Virology - Exam 1: Just Tiny Parasites

What is a virus?

Infectious, obligate, intracellular parasite made of genetic material, surrounded by protein coat

What is the "virus first hypothesis" for the origin of viruses?

Viruses originate from pre-cellular RNA replicons

What is the "progressive hypothesis" for the origin of viruses?

Viruses originate from segments of cellular nucleic acid, which acquired the ability to replicate at the host cell's expense

What is the "regressive hypothesis" for the origin of viruses?

Viruses originate from free-living organisms that lost genetic information until they became dependent on host cells

What is the fourth theory on the origin of viruses?

Viruses originated at the same time as host cells and co-evolved with them

What is the most accepted hypothesis on the origin of viruses?

The progressive hypothesis, that they originated as segments of cellular nucleic acid

What are the key properties of living organisms?

They must

Grow,

Reproduce,

Maintain internal homeostasis,

Respond to stimuli,

Carry out metabolic processes, and

Evolve

Are viruses alive?

No, they cannot maintain internal homeostasis and rely on host cells

T/F: Viruses are generally extremely small

True

What is the main way we classify viruses?

By the nature and sequence of its genome

What are other ways we can classify viruses?

Based on the

Symmetry of the capsid, the

Presence of a lipid membrane, the

Size of the particle, and its

Replication characteristics

What are the major structures of a virus?

The

Nucleic acids, the

Capsid, the

Envelope, the

Glycoproteins and Peplomers (spike proteins), and the

Matrix proteins (tegument)

What are the two types of capsid structures?

Icosahedral symmetry, and

Helical symmetry

What are the functions of structural proteins?

To protect the genome by making a protein shell, packaging the DNA, forming an envelope, and exiting the cell.

To deliver the genome by binding to cell receptors, uncoating the genome, fusing with cell membranes, transporting the genome.

What are the steps of the infectious cycle?

1. Attachment to a receptor on host.

2. Entry into the cell.

3. Uncoating of the nucleic acids.

4. Replication of the nucleic acids and synthesis of proteins.

5. Assembly of new viruses

6. Release of new viruses by budding

What does it mean if viral RNA has a positive sense?

It has the same orientation as mRNA, so that it can be read to make proteins immediately

What does it mean if viral RNA has a negative sense?

It has the complementary orientation to mRNA, thus must be copied into a positive sense by an enzyme

Production of ______ is required for the synthesis of viral proteins

Viral mRNA

How do DNA viruses synthesize viral mRNA?

They can subvert host cell transcriptases

What are the two ways that an RNA virus can make mRNA?

1. It can use it's genome directly as mRNA.

2. It can package an RNA-dependent RNA polymerase (RdRp) to make it's own viral mRNA since this enzyme is not present in host cells

(Basically, it brings it's own factory)

Why might we grow viruses?

To

Diagnose viral infections, to

Study in vitro and in vivo, and to

Make vaccines

If a cell has a receptor for a virus, that means it is ______

Susceptible, but this does not imply that it can replicate

A cell that has no receptors for a virus is _______

Resistant, and it may or may not have the mechanisms for replication

A cell that has the mechanisms for replication, regardless of whether it as a receptor for a virus, is ________

Permissive

A cell must be _______ and _______ in order for a virus to infect it

Susceptible and permissive, meaning it has a receptor and facilitates replication

Fun fact: The majority of influenza vaccines are grown in what?

Embryonated chicken eggs

What cell line takes fresh tissue in order to cultivate viruses, but can't be maintained long term due to cell death?

Primary cell lines (Like fresh produce, good but short-lived)

What cell line uses animal tissue, but last longer than primary cell lines when maintained in a lab?

Diploid cell strains (Like frozen food, lasts longer but still goes bad)

What cell line uses cells that divide indefinitely due to them coming from cancerous tumors or being genetically altered?

Immortalized cell lines (like canned food, lasts forever, but not quite the same as fresh)

What cell lines uses pieces of actual tissue outside the body, like a chunk of skin, with several cell types in order to study viruses, but not to make vaccines?

