Comprehensive Study Notes: Viruses (Chapter 13) – Structure, Classification, Replication, and Infection

Viruses vs. Bacteria

Structural and Biological Differences

Viruses and bacteria are fundamentally different in their structure, replication, and biological properties. Understanding these differences is essential for microbiology students.

• Bacteria:

  • Living cells considered as living organisms.

  • Cell structure: Possess a cell wall, cell membrane, cytoplasm, and DNA that floats freely inside.

  • Extra parts: Flagella (for movement), Pili (for attachment).

  • Size: Larger than viruses (typically 0.2–10 μm).

  • Reproduction: Independent (binary fission).

  • Antibiotics: Often effective against bacteria.

• Viruses:

  • Not considered living cells; they are particles, not complete cells.

  • Structure: Very simple; consist of genetic material (DNA or RNA) and a protein coat called a capsid. Some have an outer envelope made of fats.

  • Size: Much smaller than bacteria (20–300 nm).

  • Reproduction: Cannot reproduce on their own; must infect a living cell and use the cell’s machinery.

  • Antibiotics: Ineffective against viruses.

Key Point: Bacteria are living cells; viruses are non-living particles that require a host to replicate.

Viral Structure

Capsid and Envelope

The structure of viruses is designed to protect their genetic material and facilitate infection of host cells.

• Capsid:

  • Protein coat surrounding the viral genome.

  • Made of repeating protein subunits called capsomeres.

  • Protects nucleic acid from degradation and helps attach to host cells.

• Envelope:

  • Lipid membrane derived from the host cell during viral release (budding).

  • Contains glycoprotein spikes for host cell attachment.

  • Not all viruses have an envelope; non-enveloped viruses rely on capsid proteins for attachment.

Morphology

• Helical: Rod-shaped or filamentous capsid (e.g., Tobacco mosaic virus).

• Icosahedral: Spherical, 20-sided polyhedral capsid (e.g., Adenovirus).

• Complex: Combination of shapes, often with additional structures like tails (e.g., Bacteriophage).

Glycoprotein Spikes

• Composed of carbohydrate-protein complexes.

• Functions:

  • Attachment to host receptors.

  • May induce hemagglutination (clumping of red blood cells).

Classification of Viruses

Criteria for Classification

Viruses are classified based on several structural and genetic features.

• Type of nucleic acid: DNA or RNA, single-stranded (ss) or double-stranded (ds).

• Capsid symmetry and morphology: Helical, icosahedral, or complex.

• Number of capsomeres: Structural subunits of the capsid.

• Size of the virion.

Not used for viral classification: Biochemical tests (used for bacteria).

Viral Species: Group of viruses with the same genetic material and ecological niche.

Nomenclature: Viruses are not named using binomial nomenclature.

Viral Replication

Steps of Viral Multiplication

Viral replication involves several key steps, which may differ between DNA and RNA viruses.

1. Attachment (Adsorption): Viral glycoprotein spikes bind to specific receptors on the host cell membrane.

2. Penetration: Virus enters the host cell via:

   • Endocytosis (host cell engulfs the virus)

   • Fusion (enveloped viruses merge with host membrane)

   • Injection (bacteriophages inject nucleic acid into bacteria)

3. Uncoating: Viral capsid is removed, releasing nucleic acid into the cytoplasm.

4. Biosynthesis: Host cell machinery synthesizes viral nucleic acids and proteins.

5. Maturation/Assembly: Viral components self-assemble into new virions.

6. Release:

   • Enveloped viruses: Bud from host membrane, acquiring lipid envelope.

   • Non-enveloped viruses: Cause cell lysis.

DNA Virus Replication

• Replicate in the nucleus using host enzymes for transcription and translation.

• Steps: DNA synthesis → transcription → translation → assembly.

• Examples: Herpesviridae, Adenoviridae, Hepadnaviridae.

