Viruses: Structure, Life Cycle, and Medical Relevance

Viruses: General Characteristics

Definition and Nature of Viruses

Viruses are obligate intracellular parasites that require a host cell for reproduction. They are fundamentally different from cellular life forms, as they lack the machinery for independent metabolism and replication. Instead, viruses use host enzymes, ribosomes, and other cellular materials to synthesize progeny viruses, effectively turning the infected cell into a factory for viral production. The outer part of a virus serves as a delivery system for the viral genome, which is the core infectious component.

• Bacteriophage: A virus that infects bacteria.

• Host Range: Determined by specific interactions between viral and host cell-surface proteins; only certain species or cell types can be infected by a given virus.

Virus Structure

Basic Components

Viruses are composed of genetic material (DNA or RNA) encased in a protein shell called a capsid. Some viruses also possess an outer lipid envelope derived from the host cell membrane.

• Capsid: Protein shell that encases the viral genome.

• Envelope: Lipid membrane surrounding some viruses, acquired from the host cell during viral budding.

• Genome: Can be DNA or RNA, single- or double-stranded, linear or circular, whole or segmented.

Bacteriophage Structure

Bacteriophages have a complex structure with a head (capsid) containing DNA, a tail sheath, tail fibers, and an endplate for attachment to bacterial cells.

Virus Size and Simplicity

Viruses are much smaller and structurally simpler than bacteria. For example, E. coli is about 1 µm (1000 nm) long, while viruses are typically tens to hundreds of nanometers in size.

Types of Viruses

Nonenveloped vs. Enveloped Viruses

• Nonenveloped (Naked) Viruses: Consist only of a capsid and nucleic acid.

• Enveloped Viruses: Possess an additional lipid membrane (envelope) with embedded proteins (spikes). Most disease-causing viruses are enveloped (e.g., HIV, influenza, herpesviruses, coronaviruses).

Host Range and Examples

• Bacteriophages: Infect bacteria (e.g., T4 phage infecting E. coli).

• Plant Viruses: Cause significant agricultural losses (e.g., Tobacco Mosaic Virus).

• Animal Viruses: Infect animals and humans (e.g., measles, influenza, HIV).

Viral Life Cycle

General Steps

The viral life cycle consists of several key steps, reflecting the obligate intracellular nature of viruses:

1. Attachment to and entry into the host cell

2. Replication of viral nucleic acid

3. Translation of viral proteins

4. Assembly of new viral particles

5. Release (export) of progeny viruses

Host Range Determinants

The host range of a virus is determined by the compatibility between viral surface proteins and host cell receptors. For example, HIV infects only human CD4+ T cells due to specific receptor interactions.

Viral Genomes and Replication Strategies

Types of Viral Genomes

• Double-stranded DNA (dsDNA): e.g., herpes simplex virus, smallpox

• Single-stranded DNA (ssDNA): e.g., canine parvovirus

• Double-stranded RNA (dsRNA): e.g., rotavirus

• Single-stranded RNA (ssRNA): e.g., coronavirus, HIV, influenza

Central Dogma and Viral Replication

The central dogma of molecular biology describes the flow of genetic information: DNA → RNA → Protein. Viruses, especially RNA viruses, often require additional steps or enzymes (e.g., RNA-dependent RNA polymerase, reverse transcriptase) to replicate their genomes and produce proteins.

Plus Sense vs. Minus Sense RNA Viruses

• Plus-sense (+) RNA: Genome can serve directly as mRNA for translation.

• Minus-sense (-) RNA: Genome must first be transcribed into complementary (+) RNA by viral RNA-dependent RNA polymerase before translation.

Viral Entry and Uncoating

Mechanisms of Entry

Viruses enter host cells through various mechanisms, including membrane fusion and endocytosis. The site of uncoating (release of the viral genome) depends on the virus type and entry mechanism.

• Membrane Fusion: Common for enveloped viruses (e.g., measles, HIV).

• Endocytosis: Virus is taken up into endosomes, where acidification triggers uncoating (e.g., hepatitis C).

Viral Evolution and Immune Evasion

Genetic Reassortment and Mutation

Viruses, especially those with segmented genomes like influenza, can undergo genetic reassortment, leading to new strains with altered virulence and transmissibility. High mutation rates in RNA viruses also contribute to immune evasion and the need for updated vaccines.

• Antigenic Drift: Gradual accumulation of mutations in viral genes.

• Antigenic Shift: Reassortment of genome segments, producing novel viruses.

Medical Relevance of Viruses

Diseases Caused by Viruses

• Acute infections (e.g., influenza, measles)

• Chronic infections (e.g., hepatitis C)

• Latent infections (e.g., varicella-zoster virus: chickenpox and shingles)

• Oncogenesis (integration of viral DNA can lead to cancer)

Antiviral Drugs and Targets

Antiviral drugs target specific steps in the viral life cycle, such as viral polymerases (e.g., Remdesivir for SARS-CoV-2), proteases (e.g., Paxlovid), or neuraminidase (e.g., Tamiflu for influenza). The effectiveness of these drugs depends on early administration and the specific viral enzymes present.

Viruses as Tools in Medicine

Viruses are used as vectors in gene therapy to deliver therapeutic genes to patients with genetic disorders. However, integration of viral DNA can sometimes lead to unintended consequences, such as oncogenesis.

Summary Table: Virus Types and Examples

Genome Type	Example Virus	Host	Notes
dsDNA	Herpes simplex virus	Humans	Latent infection possible
ssDNA	Canine parvovirus	Dogs	Nonenveloped
dsRNA	Rotavirus	Humans	Segmented genome
+ssRNA	Poliovirus, SARS-CoV-2	Humans	Genome serves as mRNA
-ssRNA	Influenza virus	Humans, animals	Requires RdRP in virion
Retrovirus (+ssRNA)	HIV	Humans	Reverse transcription, integration

*Additional info: This guide expands on the provided slides with definitions, examples, and a summary table for clarity and completeness.*