Final: Ch 5: Viruses Flashcards
Viruses
infect EVERY type of cell
seawater can contain 100 million viruses per mL
latin for “poison”
Scientists involved in discovery of viruses
Pasteur - postulated that a living thing smaller than bacteria caused diseases
Ivanovski and Beijerinck - showed that a disease in tobacco was caused by a virus
Loeffler and Frosch - discovered an animal virus causes foot-and-mouth disease in cattle
Filterable virus
early researchers found that when fluids from host organisms passed thru porcelain filters designed to trap bacteria, the filtrate remained infections
result
proved that a cell-free fluid could contain agents that could cause infection
General characteristics of viruses
“active” or “inactive”
obligatory intracellular parasites: require living host cells in order to multiply
not cells: no cell wall, cytoplasm or organelles
very small - submicroscopic (visible only w/electron microscope)
Role of viruses in evolution
infect cells and influence their genetic makeup
shape the way cells, tissues, bacteria, plants and animals have evolved
10% of the human genome consists of sequences that come from viruses
10-20% of bacterial DNA contains viral sequences
Distinctive features of viruses
bear no resemblance to cells
lack any of the protein synthesizing machinery found in cells
structure is composed of regular, repeating subunits that give rise to their crystalline appearance
contain only the parts needed to invade and control a host cell: external coating, core containing one or more nucleic acid strains of RNA or RNA, sometimes 1-2 enzymes
Virus particle: Central core
Nucleic acid core - DNA or RNA, single or double stranded
Matrix proteins
Enzymes (not found in all)
Virus particle: covering
protect nucleic acid from host’s acid and protein digesting enzymes
assist in binding and penetrating host cell
stimulate the host’s immune system
Capsid - protein coat
Envelope (not found in all) - the virus steals some of the host plasma membrane when it leaves the cell
Spikes (some) - some have carbo-protein complexes projecting from the surface
General morphology
Helical
Icosahedral
Complex
enveloped or not
Helical capsid
Naked helical:
nucleocapsid is rigid and tightly wound into a cylinder-shaped package
Enveloped helical:
nucleocapsid is more flexible
Icosahedral capsid
3D, 20 sided w/12 evenly spaced corners
variation in capsomer #
can be naked or enveloped
Complex viruses
structure more complex than helical or icosahedral Pox virus: several layers of lipoproteins causes smallpox, cowpox, chicken pox Bacteriophage: virus that attacks bacteria polyhedral head w/tail fibers looks like space craft
Viral nucleic acid
contain either DNA or RNA nucleic acid can be in different forms: -single stranded (ss) DNA -double stranded (ds) DNA -ssRNA -dsRNA possess only the genes to invade and regulate the metabolic activity of the host cells no viral metabolic genes b/c the virus uses the hosts metabolic resources
Enzymes in the virus particle
enzymes for specific operations within their host cell
- polymerase that synthesize DNA and RNA
- replicases that copy RNA
- reverse transcriptase synthesizes DNA from RNA
- retroviruses carry their own enzymes to create DNA out of RNA
Classifying viruses
look at commonalities in:
- genetic makeup (DNA, RNA, genetic sequence)
- structure (naked, enveloped, helical, icosahedral)
- chemical composition
- host relationship
- type of disease
3 orders
73 families
283 genera
Viral multiplication cycle (viral life cycle)
depends on the type of virus and species infected
animal viruses
retroviruses
bacteriophages
Viral multiplication cycle: Animal Viruses
- adsorption/attachment
- penetration
- uncoating
- synthesis
- assembly
- release
if you can stop just one step, you can stop the multiplication of viruses
- Adsorption/attachment
viral particles (virion) attaches to specific receptor on outside of host cell
surface viral particles bind to specific membrane proteins on the host cell membrane
this is how rhinovirus identifies and targets the nose while HIV binds to immune cells
- Penetration
virion penetrates the host cell thru the cell membrane and/or cell wall to enter cytoplasm
occurs by endocytosis or fusion btwn the viral envelope and the host cell membrane
- Uncoating
removal of the capsid and envelope, if present, exposes the nucleic acid of the virus
- Synthesis
production of the virion parts (nucleic acid, capsid, spikes)
synthesis of the viral nucleic acid and proteins occurs
- Assembly
parts made in step 4 are put together like a toy at christmas to make a new virus
assembly of the viral proteins and nucleic acid into its capsid and envelope, if present
making a new virus
- Release
virion exits the cell by lysing the cell or exocytosis and may steal some of the host membrane for its envelope
the mature virus leaves the cell
at this point, it can obtain an envelope by budding off the host cell membrane
Cytopathic effects (CPEs) of animal viruses
damage that occurs to the host cell due to a viral infection
can include:
-inclusion bodies: compacted masses of viruses
-syncytia: many cells fused into a large clump cell
-chronic latent stage: virus lays in wait until its reactivated
-transformation: cancer; increased rate of growth, alterations in DNA, continuous cell division, loss of contact inhibition
Viral Multiplication cycle: Retroviruses
- adsorption/attachment
- penetration
- uncoating
- synthesis by RT
- Integration
- synthesis
- assembly
- release
after uncoating –> use RT to make DNA from RNA –> DNA integrated into genome of host –> can lay latent for years until it goes thru second synthesis step where the viral RNA and proteins are produced
Persistent infections
accumulated damage from a virus infection kills most cells
persistent infections:
-cell harbors the virus
-not immediately lysed
-can last from a few weeks to the remainder of the hosts life
-can remain latent in the cytoplasm
-can be inactive viral infections or retrovirus
-inactive: the virus wont replicate itself but thru replication of its host cell
-endogenous retroviruses are always in the state of a provirus
