Viruses and Prions Study Guide

General Characteristics of Viruses

• Acellular

• Obligatory intracellular parasites

  • Require living host cells to multiply

• Contain DNA or RNA

• Contain a protein coat, not plasma membrane

• No ribosomes

• No ATP-generating mechanism

  • Lack genes for many products needed for reproduction, requires exploitation of host-cell genomes to reproduce

What determines host range in a virus?

• The spectrum of host cells a virus can infect

• Most viruses infect only specific types of cells in one host

  • Determined by specific host attachment sites and cellular factors

• Bacteriophages—viruses that infect bacteria

• Size ranges from 20nm to 1000nm in length

Define Enveloped virus

Lipid, protein, and carbohydrate coating on some viruses

Define Naked virus/Non-enveloped virus

Lack an envelope/outer lipid bilayer

• Enclosed with a capsid

Define Spikes

Projections from outer surface—means of attachment, viral recognition

Define Virion

Complete, fully developed viral particle

Define Phage conversion

The host cell exhibits new properties

• Ex: Cotynebacterium diphtheriae, which causes diphtheria, is a pathogen whose disease-producing properties are related to the synthesis or a toxin

Can a virus infect any cell?

No, because they are highly specific

Define Latent vs chronic viral infection

Latent: Sleeping virus

• Virus remains in asymptomatic host cell for long periods

• Viral genome may be integrated into hose genome (HIV) or be separate piece of DNA (VZV)

• May reactivate due to changes in immunity, age, stress, etc.

Chronic:

• Once infected the virus can be detected in tissues and continously thereafter

• Virus becomes inactive (dormant) inside host cells and later reactivates

Define Virusoid

Subviral particles best described as non-self-replicating ssRNAs

• Requires that the cell also be infected with a specific “helper” virus

Define Generalized vs Specialized transduction

Generalized Transduction: Random piece of bacterial chromosomal DNA is transferred by the phage during the lytic cycle.

Specialized Transfuction: At the end of the lysogenic cycle, when the prophage is excised and the bacteriophage enters the lytic cyle

Specialized only occurs in a lysogenic cycle, Generalizes occurs in a lytic cycle.

What are the steps to specialized transduction?

1) Prophage exists in galactose-using host (containing the gal gene)

2) Phage genome exercises, carrying with it the adjacent gal gene from the host (picks up DNA flanking from the entrance of the host cell)

3) Phage matures and cell lyses, releasing phage carrying gal gene

4) Phage infects a cell that cannot utilize galactose (lacking gal gene)

5) Along with the prophage the bacterial gal gene becomes integrated into the new host’s DNA

6) Lysogenic cell can now metabolize galactose

Define Receptor mediated endocytosis

A mechanism for a virus to penetrate into a cell, happens to non-enveloped and enveloped viruses.

Define Membrane fusion

A mechanism for a virus to penetrate into a cell, happens enveloped viruses.

Define RNA dependent RNA polymerase

RNA dependent RNA polymerase requires a template to replicate RNA

Define Reverse transcriptase, integrase, protease (3 enzymes important for Retroviruses)

Reverse transcriptase: Allows for reverse transcription to occur, makes a copy that copies RNA → DNA → dsDNA

Integrase: DNA can walk into the nucleus to become part of the host genome

Protease: Needed to modify proteins that are unique to the virus for it to mature.

Define Oncovirus

A type of retrovirus, virus capable of causing cancer

Define Antigenic drift

Gradual accumulation of small genetic mutations in a virus

Define Antigenic shift

Abrupt, major change in influenza A virus, creating a new subtype

Describe the following viral component: Capsid or coat

• Protein coat made of capsomeres (subunits)

Describe the following viral component: Nucleic acid

Core genetic material contained in a virus, serves as a blueprint to colonize a host cells machinery for replication

Describe the following viral shapes: Helical, Icosahedral or polyhedral, Complex

• Helical: Hollow, cylindrical capsid

• Polyhedral: Many sides

• Complex: Complicated structures

Describe viral replication of bacteriophage (both the lytic and lysogenic cycles) in detail.

• Include the steps in order and a brief description of what is happening at each step.

• Lysogenic cycle lets the cell live before the virus makes copies and destroys the cell

• Lysogenic cycle the viral DNA becomes part of the host DNA (prophage)

• Induction: When the virus feels threatened and stressed and enters a lytic cycle

How do bacteriophages get nucleotides and amino acids to make their body parts, if they dont have any metabolic enzymes?

From the host cell they infect

Some bacteria produce toxins and can cause disease only when it is lysogenic. What does this mean?

They use phage conversion by creating new properties.

ssDNA (single stranded DNA) Replication

dsRNA (Double stranded RNA) Replication

Positive single strand RNA (+ssRNA) Replication

• Can work the same way mRNA does, and immediately use mRNA to make proteins, makes complementary strand

• Can either come with its own RdRp or make its own, meaning it doesn’t not need to bring the enzyme with it

Negative single strand RNA (-ssRNA) Replication

• Complementary to mRNA, would have to convert the -ssRNA to a + strand to make protein

• Has to travel with its own RNA-dependent RNA polymerase to make a complementary + strand

  

Retrovirus replication

• A chronic virus because the virus becomes a part of the host genome

Why do we need a flu vaccine every year?

Because influenza viruses can go through antigenic shift and antigenic drift

Describe how antiviral drugs are able to inhibit viral replication

Describe latency of animal viruses

Latent virus = sleeping virus

• virus remains in asymptomatic host cell for long

periods

• Viral genome may be integrated into host genome (HIV) or be a separate piece of DNA (VZV)

List the steps followed by typical enveloped, double stranded DNA virus when it encounters its host.

Describe the role of viruses in contributing to certain cancers

• Causes persistent infection

• Inserts genetic material

• Induce chronic inflammation

A scientist discovers that a virus increases cell proliferation without integrating into DNA. Suggest two possible mechanisms.

Define prion

• Proteinaceous infectious particles

• Inherited and transmissible by ingestion, transplant, and surgical instruments

Why is Prion considered infectious?

They are misfoleded proteins that can force normal proteins in the brain to adopt their abnormal, disease-causing shape

How to destroy Prions?