Translation is the cellular process responsible for synthesizing polypeptides, the building blocks of proteins, based on the information encoded in mRNA.
This process involves several steps and components:
1. Charging of tRNA (Aminoacylation): Prior to translation, amino acids are activated by attaching them to their corresponding transfer RNA (tRNA) molecules in a process called charging or aminoacylation. This step is facilitated by aminoacyl-tRNA synthetases and requires ATP.
2. Initiation: Translation begins with the small ribosomal subunit binding to the mRNA at the start codon, which is typically AUG. The initiator tRNA carrying methionine (Met) binds to this start codon. The large ribosomal subunit then associates with the small subunit to form the functional ribosome.
3. Elongation: During the elongation phase, the ribosome moves along the mRNA in the 5' to 3' direction. Codons on the mRNA are read sequentially, and corresponding tRNA molecules, with their attached amino acids, enter the ribosome. The ribosome catalyzes the formation of peptide bonds between adjacent amino acids, creating a growing polypeptide chain.
5. Termination: Translation continues until a stop codon (UAA, UAG, or UGA) is encountered on the mRNA. There are no tRNAs with anticodons complementary to these stop codons. Instead, release factors bind to the ribosome when a stop codon is encountered, leading to the termination of translation. The polypeptide chain is then released from the ribosome.
6. Ribosome: The ribosome is a complex molecular machine composed of ribosomal RNA (rRNA) and numerous proteins. It plays a central role in facilitating the binding of tRNA molecules, the formation of peptide bonds, and movement along the mRNA.
7. Peptide Bond Formation: Peptide bond formation occurs between the amino group of the incoming amino acid on the A-site tRNA and the carboxyl group of the growing polypeptide chain on the P-site tRNA. This reaction is catalyzed by ribosomal RNA (23S rRNA in bacteria).
8. Translational Unit: A translational unit in mRNA includes the sequence flanked by the start codon (AUG) and the stop codon. This sequence codes for a specific polypeptide. UTRs located at both ends play roles in initiation and regulation.