Virology Flashcards
Reasons why it is important to understand viruses
They make up most of Earth’s living matter
We carry viral genome as part of our own genome
Can be harmful to host
Can enhance health of hosts
provide tools and model systems for our discovery of the fundamental principles of molecular biology
We can now engineer viruses to deliver gene therapy and kill cancers or deliver vaccines
Characteristics of viruses
Not living, cannot reproduce independently
Obligate intracellular parasite
What is a virion?
A virion or virus particle is the extracellular form of a virus
Role of virion
Form to exist outside the host and to facilitate transmission from one host cell to another
When does replication/repriduction occurs?
The replication of viruses only occurs upon the infection of host cell
What do viruses contain?
Contains nucleic acids-either DNA or RNA
Elements that viruses do NOT contain?
cytoplasmic membrane or organelles
Characteristics of the extracellular state?
Has a protein coat surrounding the nucleic acid
Some have phospholipid bilayer from host cell membrane and viral proteins
Characteristics of intracellular state?
Capsid is removed
Virus exists as nucleic acid
Meaning of the term viruses in Latin
Poison or slimy liquid
Who showed that tobacco mosaic disease (TMD) is transmissible from a diseased plant to a healthy plant?
Chemist Adolf Mayor
Who filtered the sap of a diseased plant through a porcelain filter design to retain bacteria?
Bacteriologist Dimitri Iwanowski
What did Dimitri Iwanowski showed with his experiment?
He showed that the virus is smaller than the bacteria as the virus passed through the filter
Why did Chemist Wendell Stanley earned a Nobel Prize?
He was able to isolate, characterize and crystallize the Tobacco Mosaic Virus (TMV)
What concept did Bacteriologist Sarah Stewart pioneered?
She pioneered the concept that some viruses can lead to cancer
Is there a definitive answer to the origin of viruses?
No, there is no definitive explanation for the origin(s) of viruses
When have viruses arisen ?
Viruses may have arisen either prior to or from cells ~4 billion years ago
What are the possible hypotheses for the rise and evolution of viruses?
- Viruses require host cell meaning that viruses evolved after cells first appeared
- Proof-Polio Virus from Egypt
- Viruses were around during the “RNA world” a period (pre cellular era), free living, in evolution when RNA is hypothesized to have been the sole carrier of genetic information
- Viruses are drivers for evolution-Mechanism to quickly move genes
- Viruses enriched genetic diversity of hosts by facilitating gene transfers
- Especially relevant for prokaryotic cells
- Possible that earliest viruses co-existed with the host cells evolved to kill host cell later
- Viruses enriched genetic diversity of hosts by facilitating gene transfers
- RNA to DNA transition
- Some RNA viruses evolved DNA genomes to protect their genomes from cellular ribonucleases (cellular enzymes that destroy foreign RNA)
- DNA is more stable than RNA
- Use of cellular replication machinery to replicate
Where can we find viruses ?
- Viruses occupy nearly all organisms and are found in virtually every type of habitat (ex in air and ocean)
- They are the most abundant but they take the least amount of space(represent ~5% of the biomass)
- They comprise ~94% of the nucleic-acid-containing particles
How does climate change affect the spread of human pathogenic disease?
- Climate change is increasingly driving new encounters between mammal species, raising the risks of novel disease spread
- 58% of infectious diseases confronted by humanity worldwide have been at some point aggravated by climatic hazards
How do we see virus ?
Viruses are so small that we require electron microscopy as we can not see them with a light microscope
First Giant Virus found and its characteristics?
- First giant virus was discovered in 2003 and named Mimivirus (meaning mimicking microbe)
- Mimivirus was mistaken for a bacterium
- Large capsid of ~400 nm with protein filaments extending ~100 nm from the surface
- Large DNA genome encoding ~1000 genes
- Giant viruses also known as Nucleocytoplasmic Large DNA viruses (NLDV) bc they are often able to replicate in both the host’s cell nucleus and cytoplasm
- Giant viruses have played a significant role in evolution of eukaryotes through gene exchange
what the first virophage discovered and the characteristics of this group?
- Sputnik: First virophage discovered and it only replicates in Mimivirus-infected amoeba cells
- Small DNA viruses that replicates only in cells co-infected with a giant virus
- Use viral replication factory established by Giant virus
- Have a parasitic relationship with the co-infecting giant virus.
- example: Sputnik reduces Mimivirus replication by ~70%
- While they interfered with giant viruses, they improve the survival of the host.
- Example : By preventing cell lysis by the giant virus
What is the crucial role of viruses in ecosystems? How do they do it ?
They help control the bacterial populations and influence the evolution and diversity of their host organisms
They do so with Bacteriophages which are viruses that can infect and kill bacteria
- Bacteriophages can be use in phage therapy to combat infections in humans
Estimate of biologists about viruses in and on our bodies?
