Chapter 5

•____: virus is put together using "parts" manufactured during the synthesis process. •______:the number of viruses released by infected cells is variable, controlled by: 2 •_____-infected cell: 3,000 - 4,000 virions •_____-infected cell: 100,000 virions •Immense potential for rapid viral proliferation

Assembly Release -Size of the virus -Health of the host cell Poxvirus Poliovirus

Viruses That Infect Bacteria •________: "bacteria eating" -Most contain double-stranded DNA, but some RNA types exist as well. -Every bacterial species is parasitized by various specific bacteriophages. -Often make the bacteria they infect more ______

Bacteriophage pathogenic

Viral Components (cont'd) •______: protein shell that surrounds the nucleic acid -_____: the capsid together with the nucleic acid -______ ___consist only of a nucleocapsid. •______:external covering of a nucleocapsid, usually a modified piece of the host's cell membrane •____ can be found on naked or enveloped viruses. -Project from the nucleocapsid or the envelope -Allow viruses to dock with host cells •______: a fully formed virus that is able to establish an infection in a host cell

Capsid Nucleocapsid Naked viruses Envelope Spikes Virion

Viral Capsid and Envelope •_____ -Most prominent feature of viruses -Composed of identical protein subunits called ______ -Capsomeres spontaneously self-assemble into the finished capsid. •Two different types: 2

Capsid capsomeres -Helical -Icosahedral

Damage to the Host Cell •_______ _____: virus-induced damage to the cell that alters its microscopic appearance •Types of CPEs include: -Gross changes in shape and size -Development of intracellular changes -______ ____ compacted masses of viruses or damaged cell organelles in the nucleus and cytoplasm -______: fusion of multiple host cells into single large cells containing multiple nuclei •Accumulated damage from a virus infection kills most cells.

Cytopathic Effects (CPEs) Inclusion bodies Syncytia

Synthesis: Replication and Protein Production •______ ___ -Enter the host cell's nucleus and are replicated and assembled there •_____: -replicated and assembled in the cytoplasm •Retroviruses turn their ____genomes into____.

DNA viruses: RNA viruses: RNA DNA

T-Even Bacteriophage •Infect________ •Structure: -_______ capsid containing DNA -Central tube surrounded by a_____ -Collar -Base plate -Tail pins -Fibers

E. coli Icosahedral sheath

Viral Capsid and Envelope (cont'd) •_____ ___ -Take a bit of the cell membrane when they are released from a host cell. •Enveloped viruses can bud from: 3

Enveloped viruses -Cell membrane -Nuclear envelope -Endoplasmic reticulum

Viruses and Human Health •Common causes of____ infections: -Colds, hepatitis, chickenpox, influenza, herpes, warts •Prominent____ infections worldwide: -Dengue fever, Rift Valley fever, yellow fever •Infections with high_____ rates: -AIDS, Ebola •Infections that cause long-term_____: -Polio, neonatal rubella •Connection to_____ infections: -Type 1 diabetes, MS, various cancers, Alzheimer's, obesity

acute viral mortality disability chronic

______ •A virus can invade its host cell only through making an exact fit with a specific host molecule. •_____ ____: the limited range of cells that a virus can infect: -_____ _ liver cells of humans -_____: intestinal and nerve cells of primates -_____: various cells of all mammals •Cells that lack compatible virus receptors are resistant to adsorption and invasion by that virus. •______: specificities of viruses for certain tissues

Adsorption Host range Hepatitis B: Poliovirus Rabies Tropisms

Multiplication Cycles in Animal Viruses •General phases of the animal viral replication cycle: 6 •The length of the replication cycle varies from 8 hours in polioviruses to 36 hours in herpesviruses.

-Adsorption -Penetration -Uncoating -Synthesis -Assembly -Release

Viral Components •Viruses bear no resemblance to cells and lack any of the protein-synthesizing machinery found in cells. •Viral structure is composed of regular, repeating subunits that give rise to their crystalline appearance. •The structure contains only those parts needed to invade and control a host cell: 3

-External coating -Core containing one or more nucleic acid strains of DNA or RNA -Sometimes one or two enzymes

How Viruses are Classifiedand Named •For many years, animal viruses were classified on the basis of their hosts and the diseases they caused. •Newer classification systems emphasize the following: 4 •International Committee on the Taxonomy of Viruses: 3

-Hosts and diseases they cause -Structure -Chemical composition -Similarities in genetic makeup -3 orders -73 families -283 genera

Variety in Viral Nucleic Acid •DNA viruses 4 •RNA viruses -Double-stranded, but ___ ___ single-stranded -_________ ready for immediate translation -______: must be convertedbefore translation can occur -Segmented -_____: carry their own enzymes to create DNA out of RNA

