Translation is a fundamental biological process through which cells create proteins, decoding genetic instructions carried by messenger RNA (mRNA) into a sequence of amino acids, the building blocks of proteins. Proteins perform a wide array of cellular functions. Prokaryotes are single-celled organisms that include bacteria and archaea. Unlike eukaryotic cells, prokaryotes do not possess a membrane-bound nucleus or other specialized compartments called organelles. This simpler structure impacts processes like protein synthesis.
The Cell’s Protein Factories
In prokaryotic cells, translation occurs entirely within the cytoplasm. The cytoplasm is the jelly-like substance filling the cell. The primary machinery for protein synthesis in this environment are the ribosomes.
Ribosomes are molecular machines that float freely throughout the prokaryotic cytoplasm. Prokaryotic ribosomes are composed of two subunits, 30S and 50S, which form a complete 70S ribosome during protein synthesis. These subunits are made up of ribosomal RNA (rRNA) and various ribosomal proteins. A typical bacterium, such as Escherichia coli, can contain as many as 15,000 ribosomes, making up a significant portion of the cell’s mass. These ribosomes translate genetic instructions into functional proteins.
The Molecular Players
Key molecules come together in the cytoplasm for translation. Messenger RNA (mRNA) molecules carry the genetic code from the DNA, acting as a direct template for protein assembly. This mRNA sequence dictates the specific order of amino acids for a protein.
Transfer RNA (tRNA) molecules act as adapter molecules. Each tRNA molecule has a specific site for attaching an amino acid and a region called an anticodon. The anticodon on the tRNA recognizes and binds to a complementary three-nucleotide sequence, known as a codon, on the mRNA. This precise matching ensures the correct amino acid is delivered to the ribosome.
Amino acids are the fundamental building blocks of proteins. There are 20 common types of amino acids, whose specific arrangement determines protein structure and function. These amino acids are sequentially added to a growing chain, forming a polypeptide that folds into a functional protein.
Assembling the Protein
Protein assembly at the ribosome involves three main stages: initiation, elongation, and termination. Initiation begins with the assembly of ribosomal subunits, mRNA, and the first tRNA. In prokaryotes, a sequence on the mRNA called the Shine-Dalgarno sequence helps the small ribosomal subunit bind at the correct starting point.
Following initiation, elongation commences as the polypeptide chain grows. The ribosome moves along the mRNA molecule, reading codons one by one. As each new codon is read, a corresponding tRNA molecule, carrying its specific amino acid, enters the ribosome. A peptide bond forms between the newly arrived amino acid and the growing polypeptide chain, extending the protein. This cycle of codon recognition, peptide bond formation, and ribosomal movement adds amino acids sequentially.
The process concludes with termination when the ribosome encounters a stop codon on the mRNA. Stop codons do not code for amino acids; they signal the end of protein synthesis. Release factors bind to the ribosome, triggering the release of the newly synthesized polypeptide chain from the ribosome. The ribosomal subunits then dissociate from the mRNA, ready for another round of translation.
The Coupled Process
A distinctive feature of translation in prokaryotes is its coupling with transcription. Transcription is the process where genetic information from DNA is copied into an mRNA molecule. Since prokaryotic cells lack a membrane-bound nucleus, both transcription and translation occur in the cytoplasm.
This arrangement allows ribosomes to begin translating mRNA even while it is being transcribed from DNA. As the mRNA strand emerges, ribosomes can immediately attach and initiate protein synthesis. This simultaneous occurrence, known as transcription-translation coupling, enables a rapid and efficient response to cellular needs. This coupling ensures proteins are produced quickly, advantageous for organisms with fast reproductive cycles.