Viral Multiplication and Biosynthesis: Bacteriophages and Animal Viruses

Viral Multiplication Cycles

T-Even (Tequatrovirus) Bacteriophages: The Lytic Cycle

The lytic cycle is a process by which bacteriophages infect and destroy bacterial cells, resulting in the release of new viral particles.

• Attachment: Phage attaches to the host cell via tail fibers.

• Penetration: Phage lysozyme opens the cell wall; the tail sheath contracts to inject phage DNA into the cell (hydropower syringe mechanism).

• Biosynthesis: Phage DNA and proteins are produced; host cell protein synthesis is halted.

• Maturation: Assembly of phage particles occurs.

• Release: Phage lysozyme breaks the cell wall, releasing new phages.

Example: T4 bacteriophage infecting Escherichia coli.

Bacteriophage Lambdavirus (λ): The Lysogenic Cycle

The lysogenic cycle allows phage DNA to integrate into the host genome, remaining dormant until activation.

• Infection: Lambda phage may initiate a lytic or lysogenic cycle.

• Lysogeny: Phage DNA integrates into host cell DNA as a prophage; no lysis occurs.

• Replication: When the host cell replicates, it also replicates prophage DNA.

Example: Lambda phage in E. coli.

Three Potential Outcomes of Lysogeny

• Immunity to Reinfection: Lysogenic host cells are immune to reinfection by the same phage.

• Phage Conversion: Host cell exhibits new properties encoded by prophage DNA (e.g., production of diphtheria toxin by Corynebacterium diphtheriae).

• Specialized Transduction: Specific bacterial genes are transferred to another bacterium via a phage, changing genetic properties of the recipient.

Lytic Cycle	Lysogenic Cycle
Phage DNA does not integrate; host cell lysed	Phage DNA integrates as prophage; host cell survives
Immediate production of new phages	Phage DNA replicated with host DNA
No genetic change in host	Possible new traits (phage conversion)

Multiplication of Animal Viruses

General Steps in Animal Virus Multiplication

Animal viruses follow a series of steps to infect host cells and produce new virions.

• Attachment: Virus attaches to the cell membrane.

• Entry: Via receptor-mediated endocytosis or fusion.

• Uncoating: Viral nucleic acid is separated from the capsid by viral or host enzymes.

• Biosynthesis: Production of nucleic acid and proteins.

• Maturation: Nucleic acid and capsid proteins assemble.

• Release: By budding (enveloped viruses) or rupture (nonenveloped viruses).

Naked vs Enveloped Viruses

Viruses can be classified based on the presence or absence of an envelope, affecting their stability and transmission.

• Naked Capsid Viruses:

  • More resistant to drying, heat, detergents, and acids.

  • Can survive better outside the host.

  • Must lyse host cells to be released.

  • Antibody response can neutralize infection.

• Enveloped Viruses:

  • Sensitive to environmental factors.

  • Do not need to lyse host cells for release (budding).

  • Usually require antibody and cell-mediated immune responses.

  • Cannot survive gastrointestinal tract; need to stay wet during transmission.

Property	Naked Virus	Enveloped Virus
Environmental Resistance	High	Low
Release Mechanism	Cell lysis	Budding
Immune Response	Antibody	Antibody & CMI

Biosynthesis of DNA Viruses

General Mechanism

DNA viruses replicate their DNA in the nucleus of the host cell using host enzymes, while capsid proteins are synthesized in the cytoplasm.

• Capsid proteins migrate into the nucleus for assembly.

Figure: Replication of a DNA-containing animal virus involves attachment, entry, uncoating, biosynthesis, maturation, and release.

Major Families of DNA Viruses

• Adenoviridae: Double-stranded DNA, nonenveloped; causes respiratory infections and tumors in animals.

• Poxviridae: Double-stranded DNA, enveloped; causes skin lesions (e.g., smallpox, vaccinia, MPOX virus). Assembly occurs in the cytoplasm.

• Herpesviridae: Double-stranded DNA, enveloped; includes:

  • Simplexvirus (HHV-1, HHV-2): cold sores

  • Varicellovirus (HHV-3): chickenpox

  • Lymphocryptovirus (HHV-4): infectious mononucleosis

  • Cytomegalovirus (HHV-5)

  • Roseolovirus (HHV-6, HHV-7)

  • Rhadinovirus (HHV-8): Kaposi's sarcoma

• Papovaviridae: Double-stranded DNA, nonenveloped; includes Alphapapillomavirus (causes warts, some species can transform cells and cause cancer).

• Hepadnaviridae: Double-stranded DNA, enveloped; includes hepatitis B virus, uses reverse transcriptase to make DNA from RNA.

Biosynthesis of RNA Viruses

General Mechanism

RNA viruses replicate in the host cell's cytoplasm using RNA-dependent RNA polymerase.

• ssRNA (+) strand: Viral RNA serves as mRNA for protein synthesis.

• ssRNA (−) strand: Viral RNA is transcribed to a + strand to serve as mRNA.

• dsRNA: Double-stranded RNA.

Major Families of RNA Viruses

• Coronaviridae: Includes SARS-CoV-2 (COVID-19) and other mild respiratory viruses; single-stranded RNA, + strand, enveloped; assembly involves ER and Golgi.

• Togaviridae: Single-stranded RNA, + strand, enveloped; includes Alphavirus (eastern equine encephalitis) and Rubivirus (rubella).

• Rhabdoviridae: Single-stranded RNA, − strand; includes Lyssavirus (rabies) and other animal diseases.

• Picornaviridae: Single-stranded RNA, + strand, nonenveloped; includes Enterovirus (poliovirus, coxsackievirus), Rhinovirus (common cold), and hepatitis A virus.

Biosynthesis of RNA Viruses That Use DNA

Retroviruses

Some RNA viruses use reverse transcriptase to produce DNA from their RNA genome, which integrates into the host chromosome as a provirus.

• Provirus is protected from host immune system and antiviral drugs.

• Retroviridae: Includes Lentivirus (HIV) and Oncoviruses.

Equation:

$\text{RNA} \xrightarrow{\text{reverse transcriptase}} \text{DNA} \xrightarrow{\text{integration}} \text{provirus}$

Additional info: The notes cover topics from Chapter 13 - Viruses, Viroids, and Prions, and are highly relevant for college-level microbiology courses.