Lecture 2 Flashcards
What are two factors associated with scientific discovery?
1) Availability of technologies
2) The ability to recognize and the courage to accept something new
Who discovered the first ever virus?
Adolf Mayer
What happened in 1879 with the first ever virus?
Extracts from tobacco showing mosaic symptoms were infectious
Unable to culture on Petri dish or observe bacterial growth
Fungus was not involved
What happened in 1882 with the first ever virus?
Initial Conclusion: “soluble, enzyme like contagium”
What happened in 1886 with the first ever virus?
Publication: “An unknown bacterium” –> but actually not a bacteria
What was the first ever virus, and what was the issue?
Tobacco Mosaic Virus, and their was a decline of production in tobacco due to a disease, so what was happening
In 1892 what happened with the first ever virus?
Dmitri Ivanovsky filtered tobacco extract through a bacteria proof filter, but the filtrate was still infections, and he reported to the Russian Academy of Sciences that the “filter must be defective”
In 1898 what happened with the first ever virus?
Martinus Beijerinick passed the filter, diluted the filtrate, inoculated healthy tobacco and then replenished: Ultimately, “Contagium vivum fluidum” (contagious living fluid)
What happened in 1935 with TMV?
Wendell Stanley crystallized TMV and concluded that the virus is protein in nature
What happened in 1936 with TMV?
Bawden and Pirie determined that the TMV particles contained RNA (5%)
What happened in 1939 with TMV?
Helmut Ruska performed the first visualization of TMV with EM
What happened in the 1950s with TMV?
Fraenkel-Conrat group purified viruses from tobacco, separating into capsid protein and RNA, tested several combinations for infectivity
What did the Fraenkel-Conrat group discover about TMV (3)?
CP alone that assembles into virions was non-infectious, CP and RNA was infectious, and RNA was infectious
What was the overall conclusion that the Fraenkel-Conrat group made?
RNA and not protein is the genetic material in TMV
What happened with Loeffler & Frosch?
In 1898, they found that a filterable virus from cattle infected with FMD remained infectious
What are the consequences of FMD?
High fever, reduction in milk and beef, infertility of female cattle, often fatal in calves. Repeated vaccination is required to prevent disease
Describe FMD
FMD is the most contagious and dreaded viral disease of cloven-hoofed animals: Fecal-oral route
Where is the FMD free zone?
North & Central America, Australia, New Zealand, Japan, continental Europe
What was the first virus discovered in human?
Yellow fever, which was a highly fatal endemic in tropical & subtropical countries
What are the natural hosts of yellow fever?
Monkeys and mosquitos
What does yellow fever do to the body?
Damage to liver, which causes jaundice
How did yellow fever become serious?
Introduced to the New world via slaves trades and multiple epidemics in continental USA
What happened in Philadelphia in 1793?
There was a yellow fever epidemic in 1793 which killed off 15% of the population
How was yellow fever suspected to be transmitted?
Transmissible by insects, Dr. Carlos Finlay
How was yellow fever treated?
Yellow fever commission by US congress
Human volunteers recruited for vector transmission studies: 19 people were tested, 8 were infected, and 3 died, including an army doctor
Ultimately how was yellow fever caused?
by a vector borne filterable virus, discovered by Reed and his team, 1901
In regards to phages, what happened in 1915 (Twort)?
Frederick Twort attempted to culture vaccinia; contamination, glassy transformation
In regards to phages, what happened in 1915 (d’Herelle)?
Felix d’Herelle: invisible antagonistic microbe of dysentery (Shigella) among soldiers resulting in plaque formation
In regards to phages, what happened in 1939?
Delbruck and Ellis: One step growth curve
In regards to phages, what happened in 1940?
Luria and Delbruck established a “phage group”
What was d’Herelle’s dream?
To culture all human diseases caused by bacteria
What did Hershey & Chase do in 1952?
DNA as the genetic material
Explain the Hershey & Chase experiment in 1952 with Sulfur?
1) Label phage with 35S
2) Mixed phage with bacteria
3) Blend violently
4) Allowed culture to grow
5) Centrifuge
6) Measure radio-activity (75% of activity was in supernatant)
Explain the Hershey & Chase experiment in 1952 with Phosphate?
1) Label phage with 32P
2) Mixed phage with bacteria
3) Blend violently
4) Allowed culture to grow
5) Centrifuge
6) Measure radio-activity (85% activity was in pellet –> cell)
Explain the conclusions of the Hershey and Chase experiments?
DNA enters the cell, while protein stays in supernatant, so DNA is the genetic material
Explain nucleic acids
RNA or DNA as the genetic material
The majority of viruses discovered so far contain RNA as genomes
There are no viruses that use both RNA and DNA as genetic materials
Explain proteins
Structural and non-structural
Explain lipids
For enveloped viruses only, because naked viruses lack lipids and lipids are derived from cellular lipid bilayer
Explain carbohydrates
In glycoproteins and glycolipids; involved in recognizing cell receptors and attachment to host cells
Explain single stranded RNA
There is + or - ssRNA and the ssRNA can be segmented or non-segment
Explain double stranded RNA
dsRNa is almost always segmented
Explain single stranded DNA
ssDNA is used for smaller genomes and can be linear or circular
Explain double stranded DNA
dsDNA is used for larger viruses
Explain viruses compared to cells
Viruses are much bigger than cells
What are the features of a non-enveloped, aka naked viruses?
