Study Guide: Viruses, Bacteriophages, and Viral Replication

Viruses vs. Eukaryotic and Prokaryotic Cells

Comparison of Fundamental Characteristics

Viruses differ significantly from both eukaryotic and prokaryotic cells in structure, function, and life processes. Understanding these differences is essential for studying microbiology and virology.

• Cellular Nature:

  • Viruses: Not considered true cells; acellular particles.

  • Eukaryotic Cells: True cells with membrane-bound organelles (e.g., animal, plant, fungal cells).

  • Prokaryotic Cells: True cells without membrane-bound organelles (e.g., bacteria, archaea).

• Alive?

  • Viruses: Not considered alive; cannot carry out metabolism or reproduce independently.

  • Eukaryotes/Prokaryotes: Considered living; capable of independent metabolism and reproduction.

• Size:

  • Viruses: 20–300 nm (nanometers), much smaller than cells.

  • Prokaryotes: 0.5–5 μm (micrometers).

  • Eukaryotes: 10–100 μm.

• Structure:

  • Viruses: Composed of genetic material (DNA or RNA), protein capsid, sometimes an envelope with spikes.

  • Cells: Contain cytoplasm, plasma membrane, ribosomes, and (in eukaryotes) organelles.

• Replication:

  • Viruses: Require host cell machinery for replication.

  • Cells: Replicate independently via cell division (mitosis, meiosis, or binary fission).

• Metabolism:

  • Viruses: No metabolism outside host cell.

  • Cells: Exhibit metabolism (energy production, biosynthesis).

• Genome Composition:

  • Viruses: DNA or RNA (single- or double-stranded), never both.

  • Cells: Always double-stranded DNA as genetic material.

Example: Influenza virus (RNA virus) vs. Escherichia coli (prokaryote) vs. human cell (eukaryote).

Structure and Components of Viruses

Main Components and Their Functions

• Genetic Material: DNA or RNA, encodes viral proteins.

• Capsid: Protein shell that protects genetic material; composed of capsomeres.

• Envelope (in some viruses): Lipid membrane derived from host cell; contains viral glycoproteins (spikes).

• Spikes: Surface proteins for attachment to host cells.

Example: HIV has an RNA genome, a capsid, an envelope, and glycoprotein spikes (gp120, gp41).

Antigenic Shift vs. Antigenic Drift

Impact on Influenza Virus Evolution and Outbreaks

• Antigenic Drift: Gradual accumulation of mutations in viral genes encoding surface proteins (e.g., hemagglutinin, neuraminidase). Leads to seasonal flu variation.

• Antigenic Shift: Abrupt, major change due to reassortment of gene segments, often when two different viruses infect the same cell. Can result in pandemics.

Example: 2009 H1N1 influenza pandemic was caused by antigenic shift.

Host Range and Tropism

Significance in Viral Infections

• Host Range: The spectrum of host species a virus can infect.

• Tropism: Specificity for certain cell types or tissues within a host, determined by viral surface proteins and host cell receptors.

Example: Rabies virus infects many mammals (broad host range); HIV infects only human CD4+ T cells (narrow tropism).

Bacteriophage Replication Cycles

Lytic and Lysogenic Cycles

• Lytic Cycle: Phage injects DNA, replicates, assembles new phages, and lyses host cell to release progeny.

• Lysogenic Cycle: Phage DNA integrates into host genome (prophage), replicates with host, can later enter lytic cycle.

Example: Lambda phage can undergo both lytic and lysogenic cycles in E. coli.

Features of Bacteriophage Lytic Replication

• Attachment to host cell

• Penetration of phage DNA

• Biosynthesis of viral components

• Assembly of new phage particles

• Lysis and release of progeny phages

Features of Bacteriophage Lysogenic Replication

• Phage DNA integrates into host chromosome (prophage)

• Prophage is replicated with host DNA

• Phage genes may confer new properties (lysogenic conversion)

• Environmental triggers can induce lytic cycle

Lytic vs. Latent Infections in Animal Viruses

Comparison and Examples

• Lytic (Acute) Infection: Rapid virus production, cell death, and symptom onset (e.g., influenza).

• Latent Infection: Viral genome persists in host without active replication; can reactivate (e.g., herpes simplex virus).

Example: Herpes simplex virus causes latent infections in neurons; influenza virus causes acute lytic infections in respiratory epithelium.

Viruses and Evolution

Role in Driving Evolution

• Viruses promote genetic diversity through gene transfer and mutation.

• Selective pressure from viruses can drive host evolution (e.g., immune system adaptations).

• Innovation via horizontal gene transfer and spreading of new genes.

Phage Conversion in Bacterial Pathogens

Mechanism and Impact

• Phage conversion (lysogenic conversion) occurs when a prophage imparts new traits to a bacterium.

• Can result in toxin production or antibiotic resistance.

Example: Corynebacterium diphtheriae produces diphtheria toxin only when lysogenized by a specific phage.

Growth of Bacteriophages and Animal Viruses in the Laboratory

Methods and Plaque Assay

• Bacteriophages: Grown on bacterial lawns; plaques (clear zones) indicate lysis.

• Animal Viruses: Grown in cell cultures, embryonated eggs, or live animals.

• Plaque Assay: Quantitative method to determine virus concentration by counting plaques.

Antiviral Drugs and Their Modes of Action

Examples and Mechanisms

• Acyclovir: Inhibits viral DNA polymerase; used against herpesviruses.

• Oseltamivir (Tamiflu): Inhibits neuraminidase; used against influenza viruses.

Mode of Action Example: Acyclovir is phosphorylated by viral thymidine kinase, then inhibits viral DNA synthesis.

Feature	Viruses	Prokaryotic Cells	Eukaryotic Cells
Cellular Structure	Acellular	Cellular	Cellular
Alive?	No	Yes	Yes
Genome	DNA or RNA	DNA	DNA
Metabolism	No	Yes	Yes
Replication	Host-dependent	Independent	Independent
Size	20–300 nm	0.5–5 μm	10–100 μm