Ribosomes are complex molecular machines found in all living cells that are responsible for synthesizing proteins. These structures link amino acids together in a specific order, following the instructions encoded in messenger RNA (mRNA). In eukaryotic cells—which include animals, plants, and fungi—the specific type of ribosome found in the cytoplasm is known as the 80S ribosome. They are distinct from the ribosomes found in prokaryotic organisms like bacteria.
Structure and Composition of 80S Ribosomes
The 80S ribosome is a large ribonucleoprotein complex, meaning it is composed of both ribosomal RNA (rRNA) and various proteins. It consists of two distinct, unequal parts: a large subunit and a small subunit. In eukaryotic cells, these are named the 60S subunit (large) and the 40S subunit (small). Together, these two pieces form the functional 80S ribosome that actively translates genetic code.
The naming convention, using the “S” value, refers to the Svedberg unit, which is not a measure of mass but of a particle’s sedimentation rate during ultracentrifugation. This technique separates particles based on their size, shape, and density. The sedimentation rate is influenced by surface area and friction as the particle moves through a solution. Because shape and surface area change when the two subunits combine, their Svedberg values are not additive. This is why the 40S and 60S subunits combine to form an 80S ribosome, not a 100S particle.
The small 40S subunit is primarily responsible for monitoring the correct pairing between the mRNA codon and the transfer RNA (tRNA) anticodon. It contains a single 18S rRNA molecule and approximately 33 ribosomal proteins. The larger 60S subunit is where the chemical reaction of forming peptide bonds occurs, linking amino acids into a growing polypeptide chain. This subunit contains three different rRNA molecules (28S, 5.8S, and 5S rRNA) and around 49 proteins.
The Role in Protein Synthesis
The primary function of the 80S ribosome is protein synthesis, a process called translation. During translation, the ribosome moves along an mRNA molecule, reading its sequence of codons. For each codon, the ribosome recruits a corresponding tRNA molecule carrying a specific amino acid, and the large subunit catalyzes the formation of a peptide bond to build the protein chain.
Eukaryotic cells contain two populations of 80S ribosomes, which differ in their location and the destination of the proteins they synthesize. Free ribosomes are found suspended in the cytoplasm and produce proteins that will function within the cell itself, such as enzymes involved in metabolism.
Bound ribosomes attach to the outer membrane of the endoplasmic reticulum, creating the “rough” endoplasmic reticulum. These ribosomes synthesize proteins destined for insertion into cellular membranes, packaging within organelles like lysosomes, or for export from the cell. As a protein is made, it is threaded directly into the endoplasmic reticulum for folding and modification before being sent to its final destination.
Distinguishing Eukaryotic and Prokaryotic Ribosomes
While eukaryotes have 80S ribosomes in their cytoplasm, prokaryotes like bacteria have smaller 70S ribosomes. These are composed of a 50S large subunit and a 30S small subunit. This size difference reflects a different composition of both rRNA and protein components.
The rRNA molecules within prokaryotic ribosomes are also smaller. The 70S ribosome’s small 30S subunit contains 16S rRNA, and its large 50S subunit contains 23S and 5S rRNA. These molecules are shorter and have different sequences compared to their eukaryotic counterparts.
Eukaryotic cells also contain 70S ribosomes, but only within specific organelles: the mitochondria and, in plant cells, the chloroplasts. These organelles are involved in energy production and photosynthesis, respectively, and have their own genetic material and protein synthesis machinery. The presence of 70S ribosomes in these organelles supports the endosymbiotic theory, which proposes that mitochondria and chloroplasts originated as free-living prokaryotes that were engulfed by an early eukaryotic cell.
Medical and Biological Importance
The structural differences between eukaryotic 80S and prokaryotic 70S ribosomes are medically significant. This distinction allows for the development of antibiotics that selectively target bacterial infections by exploiting the unique features of the 70S ribosome to disrupt protein synthesis, stopping bacterial growth without harming the human host.
For example, antibiotics like tetracyclines work by binding to the 30S subunit of the bacterial ribosome, preventing tRNA from attaching and halting protein production. Other antibiotics, such as erythromycin, bind to the 50S subunit and interfere with the movement of the growing polypeptide chain. Since these drugs are designed for the 70S ribosome, they do not affect the 80S ribosomes in a patient’s cells.
While targeting bacterial ribosomes is beneficial, errors in the function or assembly of our own 80S ribosomes can lead to human diseases. These conditions, known as ribosomopathies, are genetic disorders caused by mutations in genes that code for ribosomal proteins or other factors involved in ribosome assembly. These defects can impair protein synthesis, leading to a wide range of developmental and health issues.