Exam 3 Flashcards
The classification of viruses rest primarily on what 2 properties?
- Physical properties
- Chemical properties
**Physician is also required to know their disease potential and ways in which they can be controlled
Definition of virus
- What is the classic definition of a virus? What does this mean?
- do they have both DNA and RNA? - What is the end goal of virus
- Examples of hosts that virus can replicate in? (6)
- OBLIGATE INTRACELLULAR PARASITE; they are ultra microscopic and possess only one type of nucleic acid (DNA or RNA)
**Obligate intracellular means it must get into the host cell to replicate (e.g bacteriophage - virus whether bacteria)
- End goal is to reproduce
- HUMANS, ANIMALS, insects, fungi, bacteriophage and plants
Replication of virus
- Parent virus replicates in the cell to produce many progeny virus particles.
* Identify 2 infections that result in the production of progeny virus
**What 2 things combine to form progeny virus particles within an infected host cell
- LYTIC INFECTION; when virus enter cell, the cell bursts open (lyses)
- PRODUCTIVE INFECTION; produce large quantities of virus progeny). In the production of progeny virus, all the components of the parent virus (nucleic acid and proteins) must be synthesized in the new viral cell
**Production of VIRAL NUCLEIC ACIDS plus new VIRAL PROTEINS within an host cell combine to form progeny virus particles
Structural features of extracellular virus
3 parts
- Protein shell. What forms this? 2 functions
- Composed of nucleic acid? (Be precise)
- Optional. Not all viruses have this. *what sensitivity does it account for? 2 functions? **what component on the envelope does all these?
- CAPSID; protein shell
- composed of subunits (Capsomers); many capsomeres = capsid.
- 2 functions; i) protect nucleic acid. II) important for attachment of virus to cell surface RECEPTORS - VIRAL GENOME; viral nucleic acid
- RNA or DNA but not both - ENVELOPE; optional membranous (lipid bilayer) outer structure of some viruses that surround capsid.
- accounts for ETHER SENSITIVITY of many enveloped viruses
- PROTEIN SPIKES on envelop are often involved in i) VIRUS ATTACHMENT to cell receptors and II) are IMPORTANT ANTIGENS
Classification of virus into families
- What is classification based on? (Not based on?)
- Give examples of various classification of virus (9)
- Based on MORPHOLOGICAL and PHYSICAL (structure and biochemistry) features not the types of diseases that are produced
- Classifications
A. SIZE; poxvirus is largest, parvovirus is smallest
B. SHAPE of virus; icosahedral, complex, helical (envelop always)
C. NUMBER of capsomeres (adenovirus has 252 capsomeres)
D. TYPE of nucleic acid (DNA or RNA)
E. STRANDEDNESS; single or double
F. SIZE of nucleic acid (size affect viability)
G. SEGMENTATION of nucleic acid
H. Other criteria
I. Viral INFECTION of susceptible cells in tissue culture
Classification of virus
- Size of virus
- which is largest
- which is smallest - Shape; 2 types
- which must always have envelope? Which s envelope optional?
- give 2 examples of complex shape - Numbers of capsomeres;
- which has 252 capsomeres? - Type of nucleic acid
- Examples of DNA vs RNA viruses - Strandedness
- which is mostly double stranded? Single stranded?
- exceptions?
- Size
- pox virus if largest
- parvovirus is smallest - Shape
A. Icosahedral; +/- envelop (most DNA virus)
B. Complex; Pox virus and retrovirus
C. Helical; must always have an envelope for helical animal virus (many RNA virus) - ADENOVIRUS has 252 capsomeres
- DNA or RNA nucleic acid but not both
- DNA; poxviridae > herpes viriridae > adenoviridae > papoviridae > parvo viridae
- RNA; rhamdoviridae, retroviridae, orthomyoviridae, paramyxoviridae, cornoviridae, picornaviridae, togaviridae - Most DNA is double stranded
- Most RNA is single stranded EXCEPT REOVIRUS which is double stranded RNA
Classification of virus (cont’d)
- Size of nuclei acids
- the largest vs the smallest encode how many proteins each? - Segmentation of nucleic acid
- 8 segments vs single genome
- Poxvirus DNA is largest and encodes 300 proteins
- Parvovirus DNA is smallest and encodes 5 proteins
- *small virus = small DNA
- 1 - 10 segments, depends on particular virus. E.g
- influenza has 8 RNA segments
- parainfluenza has only a single genome
Classification of virus (cont’d)
8. Other criteria - Genera, strains (7) A. What part of virus structure is sensitive to ether? Exception? B. Other susceptibility (4) C. 2 viral antigens D. Host cell specificity E. What does CPE show? F. Disease (same of different symptoms?) G. Genetic relatedness?