RNA Virus Replication

• Positive-sense ssRNA (+): Acts directly as mRNA for protein synthesis.

• Negative-sense ssRNA (−): Complementary RNA (+) must first be synthesized for translation.

• Retroviruses: Carry reverse transcriptase to convert RNA → DNA, which is then integrated into the host genome.

• Examples: Picornaviridae (ssRNA +), Rhabdoviridae (ssRNA −), Retroviridae (ssRNA + with reverse transcription).

Bacteriophage Replication

• Lytic cycle: Virus replicates and lyses the host cell.

• Can confer immunity to reinfection by the same phage.

• May allow specialized transduction (transfer of host genes).

• Does not immediately kill host cells in some cases (lysogeny).

Viral Infections

Types of Infection

Viruses can cause different types of infections, each with distinct characteristics and outcomes.

Type	Description	Examples
Acute	Rapid replication, short duration	Influenza, Measles
Persistent	Long-term infection, may be asymptomatic	HIV, Hepatitis B
Latent	Dormant virus can reactivate	Herpes simplex (cold sores), Varicella-zoster (shingles)
Oncogenic	Virus causes host cell transformation and tumors	HPV (cervical cancer), EBV (Burkitt’s lymphoma)

Persistent Infections

• Virus persists in equilibrium with host without immediate disease.

• Slow replication, may eventually cause disease (e.g., HIV infection).

Oncogenic Viruses

• Contain oncogenes or viral genes that can transform host cells.

• Transforming properties include chromosomal rearrangements and tumor formation.

• Examples: HPV, Epstein-Barr Virus.

Special Viral Agents

Viroids

• Infectious RNA molecules without a protein coat.

• Primarily infect plants, causing stunted growth and disease.

• Enter cells via wounds or vectors like insects.

Prions

• Infectious proteins without nucleic acid (PrPSc).

• Cause neurodegenerative diseases like Creutzfeldt-Jakob disease and Bovine spongiform encephalopathy.

Immune Evasion and Variation

Antigenic Variation

Viruses can evade the host immune system through changes in their surface proteins.

• Antigenic drift: Small mutations cause minor changes in viral genome and surface proteins.

• Antigenic shift: Major changes due to genetic reassortment, leading to new viral strains (e.g., influenza).

Laboratory Techniques

Virus Cultivation

• Viruses are cultured in living cells (cell cultures, embryonated eggs, or laboratory animals).

• Methods not suitable: Standard bacterial media (viruses require living cells).

Plaques

• Clear zones on cell cultures where viruses have lysed host cells.

Key Terms

• Plaque: Area of cell destruction due to viral infection in a cell culture.

• Continuous cell line: Cell culture that can be maintained indefinitely.

• Prophage: Bacteriophage DNA integrated into the host genome.

• Viroid: Infectious RNA molecule without a protein coat.

• Prion: Infectious protein causing neurodegenerative disease.

• Transformation (viral): Genetic change in a cell due to uptake of viral DNA.

Comparison Table: Viruses vs. Bacteria

Feature	Virus	Bacteria
Cell structure	Non-cellular	Cellular
Reproduction	Requires host	Independent
Nucleic acid	DNA or RNA, not both	DNA and RNA
Metabolism	Dependent on host	Independent
Antibiotics	Ineffective	Often effective
Size	20–300 nm	0.2–10 μm

Important Equations and Processes

• DNA Virus Replication: $\text{DNA synthesis} \rightarrow \text{transcription} \rightarrow \text{translation} \rightarrow \text{assembly}$

• RNA Virus Replication (Retrovirus): $\text{RNA} \xrightarrow{\text{reverse transcriptase}} \text{DNA} \rightarrow \text{integration into host genome}$

Additional info: These notes expand on the provided study guide and notes, offering definitions, examples, and structured tables for clarity. The content is suitable for college-level microbiology exam preparation, focusing on Chapter 13: Viruses, as outlined in standard microbiology textbooks.