Protein synthesis translates genetic information into functional proteins. This process, known as translation, converts the nucleotide sequence within messenger RNA (mRNA) into a specific sequence of amino acids, which then fold into proteins. Proteins perform a vast array of functions, from catalyzing metabolic reactions to providing structural support and transporting molecules. Translation is central to gene expression, following transcription where DNA’s genetic code is copied into RNA.
The Genetic Code
The genetic code guides the conversion of mRNA sequences into protein sequences. This code is read in units of three consecutive nucleotides, called codons, with each codon specifying a particular amino acid or a stop signal. While there are 64 possible codons, only 20 amino acids are used in proteins, meaning that most amino acids are specified by more than one codon, a characteristic known as degeneracy. This redundancy in the code provides some protection against the effects of single nucleotide changes. The genetic code is universal, meaning the same codons specify the same amino acids across nearly all forms of life.
Key Players in Translation
Several molecular components are essential for the translation process. Messenger RNA (mRNA) carries the genetic blueprint from DNA to the ribosomes. Transfer RNA (tRNA) molecules act as adaptors, each recognizing a specific mRNA codon and delivering the corresponding amino acid to the growing protein chain. Each tRNA possesses an anticodon, a three-nucleotide sequence complementary to an mRNA codon.
Ribosomes are the machinery for protein synthesis, composed of ribosomal RNA (rRNA) and proteins. These consist of two main subunits, a small subunit and a large subunit, which come together during translation. Within the ribosome, there are three sites for tRNA binding and movement: the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site. Amino acids are the fundamental building blocks linked together to form the polypeptide chain.
Initiation: Starting the Process
Translation begins with the assembly of machinery at the start codon on the mRNA molecule. The small ribosomal subunit first binds to the mRNA. It then scans the mRNA until it locates the start codon, which is AUG. An initiator tRNA, carrying the amino acid methionine, binds to this AUG start codon within the P-site of the small ribosomal subunit.
This binding establishes the correct reading frame, ensuring that subsequent codons are read accurately. The large ribosomal subunit joins the complex, completing the functional ribosome. This assembled initiation complex is ready for amino acid addition.
Elongation: Building the Protein Chain
Once initiation is complete, the elongation phase commences, adding amino acids to the growing polypeptide chain. This cyclical process consists of three main steps. First, codon recognition occurs when a new tRNA, carrying the next amino acid, enters the A-site of the ribosome, matching its anticodon to the exposed mRNA codon. This pairing ensures the correct amino acid is incorporated into the protein.
Next, peptide bonds form. The ribosome catalyzes a peptide bond between the amino acid carried by the tRNA in the A-site and the nascent polypeptide chain attached to the tRNA in the P-site. This reaction transfers the growing polypeptide chain from the P-site tRNA to the newly arrived amino acid in the A-site. The polypeptide chain, now one amino acid longer, is attached to the tRNA in the A-site.
Finally, translocation occurs, where the ribosome moves one codon along the mRNA molecule in the 5′ to 3′ direction. This movement shifts the tRNAs within the ribosome: the tRNA now holding the polypeptide chain moves from the A-site to the P-site, and the uncharged tRNA that was in the P-site moves to the E-site. The empty tRNA then exits the ribosome from the E-site, making the A-site available for the next incoming aminoacyl-tRNA. This cycle repeats, adding one amino acid at a time, until the entire mRNA sequence has been read.
Termination: Completing the Protein
Elongation continues until the ribosome encounters a stop codon on the mRNA. There are three stop codons: UAA, UAG, and UGA. Unlike other codons, stop codons do not specify an amino acid. Instead, when a stop codon enters the A-site, it is recognized by proteins called release factors.
These release factors bind to the stop codon within the A-site, triggering the hydrolysis of the bond connecting the completed polypeptide chain to the tRNA in the P-site. This hydrolysis reaction frees the newly synthesized protein from the ribosome. Following the release of the polypeptide, the ribosomal subunits, mRNA, and tRNAs dissociate, allowing these components to be recycled for future rounds of translation.