Viruses, Viroids, and Prions: Key Features and Comparisons

Obligatory intracellular parasites

Viruses that require living host cells to multiply.

Virion

Complete, fully developed viral particle.

Nucleic acid

Viral genome that is either DNA or RNA, never both.

Capsid

Protein coat made of capsomeres (subunits).

Envelope

Lipid, protein, and carbohydrate coating on some viruses.

Spikes

Projections from the outer surface of some viruses.

Helical Viruses

Viruses with a hollow, cylindrical capsid that is helical.

Polyhedral Viruses

Most are an icosahedron (20 triangular facets and 12 corners).

Enveloped Viruses

Viruses that are roughly spherical and have an envelope.

Complex Viruses

Viruses with complicated structures, such as bacteriophages.

Baltimore classification system

Classification based on virus's nucleic acid and how its mRNA is produced.

Group I

Double-stranded DNA viruses.

Group II

Single-stranded DNA viruses.

Group III

Double-stranded RNA viruses.

Group IV

(+) sense single-stranded RNA viruses.

Group V

(-) sense single-stranded RNA viruses.

Group VI

Retroviruses (RNA reverse-transcribing viruses).

Group VII

Pararetroviruses (DNA reverse-transcribing viruses).

Plaques

Clearings on a lawn of bacteria on the surface of agar formed by bacteriophages.

Plaque-forming units (PFU)

A measure of the number of infectious virus particles.

Cytopathic effect (CPE)

Changes or deterioration of cells in a cell culture due to viral infection.

Primary cell line

Cells suspended in a nutrient solution that reproduce to form a monolayer.

Diploid cell lines

Cell lines derived from human embryos maintained for about 100 generations.

Continuous Cell Lines

Cell lines derived from transformed (cancerous) cells that can be maintained indefinitely.

ELISA

A serological test to detect and identify viruses by their reaction with antibodies.

PCR

A method to identify viruses by detecting their nucleic acids.

Lytic Cycle

A mechanism of multiplication where the phage causes lysis and death of the host cell.

Lysogenic Cycle Step

Phage DNA is incorporated in the host DNA.

Phage conversion

A process where the host cell acquires new properties due to the presence of phage DNA.

Specialized transduction

A form of transduction where specific bacterial genes are transferred by a phage.

Attachment

Phage attaches by the tail fibers to the host cell.

Penetration

Phage lysozyme opens the cell wall and the tail sheath contracts to force tail core and DNA into the cell.

Biosynthesis

Production of phage DNA and proteins while host cell protein synthesis is halted.

Maturation/Assembly

Assembly of phage particles.

Release

Phage lysozyme breaks the cell wall.

Lambda Phage (𝜆)

Not all phages complete the lytic cycle; they can enter the lysogenic cycle.

Temperate Phages

Phages that can choose between the lytic and lysogenic cycles.

Lambda Phage

Adsorb and penetrate but not induce expression.

Prophage

The viral genome inserts into bacterial genome and becomes inactive; the cell is not lysed.

Induction

Can occur resulting in activation of lytic cycle, typically triggered by host cell stress.

Lysogenic Cycle

Lysogenic host cells are immune to reinfection by the same phage.

Phage Conversion

Host cell exhibits new properties encoded by the prophage DNA.

Specialized Transduction

Bacterial genes transferred to another bacterium via a phage, changing genetic properties of the recipient bacteria.

Attachment/Adsorption

Binding of virus to specific molecules on the host cell.

Genome Entry/Penetration

Viral genome enters the host cell, occasionally in an endosome.

Uncoating

Viral genome is released from the capsid.

Synthesis and Assembly

Viral components are produced and new viral particles are constructed.

Exit and Transmission/Release

Assembled viruses are released by budding (enveloped viruses) or cell lysis (naked viruses).

Biosynthesis of DNA Viruses

Replicate their DNA in the host nucleus, using host enzymes.

Adenoviridae

Double-stranded DNA, nonenveloped; causes respiratory infections in humans and tumors in animals.

Poxviridae

Double-stranded DNA, enveloped; causes skin lesions including smallpox virus and MPOX virus.

Herpesviridae

Double-stranded DNA, enveloped; includes HHV-1 and HHV-2 causing cold sores, HHV-3 causing chickenpox.

Papovaviricetes

Double-stranded DNA, nonenveloped; includes Alphapapillomavirus causing warts and some species that can transform cells and cause cancer.

Hepadnaviridae

Double-stranded DNA, enveloped; includes Hepatitis B virus which uses reverse transcriptase to make DNA from RNA.

Biosynthesis of RNA Viruses

Virus multiplies in the host cell's cytoplasm using RNA-dependent RNA polymerase.

Coronaviridae

Includes SARS-CoV-2 which causes COVID-19; single-stranded RNA, + strand, enveloped.

Togaviridae

Single-stranded RNA, + strand, enveloped; includes Alphavirus transmitted by arthropods.

Rhabdoviridae

- Strand Single-stranded RNA; includes Lyssavirus causing Rabies.

Picornaviridae

+ Strand Single-stranded RNA, nonenveloped; includes Enterovirus (Poliovirus) and Rhinovirus (Common cold).

Reoviridae

Double-stranded RNA, nonenveloped; includes Reovirus and Rotavirus.

Retroviruses

Single-stranded RNA, produce DNA using reverse transcriptase.

Oncogenic Viruses

Viruses that can cause cancer, such as Adenoviridae and Herpesviridae.

Latent Virus

Remains in asymptomatic host cell for long periods; all herpesviruses are capable of latency.

Persistent Viral Infection

Occurs gradually over a long period and is generally fatal.

Viroids

Short pieces of naked RNA that cause diseases in plants.

Prions

Proteinaceous infectious particles that cause neurodegenerative diseases.

PrPC

Normal cellular prion protein found on cell surfaces.

PrpSC

Scapie protein that accumulates in the brain, forming plaques.