Translation (Biology)

Translation is the process of converting mRNA (messenger RNA) into protein. mRNA is read 3 nucleotides at a time by the ribosome. No nucleotides are skipped. The mRNA code is non overlapping. For example, in the arbitrary mRNA AUGAAAGUAUAG, the ribosome will read it as "AUG," then "AAA," then "GUA," then "UAG." Each triplet is called a codon and codes for one amino acid. Each tRNA has an anticodon, which will bind with the mRNA codon while the mRNA is being translated in the ribosome. Each tRNA is charged (is bonded to) one of the 20 amino acids. Multiple tRNAs can be bound to the same amino acid. However, this does not mean there are 61 tRNAs. How is this accounted for? There is a third base-pair wobble, where a tRNA with the anticodon sequence 3'-CCA-5' may bind with any of the following codon sequences: 5'-GGU-3'; 5'-GGA-3'; 5'-GGC-3'; 5'-GGG-3'. Additionally, the codon AUG (which calls for methionine) is ALWAYS the first amino acid of a protein and is thus called the start codon, and the codons UGA, UAG, and UAA do not bind to tRNAs, and do not have an amino acid; they are called stop codons; they bind to release factors which signal the end of translation.

Initiation:

In bacteria, the ribosome (70S) consists of the 50S (big) and 30S (small) subunits. Within the 30S subunit, there is a 16S rRNA. During initiation, the 30S ribosomal subunit with initiation factors 3 and 1 (IF-3, IF-1) bind the mRNA. The AUG sequence isn't enough to start initiation; the 16S rRNA must bind the ribosomal-binding site in the mRNA, also called the Shine-Dalgarno sequence. Once that binding is complete, then formyl-methionine tRNA is brought to the AUG of the mRNA loaded into the 30S subunit by initiation factor 2 (IF-2). In bacteria, the first amino acid of a protein is an f-met (formyl methionine). The other methionines do not have formyl group attached. After the tRNA is properly bound, and the 50S subunit binds the 30S, tRNA, and mRNA, GTP hydrolysis occurs, and IF-1, 2, and 3 are all released. The final complex ready to do translation is the 70S initiation complex.

$$ Elongation:

The ribosome has three sites where tRNAs attach, from "left" to "right": the "E" site (Exit), the "P" site (Peptide), and the "A" site (Aminoacyl). In the beginning of translation elongation, fMet tRNA is in the P site, meaning that the tRNA is "held in place" by the P site in the 50S ribosomal subunit and the 3'-5' UAC of the tRNA is complementarily bound to the 5'-3' AUG on the mRNA held by the 30S ribosomal subunit. A new charged tRNA is carried by EF-Tu into the A-site, and the anticodon in the tRNA complementarily binds to the codon in the mRNA. This causes a molecule of GTP to be hydrolyzed. The 23S subunit of the 50S ribosomal subunit is thought to be peptidyl transferase, and the amino acid on the tRNA in the P-site is bonded to the amino acid on the tRNA in the A-site. On a diagram, the P site's amino acid is bonded to the "top" of the A site amino acid.

Now, the ribosome is ready to translocate. On a diagram, this will look like the ribosome moves itself on the mRNA 3 nucleotides to the right. So, the tRNA in the A site moves into the P site, and the tRNA in the P site moves to the E site. Translocation requires EF-G to bind the ribosome, as well as GTP hydrolysis. When translocation has been completed, the tRNA in the E site exits and can be recharged with an amino acid. This process repeats again and again until the stop codon is reached.

$$ Termination:

The three stop codons are UAA, UAG, and UGA. It is important to note that no tRNA anticodon will bind to these stop codons unless the anticodon is somehow mutated to match it. In bacteria, release factor 1 (RF1) recognizes UAA and UAG, whereas release factor 2 (RF2) recognizes UGA. The binding of RF1 or RF2 to the A site causes peptidyl transferase to release the polypeptide chain. Then, release factor 3 (RF3) binds the 50S ribosomal subunit, and through GTP hydrolysis, RF1 or RF2 is released. The ribosome recycling factor (RRF) binds to the A-site. EF-G causes the ribosome to translocate, hydrolyzing a molecule of GTP in the proces. The tRNA in the P site moves into the E site and is subsequently released; the RRF moves from the A site to the P site and also gets released, and EF-G gets released. The 50S, 30S, and mRNA are dissociated, and are subsequently recycled to be used again, or the mRNA can also get degraded.

Initiation

Elongation

Termination

tRNA charging