Initiation, Elongation and Termination in the Translation process

<a rel="nofollow" href="https://http://highered.mcgraw-hill.com/sites/dl/free/9834092339/291136/f13_16_translation_init.swf" target="_blank">https://http://highered.mcgraw-hill.com/sites/dl/free/9834092339/291136/f13_16_translation_init.swf</a>

Ribosomes are complex structures composed of both protein and RNA. About two thirds of the mass of a ribosome is RNA, making ribosomal RNA the most abundant type of RNA in the cell. Ribosomal RNA functions in binding mRNA and tRNA and then moving them precisely through the ribosome in order to add amino acids to a polypeptide chain. The small subunit of a ribosome contains a sequence of ribosomal RNA that binds mRNA at the beginning of translation. Without this binding site translation might not begin at the appropriate location on the mRNA.

<a rel="nofollow" href="https://http://highered.mcgraw-hill.com/sites/dl/free/9834092339/291136/17b_translation_elongation.swf" target="_blank">https://http://highered.mcgraw-hill.com/sites/dl/free/9834092339/291136/17b_translation_elongation.swf</a>

Ribosomes are the central structures of protein synthesis. They bring all the components needed to build a protein together in a controlled fashion that causes amino acids to link up in a chain. Ribosomes have sites that bind mRNA and tRNA and move them through the ribosome. The movement of RNA through the ribosome adds the amino acids to the chain, one at a time. Without ribosomes, the tRNA and the mRNA would have to spontaneously come together. After the association of the two kinds of RNA, the amino acids attached to the tRNA would have to link to together. Without a ribosome this would take a very long time. Ribosomes are the organizers of protein synthesis.

<a rel="nofollow" href="https://http://highered.mcgraw-hill.com/sites/dl/free/9834092339/291136/17b_translation_termination.swf" target="_blank">https://http://highered.mcgraw-hill.com/sites/dl/free/9834092339/291136/17b_translation_termination.swf</a>

When a ribosome reaches the codon UAG, UAA, or UGA on an mRNA strand then protein synthesis is terminated. A protein known as a release factor binds to the ribosome and adds a water molecule to the end of the amino acid chain. This reaction hydrolyzes the polypeptide chain from the tRNA, allowing the protein to exit the ribosome. Without the stop codon, protein synthesis would simply stop without releasing the completed protein.

Translation consists of 3 steps: initiation, elongation, and termination.

Translation: Initiation

The mRNA molecule to be decoded binds to one subunit of the ribosome, then the other ribosomal subunit binds to both of those. In the process, a tRNA with the amino acid methionine attached docks in the ribosome's P site.

Translation: Elongation

The following cycle is repeated again and again, elongating the polypeptide by 1 amino acid per cycle.

(1) a charged tRNA docks in the A site and the existing, growing polypeptide is transferred from the tRNA molecule in the P site to the new tRNA's amino acid.

(2) translocation: The ribosome moves 3 bases along the mRNA, moving the uncharged tRNA in the P site into the E (exit) site, the tRNA in the A site (with the polypeptide now attached) into the P site, and positioning a new codon into association with the now-empty A site.

(3) Back to step (1)

Translation: Termination

Eventually, translocation brings a stop codon into the position associated with the empty A site. A stop codon does not code for an amino acid, but for a release factor. The release factor binds and then frees the polypeptide and dissociates the ribosome.

<a rel="nofollow" href="https://http://highered.mcgraw-hill.com/sites/dl/free/0072507470/291136/translation.swf" target="_blank">https://http://highered.mcgraw-hill.com/sites/dl/free/0072507470/291136/translation.swf</a>