Lecture 4 Viruses and other Acellular infectious agents Flashcards
Acellular Agents
viruses, Viroids, Satellites, Prions
Type of acellular agent that is protein and nucleic acid
Viruses
Type of Acellular agent that is only RNA
Viroids
Type of Acellular agent that is only nucleic acids
Satellites
Type of acellular agent that is only proteins
Prions
Viruses are a major cause of _____
disease
Viruses are important members of ____ world
aquatic
Are viruses important in evolution
yes
Viruses are important ____ systems in molecular biology
model
Bacterial viruses are called ______
bacteriophages (phages)
There are ___ archaeal viruses
few
Most viruses are ____ viruses
eukaryotic (plants, animals, protists, and fungi)
Viruses are classified into families based on
genome, structure, life cycle, morphology, genetic relatedness
a complete virus particle is called a ____
virion
General Properties of Viruses
- a complete virus particle
- consists of greater than or equal to 1 molecule of DNA or RNA enclosed in coat of protein
- May have additional layers
- cannot reproduce independent of living cells nor carry out cell division (but can exist extracellularly)
A virion size range is about ______ nm in diameter and most viruses must be viewed with an _____-
10-400 nanometers, electron microscope
All virions contain a _____ which is composed of ______ and a ______
nucleocapsid, which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid)
What are the protein subunits of a virion capsid called
capsomeres
All Virion structure
- Nucleic Acid (DNA or RNA) (Single or double stranded)
- Capsid (Protein coat) (Protein subunits= Capsomeres)
All virions contain a nucleocapsid which is composed of ____ and a _____ but some consist only of a nucleocapsid, others have additional components called ____
Nucleic acid (DNA or RNA) and protein coat (capsid), Envelopes
The enveloped that surrounds the nucleocapsid of viruses usually has viral proteins called ____ inserted into it
spikes
What are the three types of capsid symmetry
- helical
- icosahedral
- complex
The envelope that surrounds some viruses is composed of
combination of lipids and carbohydrates , and proteins
Envelope lipids and carbohydrates are acquired from the ____. while envelope proteins are coded for by _____ and may even project form the envelope surface as ____
host, viral genes, spikes (also called peplomers)
What is the function of envelope spikes (peplomers)
attachement to host cell surface
General Characteristics of Viruses
- Obligatory intracellular parasites
- Contain DNA or RNA
- Contain a protein coat (note that the DNA/RNA and protein coat make up the nucleocapsid present in all viruses)
- Some are enclosed by an envelope
- Some viruses have spikes
- Most viruses infect only specific types of cells in one host
- Host range is determined by specific host attachment sites and cellular factors
Large macromolecular structures which serve as protein coat of virus
Capsids
Functions of virus capsid
Protect viral genetic material and aids in its transfer between host cells
Virus Capsids are made of protein subunits called
protomers or capsomeres
Viruses are classified by ___-
morphology
examples of viruses classified by morphology
- helical viruses
- polyhedral viruses
- Enveloped Viruses
- Complex viruses
Viruses shaped like hollow tubes with protein walls
Helical Capsids
In helical capsids ____ self assemble
protomers
In helical capsids the size of the capsid is a function of ______
nucleic acid
Examples of helical viruses
- Rabies
- Ebola hemorrhagic feber
a _____ is a regular polyhedron with 20 equilateral faces and 12 vertices
icosahedron
An icosahedron is a regular polyhedron with ____ equilateral faces and ___ vertices
20, 12
______ are the most efficient way to enclose a space
Icosahedral capsids
Icosahedral capsids are composed of ____, which are ring or knob-shaped units, each usually 5 or 6 protomers
Capsomers
5 subunit capsomers are called
Pentamers (pentons)
6 subunit capsomers are called
Hexamers (hexons)
Pentamers are usually at the ____ of the icosahedron, whereas hexameters generally form its ____ and ____
vertices, edges and triangular faces
Example of polyhedral virus
- Adenovirus
- Poliovirus
what is the largest animal virus
poxvirus
Viruses that do not fit into the category of helical or icohedral capsids are called
Capsids of complex symmetry
Examples of complex viruses
- Poxvirus
- Large bacteriophages
some Large bacteriophages have ____ symmetry
binal (head resembles icosahedral, tail is helical)
many viruses are bound by an outer, flexible, membranous layer called the ____
envelope
Animal virus envelopes (lipids and carbohydrates) usually arise from
host cell plasma or nuclear membranes
Examples of enveloped viruses
- Influenza virus
- Herpes simplex virus
- rabies
Envelope proteins, which are viral encoded, may project from the envelope surface as ____/____
spikes/peplomers
functions of envelope protein, spikes/peplomers?
