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Like transcription, the process of translation occurs in three stages called initiation, elongation, and termination. During initiation, mRNA, the first tRNA, and ribosomal subunits assemble into a complex. Next, in the elongation stage, the ribosome moves from the start codon in the mRNA toward the stop codon, synthesizing a polypeptide according to the sequence of codons in the mRNA. Finally, the process is terminated when the ribosome reaches a stop codon and the complex disassembles, releasing the completed polypeptide. In this section, we will examine the steps in this process as they occur in living cells.
As its name suggests, the elongation stage involves the covalent bonding of amino acids to each other, one at a time, to create a polypeptide. Even though this process involves several different components, translation occurs at a remarkable rate. Under normal cellular conditions, the translation machinery can elongate a polypeptide chain at a rate of 15 to 18 amino acids per second in bacteria and six amino acids per second in eukaryotes.
To elongate a polypeptide by one amino acid, a tRNA brings a new amino acid to the ribosome, where it is attached to the end of a growing polypeptide chain. In step (1), translation has already proceeded to a point where a short polypeptide is attached to the tRNA located in the P site of the ribosome. This is called peptidyl tRNA. In the first step of elongation, an aminoacyl tRNA carrying a single amino acid binds to the A site. This binding occurs because the anticodon in the tRNA is complementary to the codon in the mRNA. The hydrolysis of GTP by proteins that function as elongation factors provides the energy for the binding of the tRNA to the A site. At this stage of translation, a peptidyl tRNA is at the P site and an aminoacyl tRNA is at the A site, which is how the P and A sites came to be named.
In the second step, a peptide bond is formed between the amino acid at the A site and the growing polypeptide chain, thereby lengthening the chain by one amino acid. As this occurs, the polypeptide is removed from the tRNA in the P site and transferred to the amino acid at the A site, an event termed a peptidyl transfer reaction. This reaction is catalyzed by a region of the 50S subunit known as the peptidyltransferase complex, which is composed of several proteins and rRNA. Thomas Steitz, Peter Moore, and their colleagues proposed that the rRNA is responsible for catalyzing bond formation between adjacent amino acids. In other words, the ribosome is a ribozyme.
After the peptidyl transfer reaction is complete, the third step involves the movement or translocation of the ribosome toward the 3? end of the mRNA by exactly one codon. This shifts the tRNAs at the P and A sites to the E and P sites, respectively. Notice that the next codon in the mRNA is now exposed in the unoccupied A site. The uncharged tRNA exits the E site. At this point, the next charged tRNA can enter the empty A site, and the same series of steps will add the next amino acid to the polypeptide chain.
When a stop codon is found in the A site of a ribosome, translation is ended. The three stop codons UAA, UAG, and UGA, do not have tRNAs with complementary sequences. Instead, stop codons are recognized by proteins, known as release factors. Interestingly, the three-dimensional structure of a release factor protein mimics the structure of tRNAs.
In step (1), the completed polypeptide chain is attached to a tRNA in the P site. A stop codon is located at the A site. In the first step, a release factor binds to the stop codon at the A site. In the second step, the bond between the polypeptide and the tRNA is hydrolyzed, causing the polypeptide and tRNA to be released from the ribosome. Finally, in the third step the ribosomal subunits, mRNA, and release factor dissociate.
Summary
The Machinery of Translation
Translation requires mRNA, tRNAs, ribosomes, and many translation factors.
tRNA molecules have a cloverleaf structure. Two important sites are the 3? end, which covalently binds an amino acid, and the anticodon, which base pairs with a codon in mRNA.
The enzyme aminoacyl-tRNA synthetase attaches the correct amino acid to a tRNA molecule.
Ribosomes are composed of rRNA molecules and many proteins to produce a small and large subunit. Bacterial and eukaryotic ribosomes differ in their composition.
Ribosomes have three sites called the A, P, and E sites, which are locations for the binding and release of tRNA molecules.
The gene that encodes the small subunit rRNA has been extensively used in the evolutionary comparisons of different species.
The Stages of Translation
Translation occurs in three stages called initiation, elongation, and termination.
During initiation of translation, the mRNA assembles with the ribosomal subunits and the first tRNA molecule.
Polypeptide synthesis occurs during the elongation stage, one amino acid at a time.
During the termination of translation, the binding of a release factor to the stop codon causes the release of the completed polypeptide and the disassembly of the mRNA and ribosomal subunits.
A polypeptide has a directionality in which the first amino acid is at the N- or amino-terminus, while the last amino acid is at the C- or carboxyl-terminus.
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