Viruses

Why do we study viruses?

They are a major cause of human disease and death.

What are viruses?

Tiny infectious particles that can reproduce only by infecting a host cell.

Can viruses reproduce without a host?

No, viruses “commandeer” the host cell & use its resources to reproduce. They cannot replicate unless they are in a host cell using that cell’s machinery.

Capsid

Outer protein coat

Nucleic Acid Genome

Single- or double- stranded DNA or RNA

Envelope

Membrane covering capsid (not all viruses)

Major Structures of Viruses

Capsid, Nucleic acid genome, & envelope.

Who discovered viruses?

Dimitri Ivanovsky

Bacteriophage T4

* Bacteriophage infect T4 infects the bacterium, *Escherichia coli*
* Has a DNA genome+ is non-enveloped

\

Adenovirus

* Infects the human respiratory tract
* Has DNA+ is non-enveloped
* Other non-enveloped viruses

HIV Retrovirus

* Causes AIDS
* Is enveloped
* Is a retrovirus (can reverse transcribe its RNA genome into DNA form).

What are the 3 main hypotheses regarding virus evolution?

1. Devolution or regressive
2. Escapist or progressive
3. Virus first

H1: Devolution or Regressive

Viruses evolved from more complex free-living organisms that lost genetic material as they adapted to a parasite.

H2: Escapist or Progressive

Viruses may stem from pieces of RNA and DNA that escaped from a host cell and gained the ability to move between cells.

H3: Virus First

Viruses existed before all life as self-replicating entities that over time become more organized and more complex.

Filamentous

Long, thin, worm-like shape

Isometric (or Icosahedral)

Spherical-shape

Enveloped

Have membranes that surrounding the capsids

Complex/Head and Tail

Infect bacteria and have a head that is similar to icosahedral viruses and a tail shaped like helical viruses.

DNA Viruses

* Often double-stranded, but can be single-stranded
* Replication takes place in the nucleus
* A few have DNA polymerases and can complete replication in the host cell’s cytoplasm
* Example: smallpox virus

RNA Viruses

* Usually single-stranded, but can be double stranded
* Replication takes place in the cytoplasm
* Mutation happens at very high rate because RNA polymerase does not have proof-reading capabilities
* Examples: Influenza virus, coronaviruses

4 Steps of Virus Infections

1. Attachment
2. Entry
3. Replication & Assembly
4. Egress (Release)

Attachment

* Receptors on the surface of the host cell bind to virus capsid proteins or virus envelope glycoproteins.
* Viruses can attach only to cells that have the right receptor molecules… so it is cell-type specific.

Entry

Viruses may enter eukaryotic cells by endocytosis, or if enveloped, by fusion with the cell’s membrane.

Replication and Assembly: DNA

Transcribe mRNA → Make viral proteins duplicate DNA to make new viral genomes.

Replication and Assembly: RNA

Make complementary RNA if necessary to transcribe mRNA → make viral proteins copy RNA to make new viral genomes.

Replication and Assembly: RNA Retrovirus

Reverse transcribe RNA to make DNA, using *reverse transcriptase* → DNA incorporates into host genome → DNA directs synthesis and assembly of new viruses

Egress (Release)

* May involve lysis and death of the host cell.
* May involve budding, which does not directly kill the host cell.

How are vaccines prepared?

Prepare using: attenuated “live” virus, “killed” virus, & molecular subunits.

What are vaccines used for?

Preventing viral infections; triggering immune protection.

How do vaccines work?

Prime immune system to react when body is exposed to virus.

If the virus is stable and does not mutate frequently…

the vaccine can work for years without update.

If the virus mutates frequently…

vaccine may need a frequent re-design (i.e. annual flu vaccine) or vaccine may be difficult to design at all (i.e. vaccine against HIV).

Animal Viruses & their Diseases May Cause:

* Acute disease (colds, influenza, COVID-19)
* Chronic infection (hepatitis C)
* Oncogenesis (hepatitis B and C, human papillomavirus \[HPV\])

HPV can lead to --- cancer.

cervical

Corona-viridae

A family of enveloped positive-strand RNA viruses, which infect amphibians, birds, and mammals.

SARS-CoV-2 causes…

COVID-19

SARS-CoV-2

…is an enveloped, single stranded RNA virus (believed to have originated in bats).

Proteinaceous Infectious Particles

* Very small
* Contains no nucleic acid
* Cause fatal neurodegenerative diseases (i.e. Mad Cow Disease)

T or F: Prions are not destroyed by cooking.

True

Is prion disease a virus?

They are not viruses because they consist solely of protein, with no enclosed nucleic acid.

Infection & Transmission

Via respiratory droplets, touching infected surfaces, & other unknown ways.

Diagnostic Testing

Molecular Test and Antigen Test

Molecular Test

To detect genetic material (i.e. viral RNA by nasal test i.e. COVID-19)

Antigen Test

To detect protein on the surface of the virus.

Antibody Test

Looks for antibodies to determine if someone has been infected in the past. Not recommended to determine if someone is still infected in the present.

T or F: Antiviral drugs are used to cure viral diseases.

False, they are for controlling, but not curing, viral diseases.

Plant Viruses

Intracellular parasites that infect plants and cause diseases.

They do not have the machinery to replicated without a host organism.

Virus

Horizontal Transmission

Virus typically enters by way of damage plant tissue. May come from pollen, another plant, or insect bites.

Vertical Transmission

Virus is transmitted from the parent plant. May cause hypoplasia or necrosis of the plant or plant tissue.

Bacteriophages

Viruses that infect bacteria.