- involved in viral attachment to host cell
- used for identification of virus
- may have enzymatic or other activity
- may play role in nucleic acid replication
The influenza virus has spikes that consist of enzyme ________, which functions in the release of mature virions from the host cell
Neuraminidase
The influenza virus has spikes that consist of _____ proteins, so named because they bind virions to red blood cells and cause the cells to clump together- a process called ______
hemagglutinin, hemagglutination
Do virions have a variety of enzymes
yes
Some other virus enzymes are associated with the ____ or ____ but most are within the _____
envelope or capsid, but most are within the capsid
Characteristics of the viral genome
- Diverse nature of genomes
- a virus may have single or double stranded DNA or RNA
- The length of the nucleic acid also varies form virus to virus
- Genomes can be segmented or circular
What is the first step in the life cycle of a virus
attachment (often called absorption)
Steps of animal viruses life cycle
- Attachement: viruses attach to cell membrane
- Penetration/Entry: Endocytosis or fusion
- Uncoating: viral or host enzymes
- Biosynthesis: Production of nucleic acid
- Maturation: Nucleic acid and capsid proteins assemble
- Release: by buddying (enveloped viruses) or rupture (lysis)
HIV binds to what two proteins on human cells
CD4 and CCR5
attachement (adsorption) of virus is ____ specific
receptor
The receptor determines host preference for attachment (adsorption) of virus: may be ______, _______, _____, ____
may be specific tissue, may be more than one host, may be more than one receptor, maybe in lipid rafts providing entry of virus
after attachment to the host cell, the virus’s ____ or the entire ____ enter the cytoplasm
genome or entire nucleocapsid
Viral entry and uncoating varies between ___ or ____ virus
naked or enveloped
What are the 3 methods of viral entry and uncoating
- fusion of the viral envelope with host membrane; nucleocapsid enters
- endocytosis in vesicle; endosome (and endosomal enzymes) aid in viral uncoating
- Injection of nucleic acid
RNA virus must carry in or synthesize the _____ proteins necessary to complete synthesis
proteins
___ proteins are important in the assembly phase of the virus life cycle
late
The ____,___, and ___ of the of a bacteriophage T4 are assembled separately.
baseplate, tail fibers, and head components
for many viruses, so-called “virus factories” are generated. These are large ______ of either complete nucleocapsids or procapsids
paracrystalline clusters
nonenveloped viruses ___ the host cell for release
lyse
during the release of non enveloped viruses viral proteins may attack ____ or ____
peptidoglycan or membrane
Lysozyme function in non enveloped viruses
attacks peptidoglycan in the host’s cell wall
What is the function of holin in non-enveloped viruses
creates holes in plasma membrane, thus enabling virus to move from the cytoplasm to the peptidoglycan
Enveloped viruses are released via
budding
Explain enveloped viruses being released via budding
- viral proteins are placed into host membrane
- nucleocapsid may bind to viral proteins
- envelope derived from host cell membrane, but may be Golgi, ER, or other
- Virus may use host actin tails to propel through host membrane
Special features of Biosynthesis of single stranded DNA
Cellular enzyme transcribes viral DNA in nucleus
Special features of Biosynthesis of double stranded DNA
- Cellular enzyme transcribes viral DNA in nucleus
- Viral enzyme transcribes viral DNA in virion, in cytoplasm
Special features of Biosynthesis ofDNA, reverse transcriptase
- Cellular enzyme transcribes viral DNA in nucleus; reverse transcriptase copies mRNA to make viral DNA
Special features of Biosynthesis of RNA, + strand
Viral RNA functions as a template for synthesis of RNA synthesis of RNA polymerase which copies- strand RNA to make mRNA in cytoplasm
Special features of Biosynthesis of RNA, - strand
Viral enzymes copies viral RNA to make mRNA in cytoplasm
Special features of Biosynthesis of RNA, double-stranded
Viral enzyme copies - strand RNA to make mRNA in cytoplasm
Special features of Biosynthesis of RNA, Reverse transcriptase
Viral enzyme copies viral RNA to make DNA in cytoplasm; DNA moves to nucleus
Parvovirdae is what type of viral nucleic acid
DNA, single-stranded
Herpesviridae, papovavidae, and poxviridae are what type of viral nucleic acid
DNA, Double-stranded
Hepadnaviridae is what type of viral nucleic acid
DNA, reverse transcriptase
