virology 1

Characteristics unique to viruses

Smallest infectious particles (18-300 nm); contain either DNA or RNA (never both); lack an independent metabolic system; obligate intracellular parasites that require host cells to replicate

Virus life cycles

Extracellular (transmissive) and intracellular (reproductive)

Viruses vs unicellular organisms: nucleic acid

Viruses have either DNA or RNA, never both; unicellular organisms have both DNA and RNA

Viruses vs unicellular organisms: replication

Viruses are obligate intracellular and do not undergo binary fission; unicellular organisms divide by binary fission

Viruses vs unicellular organisms: cell machinery

Viruses lack ribosomes and metabolism; bacteria, rickettsiae, mycoplasmas, and chlamydiae have both

Viruses vs unicellular organisms: growth on non-living media

Viruses cannot grow on non-living media; bacteria and mycoplasmas can, rickettsiae and chlamydiae cannot

Viruses vs unicellular organisms: antibiotics

Viruses are not sensitive to antibiotics (exception: poxvirus is susceptible to rifampicin)

Viruses vs unicellular organisms: infectious nucleic acid

Viral nucleic acid can be infectious by itself (some DNA and RNA viruses); unicellular organisms' nucleic acid is not

Unicellular organisms

Protozoa, fungi, bacteria, rickettsiae, mycoplasmas, and chlamydiae

Subviral particles (unconventional viruses)

Extremely simple replicating agents made of either nucleic acid or protein: viroids, virusoids, and prions

Viroids

Plant pathogens; <400 bp RNA, no protein encoded; replicate in the nucleus

Virusoids

Plant pathogens similar to viroids; replicate in the cytoplasm

Viroid-like virus in humans

Hepatitis D virus

Prions

Proteins only (no nucleic acid genome); small proteinaceous infectious particles that cause spongiform encephalitides

Prion resistance

Highly resistant to heat, UV, irradiation, and chemicals

PrPc

Normal cellular prion protein

PrPsc

Abnormal, misfolded (scrapie) form of the prion protein that causes prion disease

Prions vs viruses: nucleic acid

Viruses have nucleic acid; prions do not

Prions vs viruses: morphology

Viruses have a defined EM morphology; prions do not

Prions vs viruses: disinfection

Viruses can be disinfected; prions cannot (highly resistant)

Prions vs viruses: host response

Viruses cause an immune response, IFN production, and inflammatory response; prions cause none of these

Prions vs viruses: incubation

Viruses have variable disease incubation; prions have long incubation

Prions vs viruses: shared features

Both are filterable infectious agents and both contain protein

Virus classification

Based on comparing morphology and multiplication cycle; provides a common basis for naming and a common clinical approach

ICTV

International Committee on Taxonomy of Viruses; classifies viruses into orders, families (subfamilies), genera (subgenera), and species

Virus nomenclature endings

Order: -virales; Family: -viridae; Subfamily: -virinae; Genus: -virus

Naming viruses

Named for virus characteristics, family members, site first isolated, disease caused, or place first isolated

Picornavirus name

"Pico" (small) + "rna" (ribonucleic acid)

Togavirus name

"Toga" (Greek for mantle), referring to the surrounding membrane

Retrovirus name

"Retro" (reverse), referring to reverse transcription

Reovirus name

Respiratory, enteric, orphan

Examples of viruses named for places

Norwalk virus (Norwalk, Ohio) and Coxsackie virus (Coxsackie, NY)

Building blocks of non-enveloped viruses

Protein subunit, structural unit, capsomer, capsid (coat or shell), and nucleocapsid

Capsomer types

Penton and hexon

Enveloped virus building blocks

Same as non-enveloped viruses plus a viral membrane (envelope) and spike glycoproteins (peplomers)

Spike (peplomer) glycoprotein functions

Host specificity, tissue tropism, and fusion with the cell membrane (infection)

Enveloped virus epidemiology

Short survival in the environment, labile lipids, easier to inactivate, often seasonal diseases

Enveloped virus pathogenesis

Bud through infected cells; chronic/persistent infections

Enveloped virus immunology

Glycoprotein antigens (virus neutralizing antibodies and cell-mediated immunity)

Non-enveloped (naked) virus structure

Only a nucleocapsid protein, no envelope

Non-enveloped virus epidemiology

More resistant, longer survival in environment, not seasonal (year-long disease), difficult to inactivate

Non-enveloped virus pathogenesis

Lytic cell infection, often associated with acute disease

Virion size

Varies from ~20 nm (Parvoviridae, Picornaviridae) to 450 x 260 nm (Poxviridae) to >1,000 nm long (Ebola); enveloped viruses are generally larger than non-enveloped

Icosahedral (cubical) symmetry

Efficient package with 12 vertices, 30 edges, and 20 triangles

Helical symmetry

All animal helical viruses are enveloped

Chemical components of viruses

Nucleic acid, proteins, carbohydrates (glycoproteins), and lipids

Chemical components of non-enveloped viruses

Nucleic acid and proteins (capsomers)

Chemical components of enveloped viruses

Nucleic acid, proteins, glycoproteins (carbohydrates), and lipids

Viral genomes

DNA or RNA, never both; single or double stranded; monopartite or multipartite

Viral ploidy

Retroviruses are diploid (2n); all other viruses are haploid (1n)

DNA vs RNA

DNA: deoxyribose, thymidine, usually double stranded; RNA: ribose, uridine, usually single stranded

Structural viral proteins

Virion proteins: capsid/nucleocapsid (protect genome) and envelope proteins (spike, matrix); includes ligands (viral attachment proteins) for cellular receptors

Nonstructural viral proteins

Enzymes and replication regulatory proteins

Polymerases (transcriptases)

Nonstructural enzymes that make mRNA from dsDNA/dsRNA

Reverse transcriptase

Nonstructural enzyme in retroviruses (makes DNA from RNA)

Viral glycoproteins

Oligosaccharides added to proteins in the ER, moved through the Golgi to the cell membrane, and into the viral membrane by budding; function as ligands, enzymes, and antigens

Viral carbohydrates

Found in glycoproteins, glycolipids, and mucopolysaccharides; most are in viral membranes

Viral lipids

Phospholipids and some cholesterol; make up 20-35% of viral mass; bilayer is of cellular origin with viral proteins and glycoproteins inserted

DNA virus with smallest diameter

Parvoviridae (~20 nm, ss DNA)

DNA virus with complex symmetry

Poxviridae (all other DNA virus families are icosahedral)

Largest RNA virus genome

Coronaviridae (~30 kb)