Organoids and explants (like a mini-kitchen, mimicking real cooking but can't be mass produced)

What are the cytopathic effects of a viral infection?

Cell lysis,

Morphologic changes,

Inclusion bodies, and

Syncytia (cell fusion)

T/F: All viruses have a cytopathic effect of some sort

False

What are inclusion bodies composed of?

Viral nucleic acid,

Viral protein, or

Altered cellular material

What are the two ways to describe an inclusion body based on location?

Intracytoplasmic or intranuclear

The fusion of cells induced by viruses are known as ______

Syncytia

The binding of red blood cells to the surface of infected cells is called _______

Hemadsorption

What are the glycoproteins responsible for hemadsorption called?

Hemagglutinins

Hemagglutinins also cause clumping of RBCs upon mixing with free virions, known as _______

Hemagglutination

The process by which a virus infects a host resulting in disease is called _______

Pathogenesis (of the virus)

What are the obligatory steps in viral infection?

Entry and primary replication,

Spreading and immune evasion,

Shedding, and

Clearance

The ability of a pathogen to infect a host and cause disease is called _______

Pathogenicity

The ability of a pathogen to invade and establish replication within a host is called ______

Infectivity (fitness)

The severity of the disease exhibited by the infected host is called _______

Virulence

The infectivity and virulence of a disease determines ______

Pathogenicity

T/F: The majority of infections will cause moderate disease, with fewer causing more serious disease and death

False. The majority of infections are asymptomatic, and the number decreases as severity increases

The number of people who become sick after being exposed refers to _______

Morbidity

The number of people who die from an infection in a certain population refers to _____

Mortality

The total number of deaths from a disease is the ______

Case fatality rate

What factors contribute to a virus's pathogenesis?

Dose,

Route,

Species,

Age,

Immune status, and

Environmental factors

What is the difference between low pathogenic influenza viruses and high pathogenic influenza viruses?

Low path has proteases that can only cleave and enter cells in the respiratory and intestinal organs, while high path has ubiquitous proteases and can infect many organs at once

An aggressive response of the immune system to infection leading to the sudden increase of pro-inflammatory cytokines is called ______

Cytokine storm

Antibodies do not allow viral attachment to a cell. If the glycoprotein of the virus is not a match, what happens?

There is enhanced binding of the virus to the cell (Think FIP)

What is the difference of a viral infection in an immunocompromised patient?

There is less of a cytokine storm and a lower immune response, but this leads to viral persistence instead of recovery, allowing for the virus to adapt

A virus with a short clinical course, rapid elimination, and increased production, leading it to be highly contagious, is characterized as ______

Acute

A virus with a prolonged course, with constant or intermittent shedding, is characterized as ________

persistant

If a persistent virus is first non-productive until specific conditions arise, causing a re-emergence, it is categorized as __________

Latent

If there is a continued presence of the virus after the primary infection, it is categorized as _________

Chronic

If a virus has a long incubation time, and progresses over time, it is categorized as _______

Slow

Give examples of a virus for the following type:

Latent.

Chronic.

Slow.

Herpesvirus is latent,

Feline Covid Virus is chronic, and

Lentiviruses are slow

What are the main routes of infection for a virus?

Skin,

Respiratory tract,

GI tract, or

Urogenital tract

Local viral spread infects what cells?

Neighboring cells

How does the mechanism of lymphatic viral spread work?

Via infected leukocytes

What does viremia mean?

Viruses spreading through the blood

What are the effects of viral infection in the GI tract?

Enterocyte destruction,

Malabsorption, and

Osmotic effects (diarrhea)

Infection of bone marrow leads to what term referring to number of a certain type of cell that you've discussed in clinical pathology?

Leukopenia

I'm struggling to write questions that don't have painfully obvious answers

If you get a virus in the the brain, you die. If you get a virus while pregnant, the fetus dies. If you get a virus that effects the skin, you get diseases associated with the skin.

