occurs Outside of the Nucleus

Eukaryotes have a membrane-bound nucleus where mRNA is transcribed from DNA. After transcription, mRNA is shuttled out of the nucleus to be translated into a chain of amino acids—a polypeptide—and eventually, a functional protein. This can take place in the cytoplasm or in the rough endoplasmic reticulum, where the polypeptides are further modified. In contrast, prokaryotes lack a nuclear compartment, so translation in prokaryotes takes place in the cytoplasm while the mRNA is still being transcribed.

The Sequence of Codons in mRNA Determines the Polypeptide Sequence

Each codon in the mRNA corresponds to one of the 20 amino acids that a cell keeps stocked, as well as stop codons that do not code for amino acids. Another RNA molecule, transfer RNA (tRNA), is responsible for providing the correct amino acid, based on the codon sequence, to the ribosomes during translation. At one end of the tRNA molecule, enzymes called aminoacyl-tRNA synthetases covalently attach the specific amino acid to the attachment site, while the anticodon sequence located at the other end of the tRNA ensures that the correct amino acid is delivered to the ribosome. Some tRNA molecules are able to bind to more than one codon sequence, allowing for coding versatility known as the wobble effect. This is due to the fact that tRNA molecules have lower binding specificity to the third nucleotide in the mRNA codon sequence as compared to the first two nucleotides.

Some Inherited Diseases Stem from Defects in Translation

Translation is a complex process that depends on a wide array of cellular components. Mutations that impact any part of this diverse toolkit can cause disease. For example, the iron-storage disease hyperferritinemia, also known as cataract syndrome, results from mutations in the 5’ untranslated region of the mRNA, a region that is important for recruiting translation initiation proteins. These mutations cause abnormally high rates of translation of the iron protein ferritin, causing it to build up in the blood and tissues of affected patients. As a result, the lenses of the eyes turn cloudy. Other diseases are linked to mutations in the genes that encode tRNAs and the ribosomal subunits. For instance, the bone-marrow disease Diamond-Blackfan anemia stems from mutations in the RPS19 gene, a component of the small ribosomal subunit.

Translation is the process of synthesizing a protein from a messenger RNA template.

This process begins when the small subunit of a ribosome, in a complex with a methionine tRNA and initiation factors, binds to the five prime end of an mRNA. The complex scans the mRNA in the five prime to three prime direction until it encounters an AUG codon that serves as the translation initiation site.

Next, the methionine tRNA pairs with the AUG codon and recruits the large subunit of the ribosome. Synthesis of the new protein starts when a tRNA carrying its amino acid binds to the next codon of the mRNA through its anticodon.

This action places the new amino acid close to the previously incorporated amino acid, and a peptide bond forms between the two amino acids.

Translation continues as the ribosome moves to the next codon in the sequence. This forward movement is called translocation and will continue until the ribosome encounters a stop codon.

Stop codons are unique in that they do not have tRNAs. Instead, proteins called release factors bind to the stop codon, inducing the ribosome to release the newly synthesized protein and causing the ribosome to dissociate.