Ribosomes are fundamental cellular machines found within all living cells, from the simplest bacteria to complex human cells. Their presence underscores a basic biological principle: the need for cells to create specific functional molecules.
What Ribosomes Are Made Of
Ribosomes are complex structures composed of ribosomal RNA (rRNA) and various ribosomal proteins. These components are organized into two distinct subunits—a larger and a smaller. These subunits remain separate when not actively involved in protein synthesis and come together only when needed.
Unlike many other cellular components, ribosomes are not enclosed by a membrane. Each ribosome is a ribonucleoprotein complex. In prokaryotes, ribosomes are roughly 40% protein and 60% rRNA, while in eukaryotes, they are about half protein and half rRNA.
Where Ribosomes Do Their Work
Ribosomes operate in different cellular locations, depending on the type of cell and the ultimate destination of the protein being produced. In eukaryotic cells, ribosomes can be found floating freely in the cytoplasm, or they can be attached to the membranes of the endoplasmic reticulum (ER) and the outer nuclear envelope. Free ribosomes synthesize proteins intended for use within the cell’s own cytoplasm, such as enzymes involved in metabolic pathways.
Ribosomes attached to the endoplasmic reticulum give it a “rough” appearance, hence the name rough endoplasmic reticulum (RER). These bound ribosomes specialize in making proteins destined for secretion outside the cell, for insertion into cellular membranes, or for delivery to specific organelles like lysosomes. The proteins produced by these ribosomes enter the ER lumen as they are synthesized, where they can be further processed and sorted for their specific cellular destinations.
How Ribosomes Build Proteins
The core function of ribosomes is protein synthesis, a complex process also known as translation. This process begins when a messenger RNA (mRNA) molecule, carrying genetic instructions from DNA, binds to the small ribosomal subunit. The ribosome then reads the mRNA sequence in specific three-nucleotide units called codons.
Each codon specifies a particular amino acid, which is delivered to the ribosome by a transfer RNA (tRNA) molecule. The tRNA molecules have an anticodon sequence complementary to the mRNA codon, ensuring the correct amino acid is brought into place. As the ribosome moves along the mRNA, it facilitates the formation of peptide bonds between the incoming amino acids, linking them together in a precise sequence to form a growing polypeptide chain. This elongation process continues until the ribosome encounters a “stop” codon on the mRNA, signaling the end of protein synthesis. Once the polypeptide chain is complete, it is released from the ribosome and begins to fold into its unique three-dimensional structure, which is necessary for it to become a functional protein.
Different Types of Ribosomes
Ribosomes in prokaryotic and eukaryotic cells, while performing the same fundamental function, exhibit notable structural differences. Prokaryotic ribosomes are smaller, designated as 70S (Svedberg units, a measure of sedimentation rate). They are composed of a small 30S subunit and a large 50S subunit. The 30S subunit contains 16S rRNA, while the 50S subunit contains 5S and 23S rRNAs.
Eukaryotic ribosomes are larger, known as 80S, and are made up of a 40S small subunit and a 60S large subunit. The 40S subunit includes an 18S rRNA, and the 60S subunit contains 5S, 5.8S, and 28S rRNAs. These structural differences are significant because they allow certain antibiotics to selectively target bacterial ribosomes, disrupting protein synthesis in bacteria without harming human cells. For instance, some antibiotics bind to the 50S subunit of prokaryotic ribosomes, while others target the 30S subunit, exploiting these variations to inhibit bacterial growth.