lec 20

RNA Viruses: Structure, Classification, and Human Health Impact

Overview of RNA Viruses

RNA viruses are a diverse group of viruses that use RNA as their genetic material. They are unique among biological agents in that their genomes are composed of RNA rather than DNA. RNA viruses are classified based on their genomic structure, presence or absence of an envelope, and the size and shape of their capsid. These viruses are responsible for a wide range of diseases in humans and animals, and their replication strategies have significant implications for disease transmission and control.

Classification of RNA Viruses

• Positive-sense single-stranded RNA (+ssRNA) viruses: Their RNA genome can be directly used by host ribosomes to synthesize viral proteins. Examples include Coronaviruses, Rhinoviruses, Poliovirus, Norovirus, Zika virus, West Nile virus, Dengue Fever virus, Yellow Fever virus, and Hepatitis C virus.

• Negative-sense single-stranded RNA (-ssRNA) viruses: Their RNA genome must first be transcribed into mRNA before translation. Examples include Ebola virus and Influenza A virus.

• Double-stranded RNA (dsRNA) viruses: These viruses have a genome composed of two complementary RNA strands. One strand is used directly as mRNA, while the other is transcribed to produce more dsRNA. Example: Rotavirus.

• Retroviruses (+ssRNA): These viruses use reverse transcriptase to convert their RNA genome into DNA, which is then integrated into the host genome. Example: HIV.

Key Properties of RNA Viruses

• Genomic Diversity: RNA viruses can have linear or segmented genomes, and their capsid structures vary widely.

• Enveloped vs. Non-enveloped: Some RNA viruses possess a lipid envelope derived from the host cell membrane, while others are naked (non-enveloped).

• Mutation Rates: RNA viruses, especially those with segmented genomes like influenza, mutate rapidly and can exchange genome segments, leading to antigenic drift and shift.

• Vectors and Zoonosis: Many RNA viruses are zoonotic (transmitted from animals to humans) and use vectors such as mosquitoes for transmission (e.g., Dengue, Zika, West Nile).

Replication Strategies of RNA Viruses

The replication cycle of RNA viruses differs significantly from that of DNA viruses, particularly during the synthesis stage. The following table summarizes the major differences between bacteriophages and animal viruses:

Step	Bacteriophage	Animal Virus
Attachment	Proteins on tails attach to proteins on cell wall	Spikes, capsids, or envelope proteins attach to proteins or glycoproteins on cell membrane
Penetration	Genome is injected or diffuses into cell	Capsid enters cell by direct penetration, fusion, or endocytosis
Uncoating	None	Removal of capsid by cell enzymes
Site of Synthesis	Cytoplasm	RNA viruses: cytoplasm; most DNA viruses: nucleus
Site of Assembly	Cytoplasm	RNA viruses: cytoplasm; most DNA viruses: nucleus
Mechanism of Release	Lysis	Naked virions: exocytosis or lysis; enveloped virions: budding
Nature of Chronic Infection	Lysogeny, always incorporated into host chromosome, may leave host chromosome	Latency, with or without incorporation into host DNA; incorporation is permanent

Examples of Medically Important RNA Viruses

• Poliovirus: A naked, positive-sense ssRNA virus that causes poliomyelitis. Vaccination efforts have dramatically reduced polio cases worldwide.

• Coronavirus (e.g., SARS-CoV-2): An enveloped, positive-sense ssRNA virus responsible for COVID-19. It is characterized by its crown-like appearance due to spike proteins.

• Influenza A Virus: An enveloped, negative-sense ssRNA virus with a segmented genome. It mutates rapidly, necessitating annual vaccination.

• HIV (Human Immunodeficiency Virus): A retrovirus that integrates its genome into host DNA, establishing latency. It targets CD4+ T-cells and macrophages, leading to immune dysfunction and AIDS.

Unique Features of RNA Virus Replication

• Direct Translation: +ssRNA viruses can be directly translated by host ribosomes.

• Reverse Transcription: Retroviruses use reverse transcriptase to convert RNA into DNA, which is then integrated into the host genome. This process is a notable exception to the central dogma of molecular biology. Equation for reverse transcription: $\text{RNA} \xrightarrow{\text{reverse transcriptase}} \text{DNA}$

• Latency: Some animal viruses, such as HIV, can integrate into the host genome and remain dormant for years. This latent state is permanent for proviruses, unlike prophages in bacteria.

Public Health Impact and Control

• Vaccination: Effective vaccines have dramatically reduced the incidence of diseases such as polio and influenza.

• Antiviral Therapy: Antiretroviral therapy (ART) allows HIV-infected individuals to live relatively normal lives, but lifelong treatment is necessary due to latent reservoirs.

• Emerging Diseases: RNA viruses are often responsible for emerging infectious diseases due to their high mutation rates and zoonotic potential.

Summary Table: Types of RNA Viruses

Type	Genome	Key Examples	Replication Feature
+ssRNA	Single-stranded, positive sense	Coronavirus, Poliovirus, Rhinovirus	Genome acts as mRNA
-ssRNA	Single-stranded, negative sense	Influenza A, Ebola	Genome must be transcribed to mRNA
dsRNA	Double-stranded	Rotavirus	One strand used as mRNA, other transcribed
Retrovirus (+ssRNA)	Single-stranded, positive sense	HIV	Reverse transcription to DNA, integration

Additional info: The images included above are directly relevant to the structure, transmission, and replication of RNA viruses, as well as their impact on human health. The tables summarize key differences and classifications for exam preparation.