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Tranlation of RNA to protein

Central Dogma


Translation


The Ribosome

How do messenger RNAs direct protein synthesis ? The cellular factory responsible for synthesizing proteins is the ribosome. The ribosome consists of structural RNA and about 80 different proteins. In its inactive state, it exists as two subunits; a large subunit and a small subunit. When the small subunit encounters an mRNA, the process of translation of the mRNA to protein begins. There are two sites in the large subunit, for subsequent amino acids to bind to and thus be close enough to each other for the formation of a peptide bond. The A site accepts a new tRNA bearing an amino acid, and the P site bears the tRNA attached to the growing chain. Here is a schematic diagram of the ribosome:

Charging the tRNA

As discussed previously, the adaptor molecule that acts as a translator between mRNA and protein is a specific RNA molecule, tRNA (transfer RNA). Each tRNA has a specific anticodon and acceptor site. Each tRNA also has a specific charger protein; this protein can only bind to that particular tRNA and attach the correct amino acid to the acceptor site. The energy to make this bond comes from ATP. These charger proteins are called aminoacyl tRNA synthetases

Initiation of Translation

The start signal for translation is the codon ATG, which codes for methionine. Not every protein necessarily starts with methionine, however. Often this first amino acid will be removed in post-translational processing of the protein. A tRNA charged with methionine binds to the translation start signal. The large subunit binds to the mRNA and the small subunit, and elongation begins.

Elongation of the New Protein

After the first charged tRNA appears in the A site, the ribosome shifts so that the tRNA is in the P site. New charged tRNAs, corresponding the codons of the mRNA, enter the A site, and a peptide bond is formed between the two amino acids. The first tRNA is now released and the ribosome shifts again so that a tRNA carrying two amino acids is now in the P site, and a new charged tRNA can bind to the A site. This process of elongation continues until the ribosome reaches a stop codon.

Termination of the Protein

When the ribosome reaches a stop codon, no aminoacyl tRNA binds to the empty A site. This is the ribosomes signal to break into its large and small subunits, releasing the new protein and the mRNA.

Post-Translational Processing

This isn't always the end of the story for the new protein. Often it will undergo post-translational modifications. It might: Discussion about where and how these processes occur can be found in the Cell Biology chapter of the hypertext.


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