Viruses must invade a living cell to replicate their genetic material. Unlike cellular life forms that utilize deoxyribonucleic acid (DNA) as their blueprint, RNA viruses use ribonucleic acid (RNA) as their genetic material. This fundamental difference in their genomic structure dictates their unique replication strategies and contributes significantly to their ability to adapt rapidly. The RNA genome enables these viruses to hijack the host cell’s machinery quickly, forcing it to produce new viral particles.
Defining Characteristics and Structure
The simplest RNA virus consists of a core of genetic material encased within a protective shell called a capsid. This capsid is a protein coat constructed from numerous smaller, identical protein subunits. Some RNA viruses are further protected by a lipid envelope, an outer layer derived from the membrane of the host cell during the process of exiting.
These enveloped viruses, which include pathogens like the influenza virus, often use specialized glycoprotein spikes embedded in this membrane to attach to and enter new host cells. Conversely, non-enveloped viruses, such as poliovirus, rely solely on their durable protein capsid for protection and cell entry. The RNA genome itself can be organized in various ways, appearing as a single, continuous strand or as multiple separate segments.
Classification by Genome Organization
RNA viruses are systematically classified based on the “sense,” or polarity, of their genomic RNA. Positive-sense single-stranded RNA (+ssRNA) viruses possess a genome that functions directly as messenger RNA (mRNA) once inside the host cell. This means the host’s ribosomes can immediately begin translating the viral genome into proteins necessary for replication.
Negative-sense single-stranded RNA (-ssRNA) viruses carry a genome that is complementary to mRNA and cannot be directly translated. These viruses must first use a viral enzyme to transcribe their RNA into a positive-sense strand before protein synthesis can occur. Double-stranded RNA (dsRNA) viruses contain two RNA strands, one positive and one negative, which remain paired within the viral core.
A distinct group, the retroviruses, uses an RNA genome that is converted into DNA. These viruses employ a unique enzyme, reverse transcriptase, to create a DNA copy of their RNA genome. This newly formed DNA intermediate is then integrated into the host cell’s chromosome, marking a permanent change in the host’s genetic information.
The Replication Cycle and High Mutation Rate
The replication of an RNA virus depends on a specialized enzyme called RNA-dependent RNA polymerase (RdRp). This enzyme is responsible for copying the viral RNA genome and producing the messenger RNA needed for synthesizing viral proteins. The RdRp enzyme is highly prone to error because it lacks exonuclease activity, a proofreading mechanism that corrects mistakes during the copying process.
Without this correction ability, RdRp inserts incorrect nucleotides at a high frequency. This high error rate creates a vast population of slightly different viral genomes. The rapid accumulation of these mutations allows the virus to evolve quickly, a process known as antigenic drift.
This adaptation can swiftly change the surface proteins of the virus, enabling it to evade the host’s immune response or become resistant to antiviral medications. This mechanism explains why vaccines and treatments for RNA viruses like influenza must be updated frequently. Some large RNA viruses, such as coronaviruses, have evolved a dedicated proofreading exoribonuclease enzyme, which allows them to maintain a much larger and more stable genome than most other RNA viruses.
Notable Examples of RNA Viruses in Humans
Among the positive-sense single-stranded RNA viruses is SARS-CoV-2, the virus responsible for the COVID-19 pandemic. Another well-known example is the Poliovirus, which is a non-enveloped virus.
The negative-sense single-stranded RNA viruses encompass the highly contagious Measles virus, the segmented Influenza virus, and Ebola virus. The retrovirus group is dominated by the Human Immunodeficiency Virus (HIV), the cause of AIDS. HIV is characterized by its ability to integrate a DNA copy of its RNA genome into the host cell’s DNA, leading to persistent, lifelong infection.