Mitochondria are structures primarily known for generating the chemical energy that powers cellular activities. A separate but related molecule, RNA, acts as a messenger, translating genetic instructions into the proteins that perform many functions. Mitochondria operate their own distinct genetic system, completely separate from the primary genetic library stored in the cell’s nucleus. This mitochondrial system includes its own DNA and, consequently, its own specialized mitochondrial RNA, which is necessary for the mitochondrion’s operation.
The Mitochondrial Genome
Mitochondrial RNA originates from mitochondrial DNA (mtDNA). Unlike the linear chromosomes housed within the cell’s nucleus, mtDNA is a small, circular molecule, an organization reminiscent of the DNA found in bacteria. The human mitochondrial genome is compact, containing 37 genes dedicated to producing components for energy production.
Of these 37 genes, 13 provide the instructions for building protein subunits of the cellular energy machinery. The remaining 24 genes code for the RNA molecules—transfer RNAs and ribosomal RNAs—that are required to assemble those proteins. This self-contained genetic toolkit allows the mitochondrion to synthesize some of its own components.
A distinguishing feature of the mitochondrial genome is its pattern of inheritance. While the nuclear DNA in our cells is inherited from both parents, mtDNA is passed down exclusively from the mother. This is because the mitochondria in a sperm cell are not incorporated into the egg during fertilization, meaning an individual’s mtDNA creates a clear maternal line of inheritance.
Defining Mitochondrial RNA
Mitochondrial RNA (mtRNA) is transcribed from the mitochondrial DNA template. Once the long, initial RNA strand is produced, it is processed into its functional forms by specialized enzymes. This processing releases the three distinct types of mtRNA, which mirror the RNA molecules found in the main part of the cell but are dedicated solely to mitochondrial tasks.
The first type is mitochondrial messenger RNA (mt-mRNA). After being transcribed from the 13 protein-coding genes in mtDNA, these molecules carry the genetic blueprints for building specific mitochondrial proteins. Each mt-mRNA molecule serves as a template that is read by the mitochondrion’s protein-building machinery.
Another category is mitochondrial transfer RNA (mt-tRNA). The mitochondrial genome codes for 22 different mt-tRNAs. These molecules act as adaptors in the protein synthesis process, recognizing the code on the mt-mRNA and fetching the corresponding amino acid to deliver it to the assembly site.
The final type is mitochondrial ribosomal RNA (mt-rRNA). The mtDNA contains two genes that produce mt-rRNA molecules. These mt-rRNAs are the core structural and functional components of mitochondrial ribosomes, known as mitoribosomes. These mitoribosomes are the protein factories within the mitochondria that read the mt-mRNA templates and link together the amino acids to build the final proteins.
Role in Cellular Energy Production
The function of mitochondrial RNA is directly tied to the cell’s ability to produce energy. The proteins assembled through the coordinated action of mt-mRNA, mt-tRNA, and mt-rRNA are integral parts of a system called the electron transport chain. This chain is a series of protein complexes embedded in the inner mitochondrial membrane, and the 13 proteins coded by mtDNA are necessary components of these complexes.
This energy production process, known as oxidative phosphorylation, is where the cell’s energy currency, adenosine triphosphate (ATP), is generated. As electrons are passed along the protein complexes of the chain, a flow of protons is established across the mitochondrial membrane. This flow creates an electrochemical gradient that powers an enzyme to synthesize ATP. The proteins built using mtRNA instructions are necessary for this movement, and without them, ATP production would be severely disrupted.
Connection to Human Disease
Because the mitochondrial genome is central to energy production, errors within it can have significant consequences for human health. Mutations in the mitochondrial DNA can result in the production of faulty or incomplete mitochondrial RNA molecules. This flawed mtRNA can lead to the incorrect assembly of proteins in the electron transport chain, impairing their function and compromising the mitochondrion’s ability to generate ATP.
This energy deficit gives rise to a class of conditions known as mitochondrial diseases. These disorders can manifest with a wide variety of symptoms because they affect cellular power. The parts of the body with the highest energy requirements are often the most severely affected. Tissues in the brain, heart, liver, and skeletal muscles depend on a constant supply of ATP to function properly.
When energy production falters due to defects originating from mtRNA, these high-demand organs can suffer damage, leading to symptoms like muscle weakness, seizures, developmental delays, and heart problems. The severity and type of disease often depend on the specific mutation and the percentage of mutated mtDNA within a person’s cells. This direct link between the integrity of mitochondrial RNA and cellular function highlights its importance in maintaining health.