Ribosomes are often described as the “protein factories” of a cell, playing a fundamental role in nearly all life processes. These complex molecular machines create proteins, performing diverse functions from structural support to catalyzing biochemical reactions. Without functional ribosomes, cells cannot produce the proteins necessary for survival, growth, and reproduction.
Structure and Composition of Ribosome Subunits
A ribosome is assembled from two parts: a large ribosomal subunit and a small ribosomal subunit. Both are intricate complexes of ribosomal RNA (rRNA) molecules and numerous ribosomal proteins (r-proteins). Their specific composition varies slightly across organisms, but the overall structural design is conserved.
The size of these subunits is measured in Svedberg units (S), which reflect sedimentation rate during centrifugation, not direct mass or volume. This explains why individual subunit Svedberg units do not simply add up to the complete ribosome’s total. For instance, a complete ribosome might be 70S, but its subunits could be 30S and 50S. This non-additive nature arises because sedimentation is influenced by factors beyond just mass, such as shape and density.
The smaller subunit interacts with messenger RNA (mRNA) and ensures accurate genetic code reading. It mediates correct pairing between mRNA codons and tRNA anticodons. The larger subunit contains the peptidyl transferase center. This site catalyzes peptide bond formation between amino acids during protein synthesis.
How Ribosome Subunits Assemble Proteins
Protein assembly, also known as translation, begins when the small ribosomal subunit binds to a messenger RNA (mRNA) molecule. This mRNA carries genetic instructions, a blueprint for the protein. The small subunit then scans the mRNA until it finds a “start” signal, a three-base sequence called a codon.
Once the small subunit is positioned on the mRNA, the large ribosomal subunit joins, forming a functional ribosome. The assembled ribosome has three sites where transfer RNA (tRNA) molecules bind. Each tRNA is a carrier, designed to pick up an amino acid and deliver it to the ribosome based on mRNA codons.
As the ribosome moves along the mRNA, it reads the codons in sequence. A tRNA with an anticodon matching the mRNA codon enters, bringing its amino acid. The large subunit catalyzes peptide bond formation, linking the new amino acid to the growing chain. This process continues until a “stop” codon on the mRNA signals the end of the protein sequence.
Ribosome Subunits in Different Cell Types
Ribosome subunits show differences between prokaryotic cells (e.g., bacteria) and eukaryotic cells (e.g., plants and animals). Prokaryotic ribosomes are smaller, known as 70S ribosomes. They are composed of a small 30S subunit and a large 50S subunit. The bacterial 30S subunit contains 16S RNA and about 21 proteins, while the 50S subunit has 5S and 23S RNA and about 31 proteins.
Eukaryotic cells, in contrast, possess larger 80S ribosomes in their cytoplasm. They consist of a 40S small subunit and a 60S large subunit. The eukaryotic 40S subunit contains 18S RNA and around 33 proteins, while the 60S subunit includes 5S, 5.8S, and 28S RNA along with about 49 proteins.
The structural differences between prokaryotic and eukaryotic ribosomes are important in medicine, particularly for antibiotics. Many antibiotics specifically target bacterial ribosomes (e.g., 30S or 50S subunits) without harming human (eukaryotic) ribosomes. For example, tetracyclines bind to the bacterial 30S subunit, and macrolides affect the 50S subunit, disrupting bacterial protein synthesis. This selectivity allows drugs to combat bacterial infections effectively while minimizing adverse effects on human cells.