380 trillion viruses are living on and inside our body right now 10x more bacteria
Structure of Viruses
- Capsid
- Protein coat surrounding nuclei acid
- Composed of protein subunit called capsomeres
- Nucleocapsid
- Nucleic acid+capsid
- Envelope
- Consist of combination of lipids and proteins-comes from host
Difference between Bacterial viruses and animal Viruses?
-Most Bacterial and plant viruses are naked/non-enveloped
-Most Animal viruses are enveloped
What are the functions of the capsid?
- Protection of the genome
- Delivery of the genome (non enveloped viruses)
- Capsid binds to host cell receptors
- Direct penetration
- Genome alone enters the cell
- Endocytosis
- Host cell endocytoses the entire virus
- Uncoating of the capsid, releasing genome
The general morphology/symmetry of capsids and their characteristics?
- Helical viruses
- Hollow, cylindrical capsid
- Spiral or Rod shaped
- Length determined by length of nucleic acid
- Width determined by size and packaging of capsomeres
- Helical capsids only package ssRNA bc of the rigidity of double-stranded nucleic acids
- Examples: tobacco mosaic virus or Ebola
- Icosahedral viruses
- Many-sided capsid
- Spherical and rigid, its a closed shell with the nucleic acid inside
- Used to package ss or dsRNA or DNA
- Identical triangular faces, edges, vertices
- 5,3 or 2 identical segments
- Proteins of the icosahedral capsid can be arranged in petamers and/or hexamers
- Most efficient arrangements of subunits- requires fewest capsomeres
- Most animal viruses have icosahedral capsids
- Examples: adenovirus, poliovirus, human papillomavirus
- Complex viruses
- Complicated structures
- Capsid symmetry id neither icosahedral or helical
- Bacterial viruses, Bacteriophages are complex viruses
- Capsid is polyhedral and the tail sheath is helical
- Poxviruses
- Overall shape is described as brick-shaped
- Does not contain clearly identifiable capsids but have several coats around the nucleic acid
How is the genome of enveloped virions delivered?
- Attachement of the virus to the host cell membrane
- Virus enters
- Endocytosis
- Membrane fusion
- Uncoating of capsid takes place to release genome
Characteristics of enveloped virions?
- Envelope is acquired during viral replication or release(Budding)
- Envelope proteins and glycoproteins play a role in host recognition
- Envelope provides some protection to the virus from the immune system
- Enveloped viruses are more fragile than naked viruses
- Lipid bilayer of the envelope is sensitive to the changes in teh environment (pH temperature)
- Easily disrupted by physical and chemical agents and ca dry out easily
How is the metastability of virions achieved?
- stable structure: Created by symmetrical arrangement of many identical proteins to provide maximum contact
- Unstable structure: structure is not permanently bonded together and can be taken apart and loosened upon infection
Explain the two experiment that proved that the viral nucleic acid genome is the genetic code
- Hershey-Chase experiment with phage T2
- DNA virus that infects bacteria
- This experiment proves that DNA is the genetic macterial of bacteriophage T2
- Some viruses had radiolabelled proteins done with radioactive sulfur
- some viruses had radiolabelled DNA done with radioactive phosphorus
- The experiment showed that DNA is inherited, meaning that DNA is the genetic material for bacteriophages
- Fraenkel-Conrat’s work with TMV
- RNA virus that infects tobacco leaves
- Experiment based on the creation of hybrid viruses
Composition of viral genomes
- dsDNA or ssDNA
- DsRNA or ssRNA
- Ss genome can be positive-sense or negative-sense
What are the shape of viral genomes?
- Linear
- Circular
- Segmented(several parts)
What do RNA viruses carry and what can they do ?
They encode or carry enzymes that can:
- Reverse-transcribe RNA into DNA using Reverse Transcriptase
- Synthesize RNA from an RNA template using RNA-dependant RNA polymerase
What do all RNA viruses require to replicate their genome and synthetize viral mRNAs?
RdRp (RNA dependent RNA polymerase)
Viruses are classified based on what attributes?
- Nature of nucleic acid in virion
- Symmetry of protein shell
- Presence or absence of phospholipid membrane
- Host range
Which organisation is responsible for the classification and nomenclature of viruses?
ICTV- International Committee on Taxonomy of Viruses
Classical hierachical system of the taxonomy of viruses?
- Order names end in -virales
- Family names end in -viridae
- Genus names end in -virus
Who created the Baltimore classification and what does it do?
David Baltimore created teh Baltimore system and it is used to classify animal viruses
What criteria are used to classify the animal viruses in the Baltimore classification?
- The nucleic acid composition of the viral genome
- The way the viral genome is replicated
- The way the virus makes mRNAs for expression of the viral proteins
What is an important function for cells and viruses?
All cells and viruses need to make messenger RNA to produce their fundamental protein components