A. ETHER SENSITIVITY
- enveloped (Lipid bilayer) virus is usually sensitive except for POXVIRUS
B. Heat, chemical, pH sensitivity and detergent susceptibility
C. Antigenicity; 2 viral antigens are;
- capsid proteins
- envelop protein spikes
D. Host cell specificity
- cells in culture or cells within tissues in VIVO are susceptible to infection by certain viruses only; tropism
E. CPE - CYTOPATHIC EFFECT
- type of morphological changes of infected cells
- there are several types of CPE; lysis, slow cell death, enhanced cell proliferation
F. Disease in host organism (limited usefulness for classification, totally different viruses can cause diseases with same symptoms)
G. Genetic relatedness - hybridization, similarity of nucleotide sequences between the nucleic acids of 2 viruses
Classification of virus cont’d
- Other criteria
A. what do you see in a confluence MONOLAYER - stained
B. confluence monolayer - not stained
C. Neutral red staining of confluence monolayer
D. Neutral red staining of single plaque
E. Neutral red staining of multiple plaques
A. what do you see in a confluence MONOLAYER - stained
- normal cells/tissues
- fibroblastic appearance of cells, elongated and spindle shaped
B. confluence monolayer - not stained
- see nothing
C. Neutral red staining of confluence monolayer
- viable/live cells absorb stain and acquire reddish appearance
D. Neutral red staining of single plaque
- PLAQUE FORMATION seen here; virus replicates in one cell - spread to surrounding cells (replicates) - kill so many cells over time - form PLAQUE (trace to first single cell)
- Point where they all meet (blue arrow) - PFU - plaque forming unit
E. Neutral red staining of multiple plaques
• This has multiple plaques
• Not all virus cause plaque formation. ONLY THOSE THAT CAN CAUSE LYTIC INFECTION OF CELLS
Classification of virus cont’d
- Differentiate between constant inhibition and tumor derived cells
- other names
- growth or no growth?
- CONTACT INHIBITION (DENSITY DEPENDNET INHIBITON); contact each other and stop replicating
- Normal cells express contact inhibition - TUMOR/TRANSFORMED; cells contact each other but don’t stop replicating so grow in MULTILAYERS
- cells grow in multilayers or (loose contact inhibition and have altered growth characteristics)
Classification of virus cont’d
- Viral infection of susceptible cells in tissue culture
A. Identify term - # of infectious virus particles added per cell to initial infection
B. What are 4 optimal conditions for infection
- what ions play role? What role?
- what inhibit attachment (4)
- what is term when virus required cells to be actively replicating
- term for cells that permit viral replication
A. MULTIPLICITY OF INFECTION (MOI); number of infectious virus particles added per cell to initiate infection
B. Optimal conditions for infections (4)
1. Ions; calcium and magnesium help virus attract to receptors on cells
- Inhibitors of adsorption (attachment); serum components, proteins, cross reacting antibodies; all use serumless media at time of infection
- Confluent vs non-confluent
- confluent; certain virus that require cells at specific state of division or metabolism
- non confluent; dont require actively diving cell - Permissive vs non permissive; influence lytic or productive infection
- permissive cells; support replication to produce progeny virus
- nonpermissive; lack essential factors for complete viral replication, may be transformed by certain tumor viruses
CPE (cytopathic effects) are due to virus infection
**Identify the 3 types of CPE
- LYSIS (cytolytic); rapid destruction of cells
- termination of host cell protein and nucleic acid synthesis (e.g herpes) - Slow cell death; with continuous release of virus over a period of time (e.g measles)
- Enhanced cell proliferation; with or without virus production
- “transformed” cells; caused by DNA or RNA tumor virus - cells do not die as a result of viral infection (loss of contact inhibition lead to formation of multilayers)
Identify the following infection types
- Small amount of virus produced but doesnt cause destruction of tissue around
- e.g chicken pox - varicella zoster (shingles caused by same virus that caused chicken pox when you are older)
- Persistent infection; Cell maintenance with or without obvious effect on the cell
- Latent infection; virus may no longer be evident yet viral genome is present
Quantitation of viruses and detection of virus infected cells
- Virus assay for infectious virus particles
- 2 virus types
- PFU vs FFU (which is measure of infectious virus) - Virus assay for non infectious and infectious (no distinction)
- what shows how many viral particles
- Quantitation
A. CYTOLYTIC VIRUSES;
