Summary
Highlights
Translation is the second step of gene expression, where information stored in DNA (genes) is used to create functional gene products, primarily proteins. These proteins are crucial for various cellular functions like repair, maintenance, energy synthesis, and enzymatic actions. Gene expression involves transcription (DNA to mRNA) in the nucleus, followed by translation (mRNA to protein) in the cytoplasm.
Besides messenger RNA (mRNA), translation requires transfer RNA (tRNA) and ribosomes. Ribosomes are cellular machinery that read the mRNA message, while tRNA transfers specific amino acids to the ribosome based on the mRNA's base pair sequence. These amino acids are then joined to form a protein.
Translation occurs in three basic steps: initiation, where the ribosome assembles around the mRNA; elongation, where tRNA brings amino acids to form a polypeptide chain; and termination, where the ribosome releases the polypeptide after reading a stop signal on the mRNA.
In prokaryotes, translation takes place in the cytoplasm. In eukaryotes, it occurs in the cytoplasm or across the endoplasmic reticulum membrane, where the newly synthesized protein can be stored, released later, or secreted immediately.
Ribosomes are complex molecular structures, found in all living cells, that act as the site for protein synthesis. They consist of a small subunit (40S in eukaryotes) and a large subunit (60S in eukaryotes), forming an 80S ribosome when joined. The small subunit reads mRNA, and the large subunit joins amino acids to form the polypeptide chain.
A codon is a sequence of three DNA or RNA bases. Ribosomes read these three-base sequences, not individual bases. Each codon corresponds to a specific amino acid. For example, UUU codes for phenylalanine, and UCC codes for serine. AUG is the start codon for methionine, signaling the beginning of protein synthesis, while UAG, UAA, and UGA are stop codons, signaling the end.
Initiation begins with a tRNA carrying methionine binding to the small ribosomal subunit. This complex then scans the mRNA from its 5' end. Upon encountering the AUG start codon, it binds to the large ribosomal subunit, forming a complete ribosome and initiating protein synthesis.
In the large ribosomal subunit, there are three sites: E (exit), P (peptidyl), and A (aminoacyl). The initial methionine-carrying tRNA occupies the P-site. The A-site receives the next tRNA carrying the amino acid corresponding to the next codon. A bond forms between the amino acids in the P and A sites. The ribosome then moves, shifting the tRNA from the P-site to the E-site (for exit) and the tRNA from the A-site to the P-site. The A-site becomes empty for the next tRNA. This process repeats, elongating the polypeptide chain.
Termination occurs when the ribosome encounters a stop codon (UAG, UAA, or UGA) on the mRNA. These codons signal the ribosome to stop protein synthesis, and the completed polypeptide chain is released from the translation complex. Following synthesis, proteins may undergo additional processing, such as amino acid deletions or folding, to form their stable, functional structure.