-Single-stranded (ss) -Double-stranded (ds) -Linear -Circular more often Positive-sense RNA: Negative-sense RNA Retroviruses

Nucleic Acids: At the Core of a Virus •_____:the sum total of the genetic information carried by an organism •Viruses contain DNA or RNA but not ____. •The number of viral genes is quite ____ compared with that of a cell: -Four genes in hepatitis B virus -Hundreds of genes in some herpesviruses -Possess only the genes needed to ____ host cells and ____ their activity

Genome both small invade redirect

Techniques in Cultivating and Identifying Animal Viruses •Viruses require living cells as their "medium." -______: laboratory-bred animals and embryonic bird tissues -______: cell or tissue culture methods •Primary purposes of viral cultivation: -______ & ______ viruses in clinical specimens. -Prepare viruses for______. -Do detailed______ on viral culture, multiplication cycles, genetics, and effects on host cells.

In vivo In vitro Isolate and identify vaccines research

Discovery of Viruses •_______ & _____ showed that a disease in tobacco was caused by a virus. •______ & _______ discovered an animal virus causes foot-and-mouth disease in cattle. •______ ________: -These early researchers found that when fluids from host organisms passed through porcelain filters designed to trap bacteria, the filtrate remained infectious. -This proved that:.

Ivanovski and Beijerinck Loeffler and Frosch Filterable virus a cell-free fluid could contain agents that could cause infection

The Danger of Lysogeny in Human Disease •Occasionally phage genes in the bacterial chromosome cause the production of toxins or enzymes that cause pathology in the human. •______ _____: when a bacterium acquires a new trait from its temperate phage -Corynebacterium diphtheriae - diphtheria toxin-Vibrio cholerae - cholera toxin-Clostridium botulinum - botulinum toxin

Lysogenic conversion

______ & _____ •The flexible cell membrane of the host is penetrated by the whole virus or its nucleic acid. •Penetration through ______ happens when an entire virus is engulfed by the cell and enclosed in a vacuole or vesicle. •___ ____: fusion of the viral envelope with the cell membrane •_____: -Enzymes in the vacuole dissolve the envelope and capsid. -Virus fuses with the wall of the vesicle. -Viral nucleic acid is released into the cytoplasm.

Penetration and Uncoating endocytosis Direct penetration Uncoating:

Detection of Viral Growth in Culture •Observation of degeneration and lysis of infected cells •_______:areas where virus-infected cells have been destroyed and show up as a clear, well-defined patches in the cell sheet -Microscopic manifestation of cytopathic effects (CPEs)

Plaques

Detection of Viral Growth in Culture (cont'd) •This same technique is used to detect and count bacteriophages: -_______develop when grown in bacterial cell culture. -Viruses released by an infected cell radiate out to adjacent host cells. -New cells become______, they die and release more viruses, and the process continues. -Plaque manifests as a macroscopic, round, clear space that corresponds to areas of dead cells.

Plaques infected

Other Substances in the Virus Particle •Enzymes for specific operations within their host cell -______ that synthesize DNA and RNA -_____ that copy RNA -_____ _____ synthesizes DNA from RNA •Completely lack the genes for synthesis of metabolic enzymes •Some viruses carry ____ substances from their host cell: -Retroviruses borrow the host's ____ molecules.

Polymerases Replicases Reverse transcriptase away tRNA

Persistent Infections •Cell harbors the virus and is not immediately lysed: -Can remain for a few weeks or the remainder of the host's life -Can remain in the cytoplasm •______: -Viruses incorporate into the DNA of the host. -Measles virus •____ ___ _____ -Periodically become activated under the influence of various stimuli -Herpes simplex and herpes zoster viruses

Provirus Chronic latent state:

_______ _____ •Dependent on other viruses for replication •____ _____ _____: -Originally thought that it could only replicate in cells infected with the adenovirus -Can also infect cells that are infected with other viruses or that have had their DNA disrupted through other means •_____ ____: -Naked circle of RNA -Expressed only in the presence of the hepatitis B virus -Worsens the severity of liver damage

Satellite Viruses Adeno-associated virus (AAV) Delta agent

Other Noncellular Infectious Agents •_______ _______: -Implicated in chronic, persistent diseases in humans and animals -Brain tissue removed from affected animals resembles a sponge -Long period of latency before first clinical signs appear -Diseases are progressive and universally fatal •______: -Agents of spongiform encephalitis infection -Deposited as long protein fibrils in the brain with no nucleic acid -Creutzfeldt-Jakob disease -Bovine spongiform encephalopathy