- can form plaques under agar (plaques are localized areas of dead cells in a cell monolayer)
- must be live cells because dead cells are not stained by neutral red
- a SINGLE infectious particle can initiate the formation of a single plaque
- one plaque = one infectious virus particle
***PFU (plaque forming units) - measure of infectious virus
1 PFU = 1 infectious virus
B. Virus with TUMOR POTENTIAL (can transform cells)
- FOCI; proliferation of transformed cels in localized areas of a cell monolayer
- FFU (focus forming unit) can be used to quantitate virus
- Virus assay - no distinction between infectious and non infectious
A. Hemagglutination; virus - erythrocytes lattice formation
- positive HA; diffuse coating of RBC on wells of plate
- negative HA; pallet/button at bottom of wells that indicate no viral-erythrocytes interaction and no lattice formation
- show how many viral particles
- Both dead and live virus cause hemagglutination. However not all viruses can cause hemagglutination
**some viruses have proteins on their surface that bind to receptors on specific RBC - this virus protein is called HEMAGGLUTININ
Identify various methods of detecting infected cells
- RBCs attach to surface of infected cells vis viral proteins (hemagglutinin) expressed on cell surfaces
- • Instead of erythrocytes you add antibodies specific to viral protein with a fluorescent dye
• You are looking for fluorescence of infected cells - Amount of virus which kills 50% of animals
- 50% of cell culture show CPE
- HEMADSORPTION
- positive hemadsorption is when RBC attach to surface of infected cells by hemagglutin proteins - Immunofluorescence (If) and other staining procedures
- “If” requires fluorescent Ab which specifically binds to viral proteins on infected cells, irradiate fluorescein with UV light, gives off visible light
- cells stained with antibody-fluorescein look bright green under microscope - Death of animals (LD50); 1 LD50 equals the another of virus that kills 50% of animals
- Infection of cells in cultures, tissue culture infectious dose 50 (TCID50), 50% of cell cultures show CPE at a particular dilution amount of virus e.g 2 of 4 monolayer show CPE
Viral Replication
Give an overview of the major events in the life cycle of infectious viruses (5)
- ATTACHMENT to receptors of susceptible cells
- PENETRATION and UNCOATING of nucleic acid (NA) - transport of nucleic acid to a specific site within the cell
- Intracellular SYNTHESIS; of virion components transcription - translation - lead to viral protein synthesis (translation) and viral NUCLEIC ACID SYNTHESIS
- ASSEMBLY/MATURATION; of virus components (viral proteins plus genomes at specific sites)
- RELEASE; of virions from the cell
Virus replication (General Aspects)
- identify first step
A. where does it occur?
B. what bind to receptors on cell surface? (2)
C. what happens if cell surface lacks receptor?
- ATTACHMENT
A. Virus attach to receptors on outer surface of the cell
B. CAPSID and ENVELOP PROTEINS bind to the receptors on the surface of cells
C. Some cell types may lack receptors and therefore the virus will not attach and will not cause infection
Virus replication (5 general aspects)
- Attachment
- identify/explain 6 factors that influence viral attachment to receptors - MOI
- Cell tropisim
- Ions
- No attachment inhibitors
- Temperature for attachment to occur
- Neutralizing antibodies
- MOI (multiplicity of infection); the number of virus infectious particles used to infect each cell
- the ratio of the quantity of virus particles : to infectious viruses (1:1 -> 2000:1) - Cell tropisim; ability to infect cell (cell must have receptor)
- Presence of appropriate receptors on cell surface - “specificity” between virus structural protein and cell receptor
- If there is no receptor - there will be no virus attachment to cell
* *Receptors determine tissue of CELL TROPISM - Ions (calcium and magnesium)
- proper ion concentration (Ca2+, Mg2+) helps binding of virus to receptors - No attachment inhibitors
- Lack of attachment inhibitors, remove serum proteins from media - Temperature for attachment to occur (4 to 37 degree Celsius)
- attachment occur over wide range of temp - Neutralizing antibodies (neutralize infectious activity)
- Neutralizing antibodies bind to surface proteins of the virus and prevent the virus from binding to the receptor so block attachment
Viral replication (general aspects)
2a. Penetration
- Identify 2 ways in which virus penetrate into the cell
* *What is the end goal of virus
* *Degree range that penetration occurs
- Viropexis (phagocytic engulfement) or endocytosis
- Cell cytoplasmic membrane engulfs virus that is attached to receptor
- Virus is taken up in VACUOLES.