Picornaviridae and Togaviridae are what type of viral nucleic acid
RNA, + strand
Rhabdoviridae is what type of viral nucleic acid
RNA, - strand
Reoviridae is what type of viral nucleic acid
RNA, double stranded
Retroviridae is what type of viral nucleic acid
RNA, reverse transcriptase
lack of information on ___ and ____ history makes viral classification difficult
origin and evolutionary
A uniform classification system for virus taxonomy was developed in 1971 by the
Internal committee for Taxonomy of viruses (ICTV)
Most current reports their are _____ viruses, ___ orders, ____ families, ___ subfamilies, and ____ genera
2,000, 6, 87, 19, 349
Virus classification according to internal committee for taxonomy of viruses (ICTV) is based on numerous characteristics such as
- nucleic acid type
- presence or absence of envelope
- Capsid symmetry
- Dimensions of virion and capsid
The alternative virus classification scheme was created by ____ and focuses on
David Baltimore, focuses on viral genome and process used to synthesize viral mRNA
The baltimore classification system is based on ___ life cycles groups based on, which are
7
- double stranded (ds) DNA
- Single stranded (ss) DNA
- dsRNA
- ssRNA (+ or - strand)
- retrovirus
Viral DNA can be integrated into a host cell chromosome as a provirus by _____
viral integrase
What is a provirus
A stage in a retrovirus life cycle when viral DNA is integrated into a host cell chromosome by viral integrase
Bacteriophages can with be _____ or _____
virulent or temperate
Virulent phages only have one reproductive option which is
begin multiplying immediately upon entering its bacterial host, followed by release form the host by lysis
T4 is an example of a _____ phage
virulent
Temperate phages have two reproductive options which are
- they can multiply like a virulent phage and lyse the host cell
- or they can remain within the host without destroying it
The relationship between a temperate phage and its host is called
lysogeny
Lysogenic bacteria reproduce and in most other ways appear to be normal. however, they have two distinct characteristic, which are
- they cannot be reinffected by the same virus, that is they have immunity to superinfection
- as they reproduce, the prophage is replicated and inherited by progeny cells
Many temperate phages integrate their genome into host genome (becoming _____ in a ______) in a relationship called lysogeny
Prophage, lysogenic bacterium
what are the two advantages to lysogeny for virus
- phage remains viable but may not replicate
- multiplicity of infection ensures survival of host cell
under appropriate conditions infected bacteria will lyse and release temperate phage particles. This occurs when conditions in the cell cause the prophage to initiate synthesis of new phage particles , a process called ____
conditions in the cell cause the prophage to initiate synthesis of new phage particles , a process called induction
Temperate phage changes ______ of its host. Phage may express ____ or ___
phenotype, pathogenic toxin or enzyme
most archaeal viruses are ____
temperate
Archaeal viruses can be ___ or __-
lytic or temperate
______ infection results in cell death through lysis
cytocidal
____ infections may last years
persistent
Cytopathic effects (CPEs)
- degenerative changes
- abnormalities
a slow release of virus without cell death is caused by ___ infection
chronic viral infection
activation of host proto-oncogene (human) or insertion of oncogene (other animals) can lead to
transformation into malignant cell
Virus remains in asymptomatic host cell for long period is called ____ infection
latent infection
Examples of latent infection
Cold sores, shingles
in ____ viral infections disease process occurs over a long period; generally is fatal
Chronic viral infection
examples of chronic viral infection
subacute sclerosing panencephalitis (measles virus)
abnormal new cell growth and reproduction due to loss of regulation
neoplasia
reversion of cell to a more primitive or less differentiated state
anaplasia
___ tumors remain in place
benign
Growth or lump of tissue
tumor
Spread of cancerous cells throughout body
Metastasis
carcinogenesis is a complex, ____ process, that often involves ____
multistep process, often involves oncogenes
oncogenes may come from the virus or may be transformed
host proto-oncogenes (involved in normal regulation of cell growth/differentiation)
Human herpesvirus 8 (HHV8) causes what kind of cancer
Several, including Kapok’s sarcoma
Epstein-Barr virus (EBV) causes what kind of cancer
