Within every animal cell, microscopic structures known as ribosomes work to build the proteins necessary for life. These molecular machines read genetic blueprints and translate them into functional proteins that carry out a vast array of tasks. From providing structural support to catalyzing metabolic reactions, the proteins created by ribosomes are involved in nearly every cellular activity. A single animal cell can contain as many as 10 million of these protein-producing sites.
What Ribosomes Are Made Of
A ribosome is a complex structure composed of ribosomal RNA (rRNA) and an assortment of proteins. These components are organized into two distinct pieces, known as the large and small subunits. In animal cells, these are referred to as the 60S (large) and 40S (small) subunits. These numerical labels refer to their sedimentation rate during centrifugation, a measure of their size and density.
The synthesis of these subunits begins in a specialized region of the nucleus called the nucleolus. Here, different types of rRNA molecules are manufactured and combined with approximately 80 different ribosomal proteins. This assembly results in the formation of the initial large and small subunits.
Once assembled in the nucleolus, the large and small subunits are exported separately into the cytoplasm. They remain apart until needed for protein synthesis. When a messenger RNA (mRNA) molecule is ready to be translated, the two subunits come together to form a complete, functional 80S ribosome.
The Role of Ribosomes in Protein Synthesis
The primary function of the ribosome is protein synthesis, a process called translation. During translation, the ribosome reads the genetic information encoded in a messenger RNA (mRNA) molecule. This process occurs in three main stages: initiation, elongation, and termination. Initiation begins when the small ribosomal subunit binds to an mRNA strand and identifies the starting point, after which the large subunit joins to form a complete complex.
Once assembled, the ribosome moves along the mRNA, reading its sequence of codons, which are three-letter genetic “words.” This is the elongation phase. For each codon, a specific transfer RNA (tRNA) molecule arrives carrying a corresponding amino acid. The ribosome has three sites—the A (aminoacyl), P (peptidyl), and E (exit) sites—that manage the flow of tRNA molecules, ensuring accuracy.
The ribosome then catalyzes the formation of a peptide bond, linking the new amino acid to the growing polypeptide chain in the P site. The entire complex then shifts one codon down the mRNA. The now-uncharged tRNA moves from the P site to the E site, from which it is released back into the cytoplasm. This cycle repeats until the ribosome encounters a “stop” codon on the mRNA, signaling the termination phase and the release of the new protein.
Where Ribosomes Are Found in the Cell
Ribosomes exist in two main locations within an animal cell, which determines the destination of the proteins they create. Many ribosomes, known as free ribosomes, float throughout the cytoplasm, the gel-like substance that fills the cell. Proteins synthesized by these free ribosomes have functions within the cytosol, such as metabolic enzymes or structural proteins that make up the cytoskeleton.
The second population of ribosomes is attached to the surface of a network of membranes called the endoplasmic reticulum (ER). This attachment gives the membrane a studded appearance, which is why this region is named the rough endoplasmic reticulum (RER). The association between a ribosome and the RER is determined by the protein being synthesized.
Proteins produced by these bound ribosomes have different destinations. They are threaded directly into the RER as they are being made. From there, they may be incorporated into the cell’s membranes, such as the plasma membrane, or packaged for secretion out of the cell. This system ensures that proteins are delivered to the specific locations where they are needed.
When Ribosomes Malfunction
Since ribosomes produce the proteins that cells need to function, errors in their assembly or function can have significant health consequences. A class of human disorders known as “ribosomopathies” arises from defects in ribosome creation or function. These conditions are caused by mutations in genes that encode for ribosomal proteins or other factors required for building a ribosome.
Though protein synthesis is a requirement for all cells, these defects often result in specific phenotypes, affecting particular cell types more than others. For instance, several ribosomopathies manifest as bone marrow failure syndromes. This leads to conditions like Diamond-Blackfan anemia, which is characterized by a failure to produce enough red blood cells.
Other developmental issues can also arise from faulty ribosomes. Problems such as craniofacial abnormalities and growth defects are seen in certain ribosomopathies. These conditions highlight how disruptions in protein production can cascade into complex developmental and physiological problems.