Viruses: Structure, Replication, and Classification (MIC 205 - Microbiology)

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Viruses: Structure, Replication, and Classification

Introduction to Viruses

Viruses are microscopic, acellular infectious agents that carry one or several pieces of nucleic acid (DNA or RNA) surrounded by a protein coat. Unlike cellular organisms, viruses are obligate intracellular parasites, meaning they require a host cell to replicate and cannot carry out metabolic processes independently.

Definition: A virus is a non-living infectious particle composed of genetic material and a protein coat.
Hosts: Viruses infect bacteria, plants, animals, and archaea.
Obligate Intracellular Parasites: Viruses must enter host cells to reproduce.
Example: Influenza virus infects respiratory tract cells in humans.

Comparing Cells to Viruses

Viruses differ fundamentally from cellular life forms in structure, metabolism, and replication.

Feature	Cells	Viruses
Cellular Structure	Yes (membrane, organelles)	No (acellular)
Genetic Material	DNA (sometimes RNA in some viruses)	DNA or RNA
Metabolism	Yes	No
Replication	Self-replicating	Requires host cell
Response to Stimuli	Yes	No
Growth	Yes	No

Relative Sizes of Viruses

Viruses are much smaller than most cells and bacteria, typically ranging from 20 to 300 nanometers in diameter.

Comparison: Red blood cells (~7,000 nm), bacteria (~1,000 nm), viruses (~20-300 nm).
Visualization: Viruses require electron microscopy for visualization due to their small size.

Structure of Viruses

Viruses consist of a nucleic acid core surrounded by a protein coat called a capsid. Some viruses also possess an envelope derived from host cell membranes.

Capsid: Protein shell that encases the viral genome; composed of subunits called capsomeres.
Envelope: Lipid bilayer derived from host cell membrane, present in some viruses.
Shapes: Helical, icosahedral, complex (e.g., bacteriophage).
Example: Influenza virus has an envelope; adenovirus does not.

The Viral Envelope

Not all viruses possess an envelope. The envelope is composed of a phospholipid bilayer and proteins, often including glycoproteins that facilitate host cell entry.

Function: Helps viruses evade host immune system and aids in attachment to host cells.
Origin: Derived from host cell membrane during viral budding.
Example: HIV and influenza viruses are enveloped; poliovirus is non-enveloped.

Hosts of Viruses

Viruses exhibit host specificity, infecting particular cell types or species.

Host Range: The spectrum of host cells a virus can infect.
Specificity: Determined by viral surface proteins and host cell receptors.
Example: Rabies virus infects neurons in mammals; tobacco mosaic virus infects plant cells.

Genetic Material of Viruses

Viruses display diversity in their genetic material, which can be DNA or RNA, single-stranded or double-stranded, linear or circular.

DNA Viruses: Herpesvirus, adenovirus
RNA Viruses: Influenza virus, HIV
Classification: Based on nucleic acid type and structure
Example: HIV has single-stranded RNA; herpesvirus has double-stranded DNA.

Other Ways to Categorize Viruses

Viruses can be classified by nucleic acid structure, host range, capsid structure, and presence or absence of an envelope.

Nucleic Acid: DNA or RNA, single or double-stranded
Host: Bacteria (bacteriophages), plants, animals
Capsid Shape: Helical, icosahedral, complex
Envelope: Present or absent

Viral Replication Mechanisms

Viruses rely on host cell machinery for replication. Two main mechanisms are lytic and lysogenic cycles.

Lytic Cycle: Virus replicates and lyses host cell, releasing new virions.
Lysogenic Cycle: Viral genome integrates into host DNA, replicates with host cell, may later enter lytic cycle.

Lytic Replication Steps

Attachment
Entry
Synthesis
Assembly
Release

Lysogenic Replication Steps

Attachment
Entry
Integration (prophage formation)
Replication with host
Induction (entry into lytic cycle)
Synthesis, Assembly, Release

Replication of Animal Viruses

Animal viruses follow similar replication steps as bacteriophages, with some differences in entry and uncoating.

Attachment: Virus binds to specific receptors on host cell membrane.
Entry: Via direct penetration, membrane fusion, or endocytosis.
Uncoating: Viral capsid is removed, releasing nucleic acid.
Synthesis: Viral genome is replicated and proteins are synthesized.
Assembly: New virions are assembled.
Release: Virions exit cell by lysis or budding.

Attachment and Entry of Animal Viruses

Attachment is mediated by interactions between viral surface proteins and host cell receptors. Entry mechanisms include:

Direct Penetration: Viral genome enters cell directly.
Membrane Fusion: Viral envelope fuses with host membrane.
Endocytosis: Host cell engulfs virus in a vesicle.

Synthesis of Animal Viruses

Viral genome replication and protein synthesis depend on the type of nucleic acid present.

DNA Viruses: Replicate in nucleus using host enzymes.
RNA Viruses: Replicate in cytoplasm; may require viral RNA polymerase.
Retroviruses: Use reverse transcriptase to convert RNA to DNA.

General Equation for Viral Replication

For lytic cycle burst size:

Comparing Strategies of Animal Viruses

Feature	DNA Viruses	RNA Viruses	Retroviruses
Genome Replication	Host DNA polymerase	Viral RNA polymerase	Reverse transcriptase
Site of Replication	Nucleus	Cytoplasm	Nucleus (after DNA synthesis)
Example	Herpesvirus	Influenza virus	HIV

Assembly and Release of Animal Viruses

Virions are assembled in the host cell and released by lysis (cell death) or budding (enveloped viruses).

Lysis: Host cell bursts, releasing virions.
Budding: Virions acquire envelope from host membrane.

Latent Replication of Animal Viruses

Some animal viruses can remain dormant within host cells, a state known as latency. Latent viruses may integrate their genome into host DNA, becoming a permanent part of the host chromosome.

Provirus: Integrated viral DNA in host genome.
Example: HIV can become latent in T cells.

Bacteriophage vs. Animal Virus Replication

Step	Bacteriophage	Animal Virus
Attachment	Tail fibers bind to cell wall	Surface proteins bind to membrane receptors
Entry	Injection of DNA	Endocytosis, fusion, or direct penetration
Uncoating	Not required	Required
Synthesis	Cytoplasm	Nucleus or cytoplasm
Release	Lysis	Lysis or budding

Example Virus: HIV

Human Immunodeficiency Virus (HIV) is a retrovirus with a single-stranded RNA genome. It infects CD4+ T cells and macrophages.

Attachment: gp120 binds to CD4 receptor.
Entry: Membrane fusion.
Reverse Transcription: RNA genome converted to DNA by reverse transcriptase.
Integration: Viral DNA integrates into host genome.
Release: Budding from host cell membrane.

Example Virus: Influenza

Influenza virus is an enveloped RNA virus that infects respiratory tract cells.

Genome: Segmented single-stranded RNA.
Attachment: Hemagglutinin binds to sialic acid on host cells.
Entry: Endocytosis.
Release: Budding, facilitated by neuraminidase.

Additional info: These notes expand on the original lecture slides by providing definitions, examples, and comparative tables for key concepts in virology relevant to a college-level microbiology course.