micro bio exam 2: chapter 6
shadowcasting
A technique to show surface texture, attaches a virus preparation to a surface and showers it with a dense metallic vapor directed from a certain angle. The thin metal coating over the surface of the virus enhances its contours, and a shadow is cast on the unexposed side
host range
A virus can invade its host cell only through making an exact fit with a specific host molecule (receptor) Restricted host range: e.g. hepatitis B only infects liver cells of humans, HIV Intermediate host range: poliovirus infects intestinal and nerve cells of primates Broad host range: e.g. rabies virus infects various cells of all mammals, Ebola
1. Structure
- lack protein synthesizing machinery, Need only those parts required to invade and control a host cell: External coating Core containing nucleic acids
endocytosis
-entire virus is engulfed by the cell and enclosed in a vacuole or vesicle; then enzymes in the vacuole dissolve the envelope and capsid, releasing the virus genome into the cytoplasm.
envelope
-made of lipid and proteins Composed of the membrane system of the host. Cell membrane or nuclear membrane: during release of animal viruses (either from the host cell membrane or from the ER), a part of the host membrane is taken. Regular membrane proteins are replaced with viral proteins, so the viral envelope differs significantly from the host's membranes.(research about lipid rafts) Spikes: protruding glycoproteins essential for attachment to the host cell -
ways to release of mature viruses
1. cell lyses or ruptures 2. budding or exocytosis
Two principal means by which animal viruses penetrate/uncoating
1. endocytosis( engulfment) and uncoating 2. fusion
1. Loeffler and Frosch 2. Ivanovski and Beijernick 3. Chamberland
1. known today as the Chamberland filter - that had pores smaller than bacteria. 2. : tobacco disease caused by a virus 3. foot-and-mouth disease caused by a virus
ways bacteriophages penetrate host cell: 1. that these phages have infected their nucleic acid through the cell wall and now have empty heads
2. after adsorption, the phage plate becomes embedded in the cell wall and the sheath contracts, pushing the tube through the cell wall and releasing their DNA into the interior of the host cell
8 hrs - poliovirus
36 hrs- herpesviruses
Lysogeney is a less deadly form of parasitism than the full lytic cycle and is thought to be an advancement that allows the virus to spread without killing the host
: Many bacteria that infect humans are lysogenized by phages. Occasionally phage genes in the bacterial chromosome cause the production of toxins or enzymes that cause pathology in the human
uncoating
: its envelope is uncoated, thereby freeing the viral genome into the cell cytoplasm
Penetration
: the virus is engulfed into a vesicle and
most active viral infections are ultimately lethal to the cell because of accumulated damage
A permanent shutdown of metabolism and genetic expression Destruction of cell membrane and organelles Toxicity of virus components Release of lysosomes
In vitro methods: viral cultivation in cell or tissue culture
A simple and effective way to grow populations of isolated animal cells in culture dishes Most viruses are propagated through cell culture Much of a virologist's work involves developing and maintaining cultures
cell lyses or ruptures
Nonenveloped and complex viruses that reach maturation in the cell nucleus or cytoplasm are released
naked helical virus
Nucleocapsid is rigid and tightly wound into a cylinder-shaped package
Bacteriophage
Parasitize every known bacterial species Often make the bacteria they infect more pathogenic for humans
in order to view viruses a electron microscope is needed.
Parvoviruses: 20 nm in diameter Mimiviruses: 450 nm in length - larger than some small bacteria, (1.2Mb genome, over 1000 genes) Cylindrical viruses: 800 nm long, but 15 nm in diameter
viral infections lead to long-term disability:
Polio Neonatal rubella
the structure of bacteriophage
Polyhedral head Helical tail Fibers for attachment A protein tailpiece attaches the phage to the host and injects the phage DNA inside
the genome of viruses tend to be.....
The number of viral genes is small compared to that of a cell Possess only the genes necessary to invade host cells and redirect their activity No viral metabolic genes, as the virus uses the host's metabolic resources
True or False: Viral infections that only occur in certain regions: Dengue fever, Rift Valley fever, Yellow fever Some have high mortality rates: Rabies Ebola
True
questions answer when studying animal viruses coating.uncoation.
