Within every living cell, microscopic structures known as ribosomes function as protein factories. These cellular machines are responsible for synthesis, the process of building the complex proteins required for almost all cellular activities. A cell depends on its ribosomes to construct the vast array of proteins that carry out tasks ranging from structural support to metabolic regulation.
The Genetic Blueprint for Proteins
To construct a protein, a ribosome requires instructions delivered by a molecule called messenger RNA (mRNA). The mRNA is a temporary copy of a gene, transcribed from the cell’s DNA blueprint stored within the nucleus. This mRNA molecule travels from the nucleus into the cytoplasm, where the ribosomes are located.
The information encoded within the mRNA is written in a simple genetic code, read in three-letter “words” known as codons. Each codon is a specific sequence of three nucleotide bases. As the ribosome reads the mRNA strand, it deciphers each codon one by one.
This genetic code is nearly universal across all known life, pointing to its deep evolutionary origins. Each three-letter codon corresponds to one of the 20 different amino acids, which are the building blocks of proteins. The sequence of codons on the mRNA strand dictates the order in which amino acids must be linked together.
The Protein Assembly Line
The process of translating the mRNA blueprint into a protein occurs in three main stages. The first stage is initiation, where the two components of the ribosome—a large and a small subunit—come together. These subunits, made of ribosomal RNA (rRNA) and proteins, clamp onto the beginning of the mRNA strand to form a complete ribosome.
Once assembled, the ribosome enters the elongation phase, where the protein chain is actively built. The ribosome moves along the mRNA template, reading one codon at a time. At this point, another type of RNA molecule, transfer RNA (tRNA), comes into play. Each tRNA molecule recognizes a specific mRNA codon and carries the corresponding amino acid.
Inside the ribosome are three distinct sites—A, P, and E—that manage tRNA traffic. A new tRNA carrying an amino acid enters the A (aminoacyl) site, where its anticodon pairs with the mRNA codon. The ribosome then catalyzes the formation of a peptide bond, linking the new amino acid to the growing chain held in the P (peptidyl) site. The ribosome then advances along the mRNA in a step called translocation, moving the tRNAs through the sites. The now-empty tRNA moves to the E (exit) site and is released, leaving the A site open for the next delivery.
This cycle of codon recognition, bond formation, and translocation repeats, adding amino acids one by one to the growing polypeptide chain. The final stage is termination, which occurs when the ribosome encounters a “stop” codon on the mRNA. These codons are recognized by proteins called release factors, which prompt the ribosome to add a water molecule to the end of the polypeptide chain, severing the connection between the chain and its tRNA. The completed protein is then released, and the ribosomal subunits detach from the mRNA, ready to be reused.
From Polypeptide to Functional Protein
The chain of amino acids released by the ribosome is known as a polypeptide. This linear sequence is not yet a functional protein and must undergo a process called protein folding. It spontaneously folds into a precise three-dimensional structure, a process that often begins while it is still being synthesized. This final shape is determined entirely by the sequence of its amino acids.
Misfolded proteins are inactive and can sometimes be harmful. In many cases, helper proteins called chaperones assist in the folding process, ensuring the polypeptide achieves its correct state without clumping together. Some proteins also undergo further edits, such as having certain amino acids removed or being joined with other polypeptide chains to form larger protein complexes.
The results of this process are diverse proteins that perform countless functions. Some become enzymes, which catalyze the chemical reactions necessary for life. Others form structural components, like the collagen in skin and bones. Another class of proteins are antibodies, which identify and neutralize foreign invaders like bacteria and viruses.
Location-Specific Building
In complex eukaryotic cells, such as those in plants and animals, ribosomes exist in two distinct populations. Their location determines the destination of the proteins they build. The first population consists of free ribosomes, which float unattached in the cytoplasm. These ribosomes synthesize proteins that are destined to function within the cytoplasm itself, including metabolic enzymes or proteins that make up the cell’s internal skeleton.
The second population is made of bound ribosomes. These ribosomes are physically attached to the outer surface of a network of membranes called the endoplasmic reticulum (ER). This attachment gives the ER a studded appearance, leading to its designation as the “rough” ER. Bound ribosomes produce proteins that have specific destinations beyond the cytoplasm.
These proteins are threaded directly into the ER as they are being synthesized. From there, they may be embedded into the cell’s outer membrane or the membranes of internal organelles. They might also be packaged and shipped to other organelles, like lysosomes, or prepared for export out of the cell, as is the case for hormones or digestive enzymes. The cell can adjust the numbers of free and bound ribosomes based on its metabolic needs.