Some viruses can cause the _______ transformation of cells, due to its interface with growth signals

neoplastic

What genes cause tumor production by viruses?

Oncogenes

What are the two types of genes that viruses mess with to cause neoplastic transformation?

V-onc genes in viruses, and C-onc genes in cells that correspond to the V-onc genes

How do viruses use genes to cause tumor production?

By integrating V-onc genes within a provirus, causing the overexpression of C-onc genes

What is necessary for the virus to bring in order to replicate it's RNA?

RNA-dependent RNA polymerase

What is the only RNA virus that does not replicate in the cytoplasm?

Retroviruses

The constant change in the viral population in the face of selection pressures is ______

Viral evolution

As the host population grows and adapts, there will be selection of viruses that continue to infect them, meaning there will be ________

A constant emergence of new viruses

T/F: Viruses can contribute to the selection of a host population

True

If a host population can't adapt to a deadly virus, they will _____

perish. I feel like that was obvious

What are the four drivers of viral evolution?

1. A large number of progeny.

2. Extremely frequent mutations.

3. Viral quasispecies (lots of closely related species)

4. Survival of the fittest.

An alteration in the nucleotide sequence due to mistakes is a ______

mutation

Why are there more mistakes in viral genomes than our own?

Because there is no proofreading, they just free ballin' it

What is the difference between a synonymous and a non-synonymous mutation?

A non-synonymous leads to a different amino acid being produced

T/F: Mutations happen for a reason, and not by random chance

False

What viruses tend to have the most mutations, and which ones have the least?

RNA viruses have the most mutation, followed by single stranded DNA viruses, then double stranded DNA viruses with the least

T/F: A majority of mutations leads to a change in phenotype

False, typically no change, only a small amount lead to a competitive advantage

A virus population with a dynamic distribution of non-identical but related replicons is called ______

A quasispecies or a mutant cloud

T/F: Viral infections are due to a single species

False, initiated by a population of particles

How does a mutant cloud lead to selectivity?

Because a bunch may enter a host, but only a few come out, where the strongest survive

In the mechanisms of virus evolution, the accumulation of mutations over time refers to _____ and the recombination/assortment between related viruses refers to _____

Antigenic Drift; Shift

______ is the jumping of polymerase from one strand to another halfway through replication, commonly in single stranded RNA viruses

Template switching

The exchange of segments of the genome between related viruses in the same host cell is ______

Viral reassortment

What are the six results of phenotypic expression of genetic change?

1. Host-range mutants.

2. Changes in pathogenicity.

3. Antibody-escape mutants.

4. Conditional-lethal mutant.

5. Defective-interfering mutants.

6. Genetic reactivation.

What are host-range mutants?

Mutants that infect species different from those of the parent virus

Why might host-range mutants pose a problem in nature?

They can cross to a new species and become highly transmissible

How are host-range mutants helpful to virologists?

They can force the virus to adapt to our own cells, making it weaker, thus allowing for vaccine development

T/F: A mutation in a virus will always increase pathogenicity

False, nobody knows what'll happen

What are the three consequences of adaptation that affect pathogenicity in a virus?

1. Improved binding to the receptor allow for increased transmissibility.

2. Changes to receptor preference allow the ability to infect other tissues.

3. Changes in replication kinetics allow for increased replication

What is an antibody-escape mutant?

A virus with a mutation in the epitope that used to allow binding to an antibody

What is a conditional-lethal mutant?

A mutant that can only replicate under defined conditions, such as a certain temperature

What are defective-interfering viral particles?

An incomplete particle lacking some components, often due deletion mutations, and can't replicate by themselves

What advantages do defective-interfering viral particles give the overall infection?

They multiply faster due to the shorter sequence, and can act as a smokescreen to the immune system, allowing the wild-types to cause more damage

What is genetic reactivation?

The production of an infections virus from the reassortment of non-infectious parents