lec 11:Introduction to Virology: Structure, Classification, and Impact of Viruses

Introduction to Virology

Definition and Scope

Virology is the scientific study of viruses, which are acellular infectious agents that require a host cell for replication. Recent viral epidemics, such as those caused by SARS-CoV-2 and Monkeypox virus, highlight the importance of understanding viral biology for public health.

Characteristics of Viruses

Obligate Intracellular Parasitism

• Viruses cannot replicate or perform metabolic processes independently; they must hijack a living host cell's resources, including ATP, enzymes, and ribosomes.

• This dependency distinguishes viruses from cellular life forms.

Acellular Structure

• Viruses are not composed of cells. A single viral particle, or virion, consists of genetic material (DNA or RNA) enclosed in a protein shell called a capsid.

Small Size

• Viruses are ultramicroscopic, typically 20–400 nm in diameter, much smaller than most bacteria and eukaryotic cells.

Diversity of Hosts

• Viruses infect a wide range of organisms, including animals, plants, fungi, bacteria (bacteriophages), and archaea.

• Many viruses are highly specific, infecting only certain cell types within a host species.

Evolution and Mutation

• Viruses mutate rapidly during replication, enabling adaptation to host defenses and the emergence of new strains, including drug-resistant variants.

Virus Structure

Genetic Material (Genome)

The viral genome can be either DNA or RNA, but never both. It may be single-stranded (ss) or double-stranded (ds), and linear, circular, segmented, or non-segmented. The genome encodes the information necessary for viral replication and assembly.

Capsid

The capsid is a protective protein shell made of repeating subunits called capsomeres. It determines the virus's shape and symmetry, which can be:

• Icosahedral (20-sided, spherical-like) – e.g., Adenovirus

• Helical (rod-shaped) – e.g., Tobacco Mosaic Virus

• Complex – e.g., Bacteriophages (head and tail structure)

Envelope (Optional)

Some viruses possess an outer lipid membrane called an envelope, derived from the host cell membrane during viral budding. The envelope often contains viral glycoproteins that facilitate host cell entry.

Glycoprotein Spikes (Optional)

Enveloped viruses may have glycoprotein spikes that protrude from the surface, enabling attachment to specific host cell receptors and mediating entry.

Historical Perspectives in Virology

Early Virus Studies

• Viruses were originally defined as agents that could pass through filters (e.g., Chamberland porcelain ultrafilters) that trapped bacteria.

• The invention of the electron microscope enabled direct visualization of viruses.

Bacteriophages and Plant Viruses

• Tobacco mosaic virus (TMV) was the first virus discovered.

• Bacteriophages were first isolated from sources like human sewage and replicate by injecting their genetic material into bacteria.

Viruses Challenging Definitions

• Giruses (giant viruses) such as Mimivirus and Mamavirus possess large DNA genomes and complex structures.

• Virophages are subviral agents that infect giant viruses, e.g., Sputnik.

Viromes and Their Impact

Human and Aquatic Viromes

• The virome refers to the total collection of viruses present in a particular environment, such as the human body or aquatic ecosystems.

• Human skin viromes may influence health and disease states.

• Metagenomics is used to study virome diversity and function.

Applications of Viruses in Health and Medicine

• Bacteriophage therapy is being explored as a treatment for antibiotic-resistant bacterial infections and biofilms.

• Gene therapy utilizes viral vectors (e.g., retroviruses, adenoviruses) to deliver functional genes to patients with genetic disorders.

Virus Architecture and Nomenclature

Virion vs. Virus

• A virion is the fully assembled, infectious form of a virus particle outside a host cell.

• The term virus can refer to the virion, the virus inside a host cell, or the viral genetic material integrated into host DNA.

Function of a Virion

• Infection: The virion is responsible for infecting host cells.

• Replication: Once inside, the virion disassembles, releasing its genome to direct the host cell to produce new virions.

Structure and Components of a Virion

• Nucleic acid: DNA or RNA, single- or double-stranded, linear or circular.

• Capsid: Protein shell made of capsomeres, providing protection and determining shape.

• Envelope (if present): Lipid membrane with embedded glycoproteins.

Capsid Functions

• Protection: Shields the genome from environmental hazards.

• Infection: Facilitates genome delivery into host cells.

• Stability: Maintains the integrity of the virion outside the host.

Viral Envelope

• Derived from the host cell membrane during viral exit.

• Contains viral glycoproteins for host cell recognition and entry.

Virus Nomenclature and Classification

Virus Nomenclature

• Viruses are named and classified using a hierarchical taxonomic system governed by the International Committee on Taxonomy of Viruses (ICTV).

• Ranks include Order (-virales), Family (-viridae), Genus (-virus), and Species.

• Binomial species naming is now standard, e.g., Orthoflavivirus zikaense for Zika virus.

Importance of Virus Nomenclature

• Prevents confusion in scientific and medical communication.

• Facilitates epidemiological tracking and public health responses.

• Supports regulatory approval of vaccines and diagnostics.

• Enables international research collaboration.

Baltimore Classification

The Baltimore classification system categorizes viruses based on their type of genetic material (DNA or RNA) and their mechanism of mRNA synthesis. All viruses must produce mRNA to synthesize proteins and replicate. The system divides viruses into seven classes:

• Class I: dsDNA viruses

• Class II: ssDNA viruses

• Class III: dsRNA viruses

• Class IV: (+)ssRNA viruses

• Class V: (–)ssRNA viruses

• Class VI: RNA reverse-transcribing viruses

• Class VII: DNA reverse-transcribing viruses

Summary Table: Virus Structure and Classification

Component	Description	Example
Genome	DNA or RNA, ss or ds, linear/circular	HIV (RNA), Herpesvirus (DNA)
Capsid	Protein shell made of capsomeres	Adenovirus (icosahedral), TMV (helical)
Envelope	Lipid membrane with glycoproteins (optional)	Influenza virus, HIV
Shape	Icosahedral, helical, complex	Bacteriophage (complex), TMV (helical)

Key Terms and Definitions

• Virion: The complete, infectious virus particle outside a host cell.

• Capsid: Protein shell enclosing the viral genome.

• Capsomere: Protein subunit of the capsid.

• Envelope: Lipid membrane derived from the host, present in some viruses.

• Virome: The total collection of viruses in a specific environment.

• Baltimore classification: System grouping viruses by genome type and mRNA synthesis strategy.

Practice Questions

• Which term is used to describe the viral nucleic acid genome enclosed within a protein coat? A) Nucleocapsid B) Protomer C) Envelope D) Capsomer

• Which of the following best describes an icosahedral virus? A. Rod-shaped with a spiral genome B. 20-sided symmetrical structure C. Enveloped with no capsid D. Circular DNA with no protein coat

• Capsomeres are: A. Lipid units of the viral envelope B. Protein subunits of the viral genome C. Structural proteins forming the capsid D. Enzymes used for viral replication

• The envelope of a virus is primarily derived from: A. The host's ribosomes B. The host cell membrane C. Viral DNA D. Bacterial cell wall

• Briefly explain the concept of a virome and its impact on human health.

• True or False? The genome of a virus can be composed of RNA or DNA, but only in double-stranded forms.

• True or False? Viral communities present on the skin may influence healthy and disease states in humans.

• A newly discovered virus has a helical capsid, RNA genome, and an outer lipid layer with spike proteins. Predict its likely mode of entry into host cells and discuss how its architecture might influence vaccine development.