Spongiform encephalopathies Prions

Lysogeny: The Silent Virus Infection •______ phages -Undergo adsorption and penetration -Do not undergo replication or release immediately •Viral DNA enters an________ prophage state: -Inserted into bacterial chromosome -Copied during normal bacterial cell division -______: a condition in which the host chromosome carries viral DNA •______: virus in a lysogenic cell becomes activated and progresses directly into viral replication

Temperate inactive Lysogeny Induction

Viruses and Cancer •Experts estimate that 20% of cancers are caused by viruses. •_____:the effect of oncogenic, or cancer-causing viruses: -Some viruses carry genes that directly cause cancer .-Other viruses produce proteins that induce a loss of growth regulation, leading to cancer

Transformation

Viruses and Cancer (cont'd) •_______ ____: -Increased rate of growth -Alterations in chromosomes -Changes in cell surface molecules -Capacity to divide indefinitely •_____: mammalian viruses capable of initiating tumors -Papillomaviruses -Herpes virus -Hepatitis B virus -HTLV

Transformed cells Oncoviruses

_______ •Virus-like agents that parasitize plants •About one-tenth the size of an average virus •Composed of naked strands of RNA, lacking a capsid or any other type of coating •Significant pathogens in economically important plants: tomatoes, potatoes, cucumbers, citrus trees, chrysanthemums

Viroids

The Position of Viruses on the Biological Spectrum •Viruses infect every type of cell, including: 6 •Seawater can contain 100 million viruses per milliliter. •For many years, the cause of viral infections was unknown:-_____ ____ postulated that a "living thing" smaller than bacteria caused these diseases. -He also proposed the term virus, which is Latin for "poison."

bacteria, algae, fungi, protozoa, plants, and animals. Louis Pasteur

The Vital Role of Viruses in Evolution •Infect cells and influence their _____ _____ •Shape the way cells, tissues, bacteria, plants, and animals have evolved •10% of the ____ ____ consists of sequences that come from viruses •10 - 20% of _____ ____ contains viral sequences •_____ _____ ______: -cannot multiply unless they invade a specific host cell and instruct its genetic and metabolic machinery to make and release new viruses

genetic makeup human genome bacterial DNA Obligate intracellular parasites

Using Cell (Tissue) Culture Techniques •Isolated animal cells are grown ___ ____ in cell or tissue culture rather than in an animal or egg. •Cell cultures: -Grown in sterile chambers with special media that contain the correct nutrients for cells to survive -Cells form a______, or single, confluent sheet of cells that supports viral multiplication. -Allows for the close inspection of culture for signs of infection

in vitro monolayer

Using Bird Embryos •Bird eggs containing embryos: -______ and___ ____ unit -_____environment -contain their own_____ •Chicken, duck, and turkey eggs are the most common choices for inoculation. •Virus is injected through the eggshell by drilling a small hole or making a small window.

intact self-supporting sterile nourishment

Life Cycle of dsDNA Viruses •Early phase: -Viral DNA enters the nucleus where genes are transcribed into a _______ -____ ____moves into the cytoplasm to be translated into____ ____ needed to replicate viral DNA. -Host cell's _____ ____ is involved in this phase. •Late phase: -Parts of the viral genome are translated into proteins required to form the ______ and other structures. -New ____ ____ and____ are assembled. -______ ____are released by budding.

messenger RNA RNA transcript viral proteins DNA polymerase capsid viral genomes capsids Mature viruses

The Viral Debate •Two sides of the debate: -Since viruses are unable to replicate independently from the host cell, they are ____ ____things and should be called _____ molecules. -Even though viruses do not exhibit most of the life processes of cells, they can direct them, and thus are certainly more than ____ and ____ molecules. •Viruses are better described as____ or ____ rather than ____ or_____.

not living infectious inert lifeless active, inactive , alive, dead

The General Structure of Viruses •Size range: -Smallest infectious agents -Smallest viruses: ______ around 20 nm in diameter -Largest viruses: _____ around 450 nm in length -Some cylindrical viruses can be relatively long (800 nm) but are so narrow in diameter (15 nm) that their visibility is limited without an electron microscope.

parvoviruses mimiviruses

Modes of Viral Multiplication •Viruses are minute parasites that seize control of the _______ & ________ machinery of cells •The nature of the viral replication cycle dictates: -The way the virus is_____ -What it does to the____ -Responses of ______ ____ -_____ ____to control viral infection

synthetic and genetic machinery of cells. transmitted host Human measures

Treatment of Animal Virus Infections •Antibiotics designed to treat bacterial infections have no effect on viruses. •Most antiviral drugs block _____ _____ by targeting the function of host cells, but with severe side effects. •DRACO: _________________________________: -Causes virus-infected cells to destroy themselves, no matter what the virus is •Vaccines are valuable, but there are a limited number available

viral replication double-stranded RNA activated caspase oligomizer