Provirus (persistent infection)
persistent infection in which the viral DNA is incorporated into the DNA of the host
-endogenous retroviruses are always in the state of a provirus
Viruses and cancer
15-20% of cancers are virus induced
viruses can alter our DNA when they insert their DNA into ours
viruses may produce viral replication proteins that stimulate cell reproduction (mitosis)
Viruses and cancer: some retroviruses
viral oncogenes incorporate into host cell DNA and produce proteins that lead to uncontrolled cell growth
Viruses and cancer: other retroviruses
viral genes affect expression of host oncogene leading to uncontrolled cell growth
Viruses and cancer: DNA tumor viruses
viral genes directly produce proteins that lead to uncontrolled cell growth
Cancer cell characteristics
DNA is mutated, chromosomal abnormalities
cell shape altered
uncontrolled growth
loss of contact inhibition: usually cells will stop growing when they contact each other. in cancer the cells keep growing and growing on top of each other to produce a tumor
DNA oncogenic viruses
papillomavirus –> cervical cancer
Epstein-Barr virus –> Burkitt’s lymphoma, nasopharyngeal carcinoma
Hep B –> liver cancer
RNA oncogenic viruses
Human T-cell leukemia virus
Human herpesvirus 8 (HHV8) –> Kaposi’s sarcoma
Bacteriophages
virus that attacks bacteria polyhedral head w/tail fibers looks like space craft makes bacteria more pathogenic from Greek word meaning "eating" every bacterial species is parasitized by various specific bacteriophages
Bacteriophage multiplication cycle
Lytic Cycle:
- attachment
- penetration
- biosynthesis
- maturation/assembly
- release
Lysogenic cycle:
- attachment
- penetration
- integration of the phage DNA into host DNA
- binary fission
- occasionally, excision of phage DNA initiates lytic cycle at stage 3
Temperate phages can switch btwn the 2 cycles
Lysogeny in human disease
occasionally phage genes in the bacterial chromosome cause the production of toxins or enzymes that cause pathology in the human
Lysogenic conversion
when a bacterium acquires a new trait (toxin) from its temperate phage
Isolation and cultivation of viruses
cant inoculate culture media w/viruses and expect them to grow b/c viruses always need a living host to replicate
plaques: areas where virus-infected cells have been destroyed and show up as a clear, well defined patches in the cell sheet
Isolation/cultivation of bacterial viruses
Bacterial viruses:
-grow bacteria, then infect w/virus
Isolation/cultivation of animal viruses
Animal viruses:
- infect living animals
- infect embryonated eggs
- infect animal cell cultures
a monolayer of monkey kidney cells is a cell culture enabling the propogation of viruses
Identifying a virus
similar to bacteria:
immunological methods: draw blood and analyze antibody-antigen interactions
examine genetic sequences
signs and symptoms
Viruses and chemotherapy
difficult in viruses
usually difficult to treat viral infections w/drugs since the virus relies on the metabolic machinery (enzymes) of the host
in order to inhibit/kill the virus w/a drug you may inhibit/kill the host cell in the process
Treatment of viral infection
antibiotics and sulfa drugs ineffective -antibiotics attack membrane bound structures (viruses lack membranes) -sulfa drugs inhibit metabolic pathways (viruses lack these) methods: -inhibit viral penetration -inhibit DNA synthesis -inhibit viral protein synthesis -interfere w/viral protein modification -inhibit viral enzymes -prevent viral production
Prevention of viral infections
Vaccination
Interferon: cell protein produced by infected cells; protects other cells by inhibiting viral infection; protection occurs naturally; can be artificially produced
Sanitation
Viruses and human health
viruses w/high mortality rates: rabies, AIDS, ebola
viruses that cause long term debility: polio, neonatal rubella
viruses w/possible connection to chronic affiliations w/an unknown cause: type 1 diabetes, MS, various cancers, Alzheimers, obesity
Prions (parasitic particles)
proteinaceous infectious particle
9 animal diseases caused by this (mad cow)
only destroyed by incineration or autoclaving in 1N Na OH
Spongiform encephalitis
large vacuoles in brain
fatal neurological degeneration, fibril deposits in brain, loss of brain matter
Cellular PrP protein
made by all mammals
normal structure w/ alpha-helices called cellular PrP
Prion PrP
disease causing form w/ Beta pleated sheets called prion PrP
changes shape of cellular PrP so it becomes prion PrP - this is its form of replication
How do viruses have high specificity for attachment to a host cell?
because they can invade the host cell only thru making an exact fit w/a specific host molecule
How do viruses multiply inside of a host cell?
by adhering to host cell, penetrating, uncoating itself to expose the nucleic acid sequence, produce parts, assemble and exit host cell
Virion
elementary virus particle in its complete morphological & thus infectious form
consists of the nucleic core surrounded by a capsid
the parts of a virus needed to invade and control a host cell
external coating
core containing one or more nucleic acid strains of DNA or RNA
sometimes 1-2 enzymes
Retroviruses
carry their own polymerase (reverse transcriptase) and transcribe RNA –> DNA
can integrate its DNA into our DNA and lay latent for years (AIDS)
Host range
the limitation imposed by the characteristics of the host cell on the type of virus that can successfully invade it
Endocytosis
- specific attachment –> 2. engulfment –> 3. virus in vesicle –> 4. vesicle, envelope and capsid breakdown; uncoating of nucleic acid
4 characteristics of transformed cells
- increased rate in growth
- alterations in chromosomes
- changes in the cells surface molecules
- capacity to divide for an indefinite period
3 principal purposes of cultivating viruses
- isolate & identify viruses in clinical specimens
- prepare viruses for vaccines
- do detailed research on viral structure, multiplication cycle, genetics, and effects as host cells
the 3 way viruses are cultivated
- using live animal inoculation
- using bird embryos
- using cell (tissue) culture techniques