- The pH of endocytosis vacuole is important for UNCOATING - Fusion; second type of penetration
- pertains to ENVELOPED VIRUSES
- cell membrane fuses with viral envelope
- depend upon viral proteins which mediate fusion. Many fusion proteins must be proteolytically cleaved to cause fusion
**Penetration Occurs optimally at 37 degree and minimally or not at all at 4 degree.
** The end goal of the virus is to reproduce - it must introduce its nucleic acid inside the cell so it become dynamic and starts replicating
Viral replication (general aspects)
2b. Uncoating
- describe process
**what 2 places in the cell can virus replication occur
UNCOATING; removal of some or all of capsid proteins - release genome in nucleus or cytoplasm (site of replication depends on virus under consideration)
- Some virus replicate in cytoplasm while some replicate in the nucleus
- Most DNA virus replicate in the nucleus (there is exception)
- Most RNA virus replicate in the cytoplasm
Viral replication (general aspects)
- Synthesis
A. Transcription/translation of “parental” nucleic acid
B. Synthesis of viral nucleic acids (3 places for occurrence)
C. Transcription/Translation of “progeny” nucleic acid
- what is the order of steps?
- what are the products?
- PARENTAL
- occur before synthesis of viral nucleic acids
- transcription and translation of parental nucleic acid (viral genomes) to form EARLY mRNAs and EARLY proteins - Synthesis of viral nucleic acids (occurs in 3 options)
A. Cytoplasm (poliovirus)
B. Nucleus (herpesvirus)
C. Both sites (influenza virus) - PROGENY
- occur after synthesis of viral nucleic acid
- transcription and translation of progeny nucleic acid to form LATE mRNAs and LATE proteins
- translation occur in the cytoplasm but proteins can be transported to the nucleus or to the cell membrane depending on the details of replication of the virus under consideration
**The early and late designations relate primarily to DNA virus replication schemes
• Everything before nucleic acid synthesis is early
• Everything after nucleic acid synthesis is later
Viral replication (general aspects)
- Maturation/Assembly
- what is the goal?
A. What come together to form progeny virus (2)
B. What is budding? What is the requirement to do budding?
- What is last step
- MATURATION/ASSEMBLY of viral components to form intact virions, occurs at specific sites within the cell
A. What come together to form progeny virus?
- Nucleic acid
- Capsid
B. Envelopment - “BUDDING” at cytoplasmic or nuclear membrane depending on virus
- pox virus is more complicated and has de novo synthesis of envelop
- only enveloped virus can do budding
- Virus is released or remain cell associated, depending on virus
VIRUS PROGENY - different possibilities depending on virus family
Identify 3 possibilities
- location?
- 2 reasons you react non-infectious progeny
- 2 types of mutations (what is DI particles?)
- Extracellular or cell associated virus particles
- Infectious vs Non-infectious virus (2 reasons progeny is non-infectious)
A. Empty Capsids; no nucleic acid
B. Improper maturation; no proteolytic cleavage of envelop proteins - Mutants (2 types)
A. Drug resistance; mutation in viral nucleic acid yields i) viral proteins not affected by specific drugs II) nucleic acids not affected by a drug
B. DI particles (drug interfering particles); produced after high MOI of cells.
- DI particles are defective in that they lack portions of their nucleic acid (missing part of genome) - so can’t replicate on their own
- They require help form non defective “helper” non defective infectious virus
- DI particles interfere with replication of infectious virus; they compete for polymerase and may initiate persistent infections
Identify the following concept
- Inclusion bodies (localize areas within infected cells cytoplasm or nucleus that contain viral particles)
- give examples - Granules and vacuoles - may or may not contain viral products
- Destruction or modification of normal cells structures (6)
Morphological and biological CHANGES OF INFECTED CELLS; CPE (cytopathic effect)
- Inclusion bodies
A. Negri - rabies (cytoplasmic)
B. Guarnieri - smallpox (cytoplasmic) - Granules and Vacuoles; may or may not contain viral products
- Destroy/modify structure of normal cell
A. Herpesvirus; breakdown of chromatin (cell DNA accumulate at periphery of nucleus)
B. Polio; dissociation of cell polysomes
C. cell and nuclear fusion syncytia; insert viral proteins into cytoplasmic/nuclear membranes corresponding to site of envelopment
D. increased methylation of tRNA, mRNA and rRNA
E. changing specificity of cell enzymes change energy generating systems
F. Cytoskeletal system (e.g microtubules) transform and dissociate