Capsid proteins, nucleic acid
What are the features of an enveloped virus?
Nucleic acid, nucleocapsid, envelope with glycoprotein spikes
Explain the various morphologies and dimensions of viruses
1) Rigid rods or flexuous filaments
2) Spherical (or isometric)
3) Irregular and complex morphology
What are some examples or viruses that contain rigid rods or flexous filaments?
TMV (18 x 300 nm)
Filoviridae (Ebola) (80 x 14,000 nm)
Closteroviridae (12 x 2,200)
What are some examples or viruses that contain spherical (or isometric) morphology?
Paroviridae (25 nm)
Picornovaridie (30 nm)
Adenoviridae (80 - 110 nm)
Herpesviridae (120- 300 nm)
Explain T-even bacteriophages
T-even bacteriophages
They have complex morphology, with two distinctive parts:
–> spherical head (containing DNA)
–> tail (helical sheath, tail finer, tail baseplate)
Explain baculoviruses of insects
Two virion forms of baculoviruses (bacilliform):
- -> Occluded virion (OV, survival in environment)
- -> Budded virion (BV, for spread within insect)
What is a virion?
Complete virus particle
What is a capsid?
The protein shell encasing the viral genome
What is the nucleo-capsid?
Nucleic acid + protein, the discrete substructure within the virion of enveloped viruses
Envelope is not part of nucleo-capsid
What is the structural unit?
The basic unit for building capsid/nucleo-capsid
May be a single subunit or multiple subunits
What is a subunit?
A single, folded polypeptide
Referred to individual CP
What is the envelope?
Lipid membrane enclosing the nucleo-capsid (only in enveloped viruses)
What is the structural protein?
Proteins that are part of the virion structure
What is the non-structural protein?
A protein encoded by the virus but not part of the virion; enzymes required for viral replication, movement & infection
What are the two basic types of symmetry of a viral capsid?
Helical and icosahedral
What is nature’s solution to capsid design?
Viruses use multiple copies of much smaller subunits to construct the capsids to contain the genome inside
Viruses use two basic symmetries to build the viral capsid:
Polyhedron or helical structure
Explain helical symmetry
Rod-shaped and filamentous viruses have structures built based on helical symmetry
P(pitch of the helix) = u(# of subunits per helical turn) x p (the axial rise per subunit)
In capsids with helical symmetry, the CP subunits have equivalent bonding relationship to one another, except those at both ends of the virion
What structure does helical symmetry have?
Helical symmetry has an ‘open structure’, with unlimited packing capacity (for insertion of foreign DNA as in viral vectors)
Explain the background of icosahedral symmetry
Architectural design of icosametric viruses is more complex
Watson and Crick first proposed that spherical viruses can be built as cubic structures (wrong)
What did Capser prove?
Basic design of spherical capsids and nucleocapsids were icosahedral symmetry, which means 20 equilateral triangles, and 20 equal faces
What structure does icosahedral symmetry have?
Closed structure, which allows packing of viral genomes with limited sizes: 2 fold, 3 fold, 5 fold rotational symmetry.
Because it is a closed structure, icosahedral symmetry allows for the packaging of only limited genome sizes
What are the three pillars of icosahedral symmetry?
1) Triangulation Number
2) Quasi-equivalence
3) Spontaneous self-assembly
What is triangulation number?
Define the possible icosahedral surface lattice
What is quasi-equivalence?
Describes the nearly (but not) identical bonding relationship among the subunits in a spherical virus
What is spontaneous self-assembly?
It is the self-assembly of individual CP subunits into virus like particles (VLPs) to identify the assembly process
Spontaneously interact with each other
What is the simplest cases of icosahedral symmetry?
The simplest viruses have a capsid built from 60 copies of a single capsid protein, arranged into 20 triangular faces
All subunits in the capsid are in equivalent (identical) bonding relationship
There are three types of rotational symmetry: 2-,3-, and 5- fold symmetry
Explain triangulation # and quasi-equivalence for larger isometric viruses
Most viruses are large, complex, and each virion contains much more than 60 subunits
Triangulation #: # of small facets (triangles) that exist within each of the 20 triangular faces of an icosahedron. Only certain multiples of 60 subunits allowed
Quasi-equivalence: in icosahedrons with T > 1, the relationships between capsid protein subunits are similar but NOT identical
The 20 faces are equilateral triangles, but the facets do not have to be equilateral
A good example is a beta-barrel jelly roll fold. What happens here?
This jellyroll is conserved among viruses with an icosahedral symmetry
Explain how interactions among CP subunits have to occur?
Viral CP subunits range between 20 to 70 kDa, so the # of subunits have to be increased to allow them to exist in a quasi-equivalent position in viruses with large capsids or nucleocapsids
How do CP subunits interact?
CP subunits self-assemble into larger structures, even intact capsid shell, with or without the help of the viral genome:
- -> The CP subunits are stabilized by the maximum # of non-covalent bonds (weak bonds) between them, leading to the lowest free energy state
- -> All subunit-subunit and subunit-RNA bonds are weak; the interaction is mainly hydrophobic and van Der Waals forces
How to calculate triangulation numbers?
Let’s say a triangulation number is 4, you will multiply 4 by 60 to get the 60T value
What is the architecture of a lambda phage?
1) Icosahedral head
2) Helical tail with tail cone (sheath) and attachment fibres)
Each part is assembled individually followed by assembly into an intact virion
