Ribosomal RNA (rRNA) is a type of nucleic acid that serves as a fundamental component of the ribosome, the molecular machine responsible for synthesizing all cellular proteins. rRNA possesses a unique and highly complex structure that is central to the mechanism of life. It makes up the majority of the ribosome’s mass and is a highly conserved molecule across all forms of life. Understanding the structure of rRNA is key to appreciating its powerful role in protein production.
The Basic Building Blocks
Ribosomal RNA starts as a long, single-stranded polymer. Like all nucleic acids, this strand is constructed from a repeating sugar-phosphate backbone with a sequence of individual units called ribonucleotides. Each ribonucleotide contains a ribose sugar, a phosphate group, and one of four nitrogenous bases: Adenine (A), Uracil (U), Cytosine (C), and Guanine (G). This linear sequence of bases is known as the primary structure of the rRNA molecule.
Unlike the familiar double helix of DNA, rRNA is a flexible, thread-like molecule. The chemical presence of a hydroxyl group on the ribose sugar prevents it from forming a stable, long double helix. This initial state is ready to adopt a much more intricate shape.
The Intricate Folding Pattern
The single strand of rRNA quickly folds back upon itself, driven by internal base pairing, which defines its secondary structure. Complementary bases along the same strand form hydrogen bonds with one another, such as Adenine pairing with Uracil (A-U) and Guanine pairing with Cytosine (G-C).
The paired sections of the molecule create rigid, double-stranded regions known as stems. The intervening sections that do not pair with other bases form flexible, single-stranded loops and bulges. More complex folding motifs, called pseudoknots, occur when bases in a loop region fold back further to pair with bases outside of the loop, intertwining the structure. This extensive folding is highly specific and creates the precise grooves and pockets necessary for the molecule’s function.
Integration into the Ribosome
The final, three-dimensional structure is achieved when the secondary structure folds into its compact tertiary structure and combines with proteins to form the complete ribosome. The rRNA is the central functional component, acting as a ribozyme—an RNA molecule with catalytic activity. The numerous ribosomal proteins (r-proteins) primarily assemble on the surface, acting like a stabilizing shell around the dense, globular rRNA core. This massive molecular machine is composed of two unequal subunits, a large and a small one, each containing specific rRNA molecules. The large subunit contains the active site, called the peptidyl transferase center, where the rRNA itself catalyzes the formation of peptide bonds between amino acids, linking them to create a protein chain.
Variations in Appearance
The size and composition of ribosomal RNA vary between different groups of organisms, most notably between prokaryotes and eukaryotes. Prokaryotic ribosomes, found in bacteria, are classified as 70S ribosomes based on their sedimentation rate, and they are smaller overall. These 70S ribosomes consist of a small 30S subunit containing a single 16S rRNA molecule, and a large 50S subunit containing 23S and 5S rRNA molecules.
In contrast, eukaryotic ribosomes, found in human and plant cells, are larger and are designated as 80S ribosomes. The small 40S subunit contains an 18S rRNA molecule. The larger 60S subunit is more complex, containing three different rRNA molecules: the 28S, the 5.8S, and the 5S rRNA.