Characteristics of Viruses

• Genome Composition: DNA or RNA, but never both
• Genome Structure: Can be single-stranded or double-stranded, circular or linear, single molecule or segmented; relatively small
• Capsid: All viruses have a protein coat; some (e.g., animal viruses) also have an envelope
• Intracellular Parasites: All viruses are categorized as such
• Shapes of Viral Particles: Helical, Icosahedral, Complex
• Naked vs. Enveloped Viruses:
  • Naked: No envelope
  • Enveloped: Has outer lipid membrane

Viral Life Cycles

Lytic Cycle of a Bacteriophage/Animal Virus

1. Attachment
2. Penetration (entry)
3. Biosynthesis (makes viral proteins)
4. Maturation (assembles viruses)
5. Release (cell bursts = lysis)

Lysogenic Cycle of a Bacteriophage

1. Attachment
2. Penetration
3. Integration of viral DNA into host (prophage formation)
4. Replication with host DNA
5. Can later enter the lytic cycle

Prophage and Lysogenic Conversion

• Prophage: Viral DNA integrated into host genome; can confer new traits (e.g., toxin production) to the host.

Replication Steps of an Animal Virus

1. Attachment
2. Entry
3. Uncoating
4. Biosynthesis
5. Assembly
6. Release

Mechanisms of Viral Entry

• Naked viruses: Generally enter via endocytosis or direct penetration
• Enveloped viruses: Enter primarily through fusion or endocytosis

Acquiring an Envelope

• Mechanism: Acquired from host cell membrane during the budding stage of release

Types of Viral Infections

1. Acute: Quick illness, cleared (e.g., influenza)
2. Latent: Virus can hide and reactivate (e.g., herpes)
3. Persistent/Chronic: Long-term replication (e.g., HIV)

General Replication Steps for Different Viral Types

• DNA-containing Animal Viruses:
  1. Enters nucleus
  2. Uses host enzymes to make RNA
  3. RNA to viral proteins
  4. DNA copied for new viruses
• Plus Strand RNA Virus: Acts like mRNA, proteins made directly
• Minus Strand RNA Virus: Needs to be copied into +RNA first
• Retroviruses (2 +RNA): Uses reverse transcriptase to convert RNA to DNA, integrates into host

Unique Properties of Retroviruses

• RNA virus that turns RNA into DNA using reverse transcriptase (e.g., HIV)

Examples of Viral Pathogens

• HPV (dsDNA): Causes warts, linked to cervical cancer
• RSV (-ssRNA): Respiratory virus, common in infants
• Polio (+ssRNA): Attacks nervous system
• SARS-CoV-2 (+ssRNA): Causes COVID-19, utilizes spike proteins to attach to ACE2
• Zika (+ssRNA): Spread by mosquitoes, associated with birth defects
• Influenza (-ssRNA segmented): Frequently changes (antigenic shift/drift)
• HIV (2 +ssRNA): Retrovirus, attacks immune cells

Prions and Viroids

• Prion: Misfolded protein that causes brain damage; lacks genetic material
• Viroid: Small, circular RNA that infects plants; lacks protein coat
• Virus: Contains DNA or RNA and a protein coat, capable of infecting various hosts.