Ribosomes are intricate cellular machines responsible for manufacturing proteins, molecules that carry out a vast array of functions within every living cell. These tiny factories ensure that genetic instructions encoded in DNA are accurately converted into proteins required for various cellular processes, from repairing damage to directing chemical reactions. Proteins are fundamental to life, performing diverse roles such as transporting substances, providing structural support, and catalyzing biochemical reactions.
The Translation Process
Protein synthesis, known as translation, begins with messenger RNA (mRNA), which carries genetic instructions copied from DNA in the cell’s nucleus to the ribosomes in the cytoplasm. Ribosomes read this mRNA sequence in specific three-nucleotide units called codons. Each codon corresponds to a particular amino acid, the building blocks of proteins. Transfer RNA (tRNA) molecules act as adapters, bringing the correct amino acid to the ribosome based on the mRNA codon sequence.
As the ribosome moves along the mRNA, it links incoming amino acids, forming a growing chain called a polypeptide. This process of adding amino acids one by one is known as elongation. The ribosome ensures each amino acid is added in the order specified by the mRNA, creating a polypeptide chain. This continuous assembly proceeds until a specific signal indicates that the protein is complete.
Recognizing the Termination Signal
The signal to stop protein synthesis comes in the form of a “stop codon” on the mRNA molecule. There are three such stop codons: UAA, UAG, and UGA. Unlike other codons, these do not specify any amino acid and no corresponding tRNA molecule exists to recognize them. When a ribosome encounters one of these stop codons in its A-site (aminoacyl site), it signals the end of the elongation phase.
Instead of a tRNA, specialized proteins called release factors recognize and bind to the stop codon. In bacteria, two Class 1 release factors, RF1 and RF2, are responsible for this recognition; RF1 recognizes UAA and UAG, while RF2 recognizes UAA and UGA. Eukaryotic cells, including human cells, utilize a single Class 1 release factor, eRF1, which can recognize all three stop codons. The binding of these release factors to the ribosome’s A-site initiates the termination process.
Polypeptide Release
Once a release factor binds to the stop codon in the ribosome’s A-site, it initiates a chemical reaction that severs the newly formed polypeptide chain from the ribosome. This reaction involves the hydrolysis of the ester bond connecting the polypeptide to the tRNA located in the ribosome’s P-site (peptidyl site). The Class 1 release factors possess a conserved GGQ motif that is essential for this catalytic activity, positioning it near the peptidyl transferase center of the ribosome to facilitate the hydrolysis.
Following its release, the polypeptide typically undergoes a process of folding into its unique three-dimensional structure, which is necessary for its specific function within the cell. It may also be directed to its particular destination or compartment within the cell, ready to perform its biological role.
Ribosome Disassembly and Recycling
After the polypeptide has been released, the components of the translational machinery must be disassembled and prepared for new rounds of protein synthesis. The large and small ribosomal subunits, the mRNA molecule, and any remaining deacylated (empty) tRNA molecules dissociate from each other. This separation prevents the accumulation of inactive ribosomal complexes and ensures the continuous availability of ribosomes for protein production.
Additional protein factors assist in this disassembly and recycling process. In bacteria, ribosome recycling factor (RRF) and elongation factor G (EF-G) work together to split the ribosome into its individual subunits. Other factors, like initiation factor 3 (IF3), can further aid in releasing the tRNA and preventing the re-association of the ribosomal subunits prematurely. These separated components are then ready to be reused in another cycle of protein synthesis.