Acellular Forms: Viruses, Viroids, and Prions

Acellular infectious agents are simpler than the smallest cells and include viruses, viroids, and prions. These agents lack many features of cellular life and rely on host cells for replication and metabolic activities.

Viruses

Viruses are non-cellular entities that infect all forms of life. They are obligate intracellular parasites, meaning they require a host cell to replicate.

• No cytoplasm, organelles, or active membrane

• No metabolism of their own; metabolic processes occur only within a host

• Cannot reproduce independently; reproduction occurs only inside host cells

• Genetic material is much less than that of cellular organisms (can be DNA or RNA, never both)

• Cannot directly respond to environment; only adjust through host mechanisms

Pattern: Viruses take over the host cell's genome and control the host's metabolism to work for the virus.

Viral Replication Process

• Host cell makes viral nucleic acid

• Host cell makes viral proteins (capsid = protein coat)

• Some animal viruses gain an outer membrane (envelope) from the host as the virus exits

Virion: The extracellular (inactive) form of a virus.

Diseases caused by viruses:

• Common cold

• Influenza (flu)

• Herpes

• SARS

• AIDS

Viral Structure

Viruses have a simple structure, typically consisting of genetic material enclosed in a protein coat (capsid). Some viruses also have an outer envelope derived from the host cell membrane.

• Capsid: Made of protein subunits called capsomers; provides protection and recognition sites for host attachment

• Envelope: A phospholipid membrane acquired from the host; not all viruses have envelopes

• Glycoproteins: Used for attachment to host cells

• Matrix proteins: Fill the region between the capsid and envelope

Functions of Capsid and Envelope

• Protection of viral genetic material

• Recognition and attachment to host cells

Envelope Virus vs. Naked Virus

• Naked virus: Lacks an envelope

• Envelope virus: Acquires envelope from host cell during release; envelope is made of phospholipids and proteins, often with glycoprotein spikes

Shapes of Viruses

Viruses are classified by their shape, which is determined by the arrangement of capsomers.

• Helical: Capsomers form a tube around the nucleic acid (e.g., tobacco mosaic virus)

• Polyhedral: Spherical shape, most commonly icosahedral (20-sided)

• Complex: Irregular shapes, such as bacteriophages (with icosahedral heads and helical tails)

Classification of Viruses

• Based on nucleic acid type (DNA or RNA, single or double stranded)

• Presence or absence of envelope

• Shape and size

Family names are derived from important characteristics (e.g., Picornaviridae for small RNA viruses, Hepadnaviridae for DNA viruses).

Nucleic Acids for Viruses

Viral genomes can be:

Prions are infectious protein particles and do not contain nucleic acids.

Viral Replication Terms

• Phage: Virus that infects bacteria

• Lysosome: Enzyme that weakens bacterial cell walls for viral entry

• Lytic replication: Virus causes lysis (breaking open) of host cell

• Plaque: Clear area on bacterial lawn where viruses have lysed cells

• Burst time: Time from attachment to release of new virions

• Burst size: Number of new virions released per lysed cell

• Prophage: Viral DNA integrated into bacterial chromosome

• Provirus: Viral DNA integrated into animal host chromosome

Viral Replication Basic Steps

1. Attachment

2. Entry

3. Synthesis

4. Assembly

5. Release

Forms of Bacterial Replication

• Lytic cycle: Virus replicates and lyses host cell (e.g., T4 bacteriophage)

• Lysogenic cycle: Viral DNA integrates into host genome and replicates with it (e.g., lambda phage)

Lytic Bacteriophage Replication (T4 & E. coli)

1. Attachment: Virus binds to bacterial receptors

2. Entry: Viral DNA injected into host, often with lysozyme

3. Synthesis: Host machinery makes viral components

4. Assembly: Viral parts assembled into new virions

5. Release: Host cell lyses, releasing new virions

Lysogenic Cycle

• Viral DNA integrates into host genome as a prophage

• Prophage genes can be expressed, causing lysogenic conversion (e.g., toxin production in diphtheria, cholera)

• Induction: Prophage may exit host genome and enter lytic cycle

Replication of Animal Viruses

• Attachment: Virus binds to host cell receptors

• Entry: Three mechanisms—direct penetration, membrane fusion, endocytosis

• Assembly: Most DNA viruses assemble in the nucleus; most RNA viruses in the cytosol

• Release: By budding (enveloped viruses) or lysis (naked viruses)

Latency of Animal Viruses

• Latency: Virus remains dormant in host cell, sometimes for years

• Some latent viruses integrate into host chromosome as proviruses (permanent)

Viruses and Cancer

• Neoplasia: Uncontrolled cell division

• Tumor: Mass of neoplastic cells (benign or malignant)

• Metastasis: Spread of cancer cells to other tissues

• Oncogenes: Genes that, when activated, can cause cancer

• Viruses can cause cancer by carrying oncogenes, activating host oncogenes, or interfering with tumor suppressor genes

Culturing Viruses in the Lab

• Viruses are cultured in bacteria, plants, animals, or cell/tissue culture

• Continuous cell lines (e.g., HeLa cells) are used for long-term studies

Viroids and Virusoids

• Viroids: Infect plants; small, circular RNA molecules without capsids

• Virusoids: Infect fungi; RNA viruses without capsids

Prions

• No nucleic acid; infectious protein particles

• Prion protein (PrP) converts normal proteins to abnormal, disease-causing forms

• Resistant to normal sterilization and cooking

Prion Diseases (Transmissible Spongiform Encephalopathies)

• All involve fatal neurological degeneration (spongy brain appearance)

• Examples: Scrapie (sheep), BSE (cows), CJD (humans), Kuru (humans)

• Outbreaks: Great Britain (BSE, 1980s), France (2000s)

• May be linked to diseases like Alzheimer's, Parkinson's, ALS

Additional info: Prions are unique among infectious agents in that they lack nucleic acids and are extremely resistant to standard decontamination procedures. Their mechanism of disease involves the conversion of normal host proteins into the misfolded prion form, leading to neurodegeneration.