Viruses, Viroids, and Prions: Microbiology Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Virus: A virus is a non-cellular infectious agent composed of genetic material (DNA or RNA) surrounded by a protein coat, and sometimes a lipid envelope. Viruses require a host cell to replicate.
Genome: The complete set of genetic material in a virus, which can be either DNA or RNA, single-stranded or double-stranded.
Capsid: The protein shell that encases and protects the viral genome. It is made up of protein subunits called capsomeres.
Host Range: The spectrum of host cells that a virus can infect. Some viruses have a broad host range, while others are highly specific.
Bacteriophages: Viruses that specifically infect bacteria.
Transmission: The mechanism by which viruses spread from one host to another (e.g., airborne, direct contact, vector-borne).
Tissue Tropism: The cells or tissues of a host that a virus can infect, determined by the presence of specific receptors.
Uncoating: The process by which the viral capsid is removed, releasing the viral genome into the host cell.

Host Range: Viruses may have a broad or limited host range depending on their ability to bind to host cell receptors and utilize host machinery.
Viral Reproduction: Viruses reproduce by infecting a host cell and hijacking its machinery to synthesize viral components, which are then assembled into new virions.
Virion vs. Virus: A virion is a complete, infectious viral particle outside a host cell, while "virus" can refer to the infectious agent in general, including its intracellular phase.

Capsomeres: Protein subunits that assemble to form the capsid.
Viral Envelope: A lipid bilayer derived from the host cell membrane, surrounding the capsid in some viruses. It is acquired during viral budding from the host cell.
Spike Proteins: Glycoproteins embedded in the viral envelope that facilitate attachment and entry into host cells.

Viral Species: A group of viruses sharing the same genetic information and ecological niche.
Subspecies: Variants within a viral species, often designated by numbers or letters.

Definition: Viroids are small, circular, single-stranded RNA molecules that infect plants. They lack a protein coat.
Host Range: Viroids typically infect higher plants, causing various plant diseases.

Definition: Prions are infectious proteins that cause neurodegenerative diseases by inducing abnormal folding of normal cellular proteins (PrPC) into a disease-causing form (PrPSc).
Difference between PrPC and PrPSc: PrPC is the normal, non-infectious form; PrPSc is the misfolded, infectious form.
Infection Mechanism: PrPSc converts normal PrPC into the abnormal form, leading to accumulation in nervous tissue.
Cellular Effects: PrPSc forms clumps in nervous tissue, causing cell death and neurodegeneration.
Disease Example: Creutzfeldt-Jakob disease in humans, scrapie in sheep, and mad cow disease (BSE) in cattle.

Helical: Rod-shaped capsids with the viral genome coiled inside.
Polyhedral: Many-sided, often icosahedral (20 faces) in shape.
Icosahedral: A specific type of polyhedral structure with 20 triangular faces.
Enveloped: Viruses with a lipid envelope surrounding the capsid.
Complex: Viruses with complicated structures, such as bacteriophages with icosahedral heads and helical tails.

Purpose: Used to quantify viruses by counting clear zones (plaques) formed by virus-induced cell lysis in a lawn of host cells.
Plaque: A clear area on a cell layer where virus-infected cells have been destroyed.
PFU (Plaque-Forming Unit): A measure of the number of infectious virus particles.
Interpretation: The number of plaques indicates the quantity of infectious virus in a sample.

Basic Steps:
1. Attachment
2. Penetration
3. Biosynthesis
4. Assembly and Release

Description: A viral replication cycle resulting in the destruction of the host cell and release of new virions.
Major Hallmarks: Rapid replication, cell lysis, and release of progeny viruses.
Virulent Virus: A virus that only replicates via the lytic cycle, causing cell death.
Steps:
1. Attachment
2. Penetration
3. Biosynthesis
4. Maturation
5. Release

Definitions:
- Lysogeny: The integration of viral DNA into the host genome without immediate lysis.
- Temperate Virus: A virus capable of both lytic and lysogenic cycles.
- Prophage: Viral DNA integrated into the host genome.
Steps: Attachment, penetration, integration of viral DNA (prophage), replication with host genome, induction, and entry into the lytic cycle.
Cellular Effects: If the host cell becomes aged or deteriorates, the prophage may be induced to enter the lytic cycle.
Latent vs. Persistent Infection: Latent viruses remain dormant within the host; persistent infections involve continuous viral production.

Host Range: Infects epithelial cells, especially in the skin and mucous membranes.
Disease: Causes warts and is associated with cervical and other cancers.

Disease: Causes Acquired Immunodeficiency Syndrome (AIDS).
Target Cells: Infects CD4+ T cells, macrophages, and dendritic cells.
Immune System Impact: Destroys immune cells, leading to immunodeficiency and increased susceptibility to infections.
Structure: Enveloped virus with two copies of single-stranded RNA, reverse transcriptase, integrase, and protease enzymes.
HIV Replication:
- Replication involves reverse transcription of RNA to DNA, integration into the host genome (provirus), and production of new virions.
- Provirus: Integrated viral DNA in the host genome.
- HIV spreads from cell to cell via direct contact and release of virions.

Description: An enveloped, segmented, negative-sense RNA virus causing seasonal flu epidemics.
HA (Hemagglutinin) and NA (Neuraminidase): Surface proteins involved in viral entry (HA) and release (NA) from host cells.
Spanish Flu Pandemic (1918-1919): A global outbreak of Influenza A that caused millions of deaths worldwide due to a highly virulent strain.

Type	Description	Example
Helical	Rod-shaped, genome coiled inside	Tobacco mosaic virus
Polyhedral/Icosahedral	20-sided, symmetrical	Adenovirus
Enveloped	Lipid membrane surrounds capsid	Influenza virus
Complex	Multiple structures (head, tail)	Bacteriophage

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard microbiology curriculum.