What gets in to cell? Vesicle or not? Where fusion occurs?
multiplication cycles in animal viruses
adsorption, penetration, uncoating, synthesis, assembly , release
Creutzfeldt-Jakob disease
affects the central nervous system of humans, causes gradual degeneration and death. Loss of brain function resembles Alzheimer's disease, but is very rapid in progression. Complete dementia usually occurs by the sixth month, death follows quickly.
Prions
are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals Prions propagate by converting normal proteins into the prion version
prions
are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals Prions propagate by converting normal proteins into the prion version
Viruses can infect every type of cell including themselves. 70% of diseases are caused by diseases
bacteria, fungi, algae,protozoa, plants, animals( including heminth), viruses
lysogenic cycle
bacteriophage becomes incorporated into the host cell DNA. Found only in some bacteriophages
fusion
between the viral envelope and the host cell membrane (e.g. mumps virus & HIV, b), no vesicle formation
viruses genome can be
dsDNA, ssDNA, dsRNA or ssRNA
An enveloped virus is composed of a nucleocapsid surrounded by a flexible membrane called an envelope. The envelope usually has special receptor spikes inserted into it.
enveloped virus
release
enveloped viruses bud off of the membrane, carrying away an envelope with the spikes. This complete virus or virion is ready to infect another cell.
synthesis
genome replication and protein production. Under the control of viral genes, the cell synthesizes the basic components of new viruses : viral genomes, capsomers, spikes.
viruses are considered
infectious particles not organism. when analyzing there activity use: active or inactive
every bacteria has the lytic cycle:
life cycle of bacteriophage that ends in destruction of the bacterial cell. Found in all bacteriophages
translation apparatus protein encoding genes are found in
mimivirus. since viruses lack ribosomes they use host machinery to produce proteins.
How to see viruses in microscopes:
negative staining, positive staining, and shadowcasting.
positive staining
reveal internal details by staining of specific parts of the virus such as protein or nucleic acid
the envelope continued
spherical and filamentous
Adsorption :
the virus attaches to its host cell by specific binding of its spikes to cell receptors.
Adsorb: to attach (like a virus) NOT Absorb: to soak in (like a paper towel
true
Primary purposes of viral cultivation Isolate and identify viruses in clinical specimens. Prepare viruses for vaccines. Do detailed research on viral structure, multiplication cycles, genetics, and effects on host cells.
true
Viral nucleic acid takes control over the host's synthetic and metabolic machinery. Mechanism varies depending on whether the virus is a DNA or RNA virus Most RNA viruses replicate in the cytoplasm Most DNA viruses replicate in the nucleus Exception exist !
true
temperate phages
undergo adsorption and penetration into the bacterial host but do not immediately undergo replication or release.
negative staining-
uses very thin layers of an opaque salt to outline the shape of the virus against a dark background and to enhance textural features on the viral surface. (Fig.A: an orfvirus, a type of poxvirus)
assembly
viral spike proteins are inserted into the cell membrane for the viral envelope; nucleocapside is formed from viral genome and capsomers.
damage to host cell
A. inclusion bodies B. syncytia
Viruses must use the host cell machinery in order to multiply, They have very small genomes.
Cannot multiply unless they invade a specific host cell (host range, cross-species barrier) Must instruct the genetic and metabolic machinery of the host cell to make and release new viruses
Detecting viral growth in culture: Placques
Clear, well-defined patches in the cell sheet Develops when viruses released from an infected cell radiate out to surrounding cells and infect them Infection spreads gradually and symmetrically from the original point of infection
Inclusion bodies
Compacted masses of viruses or damaged cell organelles in the nucleus or cytoplasm
Informal classification system: Animal, plant, or bacterial viruses DNA or RNA viruses Helical or icosahedra
Criteria of a formal classification system: Structure Chemical composition Similarities in genetic makeup
budding/exocytosis
Enveloped viruses are liberated by budding or exocytosis from the membranes of the cytoplasm, nucleus, ER, or vesicles. During this process, the nucleocapsid binds to the membrane, which curves completely around it and forms a small pouch. Pinching off the pouch releases the virus with its envelope. Budding of enveloped viruses causes them to be shed gradually
Syncytia
Fusion of multiple host cells into single large cells containing multiple nuclei (what's for?) Application: diagnosis
"T-even" bacteriophage infect E. coli T-2 and T-4 Most widely studied bacteriophage
Go through similar stages as animal viruses, different in penetration and release
Filterable virus
Infectious fluids were passed through filters designed to trap bacteria The cell-free filtered fluid remained infectious This proved that an agent smaller than bacteria was the cause of disease
prohage
Instead, the viral DNA enters an inactive prophage state, during which it is inserted into the bacterial chromosome. This viral DNA will be retained by the bacterial cell and copied during its normal cell division so that the cell's progeny will also have the temperate phage DNA. This condition, in which the host chromosome carries bacteriaphage DNA, is termed lysogeny. Because viral particles are not produced, the bacterial cells carrying temperate phages do not lyse, and they appear entirely normal. On occasion, in a process called induction, the prophage in a lysogenic cell will be activated and progress directly into viral replication and the lytic cycle.