Several, including Burkitt’s lymphoma and nasopharyngeal carcinoma
Hepatitis B virus causes what kind of cancer
Hepatocellular carcinoma
Hepatitis C virus causes what kind of cancer
Liver cancer
Human papillomaviruses (HPV) strains 6, 11, 16, and 18 cause what kind of cancer
Cervical cancer
Human T-cell lymphotropic virus 1 (HTLV-1) causes what kind of cancer
T-Cell leukemia
Possible mechanisms by which viruses cause cancer
- Viral proteins bind host cell tumor suppressor proteins
- Carry oncogene into cell and insert it into host genome
- Altered cell regulation
- Insertion of promotor or enhancer next to cellular oncogene
The cultivation of viruses requires _____ of appropriate living host
inovulation
Bacterial and Archaeal viruses are usually cultivated in ___ or ____ cultures of suitable, young, actively growing bacteria
- broth
- agar
Broth cultures lose ____ as viruses reproduce
turbidity
____ are observed on agar cultures
Plaques
Animal viruses are cultivated using ____ cultures
Tissue (Cell)
Localized area of cellular destruction and lysis that enlarges as the virus replicates (on tissue (cell) culture)
Viral plaques
Microscopic or macroscopic degenerative changes or abnormalities in host cells and tissues
Cytopathic effects (CPEs)
Hosts for plant viruses
- Plant tissue cultures
- Plant protoplast cultures
- Suitable whole plants
Plant viruses in suitable whole plants may cause ____ lesions or ______
localized necrotic lesions or generalized symptoms of infection
How to quantify virus on culture
- direct counting- count viral particles
- Indirect counting by an observation of virus- hemagglutination assay and plaque assays
PFU/ml=
number of plaques/sample dilution
explain plaque assays
- indirect assay that determines virion numbers based on their infectivity.
- dilutions of virus preparation are made and planted on lawn of host cells. Then the plaques are counted. results expressed as plaque-forming units (PFU)
- PFU/ml = number of plaques/sample dilution
what does a high multiplicity of infection mean (MOI)
more phages in an environment than there are host cells
In plaque assays when the multiplicity of infection (MOI) is very low, each plaque in a layer of host cells is assumed
to have arisen from the multiplication of a single virion
The ____ dose is the dilution that contains a concentration (dose) of virions large enough to destroy 50% of the host cells or organisms
lethal dose (LD50)
The _____ dose is the dose that causes 50% of the host organisms to become infected
infectious dose (ID50)
____ are covalently closed, circular ssRNAs, about 250 to 370 nucleotides long
Viroids
Viroids replicate by a host cell enzyme called ________
DNA-dependent RNA polymerase
Viroids cause ____ disease
plant
Do viroids encode gene products
No!!!
some data suggest that viroids cause disease by triggering a eukaryotic response called _____, which normally functions to protect against infection by darn viruses
RNA silencing
Infectious nucleic acids (DNA or RNA)
infectious nucleic acids
Satellite viruses encode their own _____ proteins when helped by a ____ virus
capsid, helper
Satellite RNAs/DNAs do not encode their own ______
capsid proteins
satellites differ from viroids in that they
- may encode one or more gene products
- need a helper virus to replicate and infect host cells
satellites are divided into what three types
Satellite viruses, Satellite RNAs, and satellite DNAs
Satellite viruses encode their own ______, whereas satellite RNAs and DNAs do not
capsid proteins
Human Hepatitis D virus is a _____ and requires ______ as a helper virus for replication
human hepatitis D virus is a satellite and requires Human hepatitis B virus as a helper virus for replication
Proteinaceous infectious particle
Prions
Examples of prions
- Scrapie in sheep
- Bovine spongiform encephalopathy (BSE) or mad cow disease
- Creutzfeldt-Jakob disease (CJD) and variant CJD (vCJD) in humans
- Kuru in humans
priions cause a variety of
degenerative diseases in humans and animals
PrP^c stands for
prion protein in normal form
PrP^Sc stands for
(scrapie-associated) prion protein in the abnormal form
All prion caused diseases have ____ effective treatment, and result in
No effective treatment, and result in progressive degeneration of the brain and eventual death
Interaction of PrP^Sc with PrP^c may cause PrP^c to ____ and ____
crosslink and trigger apoptosis
What distinguishes a chronic viral infection from a latent viral infection?
The infectious virus gradually builds up over a long period of time