adsorption and host range
Invasion begins when the virus encounters a susceptible host and adsorbs specifically to receptor sites on the cell membrane.....host range
capsid
Protective outer shell that surrounds viral nucleic acid Composed of capsomer subunits. Capsomeres: Identical protein subunits that spontaneously self assemble to form the capsid Two types of capsids Helical capsid (naked or enveloped): Rod-shaped capsomeres that form a continuous helix around the nucleic acid Icosahedral capsid (naked or enveloped): Three-dimensional, 20-sided figure with 12 evenly spaced corners Most viruses have either one of the above two types of capsids except another category of capsid that is called "complex"
negative sense rna
RNA genomes that need to be converted into the proper form to be made into proteins
Viruses mutate at a rapid rate. (especially RNA viruses, e.g. HIV, influenza virus) Difficult to design therapies against viruses
Scientists focus on developing vaccines against viruses since so few antiviral drugs are available and antibiotics are ineffective. Interferon (IFN): Naturally occurring human cell product Used with some success in preventing and treating viral infections
positive sense rna
Single-stranded RNA genomes ready for immediate translation into proteins
Capsid: Present in all viruses A protein shell surrounds the nucleic acid Nucleocapsid: capsid and nucleic acid together Envelope: additional covering external to the capsid, not found in all viruses Usually a modified piece of the host cell membrane
Spikes: Found on all viruses Project from either the nucleocapsid or envelope Allow viruses to dock with their host cells Virion: Fully formed virus able to establish infection in a host
complex capsids : bacteriophage
Structure is more intricate than helical and icosahedral capsid. Found in bacteriophage, the viruses that infect bacteria May have multiple types of proteins and take shapes that are not symmetrical.
lysogenic conversion
The acquisition of a new trait from a temperate phage
Impossible to measure the number of viral infections worldwide. About 70% of infectious diseases are caused by viruses. About 15% cancers are caused by viruses. Most common cause of acute infections that do not result in hospitalization: Colds, chickenpox, influenza, herpes, warts, Respiratory syncytial virus
Viral infections that only occur in certain regions: Dengue fever, Rift Valley fever, Yellow fever Some have high mortality rates: Rabies Ebola Other viral infections lead to long-term disability: Polio Neonatal rubella
Nucelocytoplasmic large DNA viruses: Closely related to each other Distantly related to other viruses May constitute a new order of viruses
Virus families: Suffix -viridae Virus genera: Suffix -virus
Transformation
Virus produces proteins that directly induce a loss of growth regulation in the cell Viral genes affect expression of host oncogenes or tumor suppressor genes leading to uncontrolled cell growth
Cytopathic Effects (CPEs)
Virus-induced damage to the cell that alters its microscopic appearance Cells can become disoriented, undergo major changes in shape or size, or develop intracellular damage
viroids ( simplest infectious agents )
Virus-like agent that infect plants & disrupt their growth Significant pathogens in economically important plants: tomatoes, potatoes, cucumbers, citrus trees, and chrysanthemums Potatoes Spindle Tuber Viroid, Tomato Chlorotic Dwarf Viroid, Cucumber Pale Fruit Viroid Naked strands of circular RNA, no capsid or other type of coating 1/10th the size of normal viruses
