Chapter 24

bacteriophages

("bacteria eaters"), more simply referred to as phages. *Their most common struc- ture consists of a long nucleic acid molecule (usually dsDNA) coiled within a polyhedral head. Most have a tail, which may be contractile and may function in penetration of the host cell. *Phages have been clinically used to treat infection for close to a century *Antibiotics, which have long been more dependable and easier to use, are increasingly less effective than in the past due to the wide- spread and escalating problem of bacterial resistance. -typically have polyhedral head with dsDNA genome and helical tail -tail helps penetrate host cell -archaeal viruses similar structure

subviral agents

infective agents smaller and simpler than viruses examples are satellites, viroids, defective interfering particles (DIPs), and prions.

prophage/ provirus

integrated virus can alternate between lysogeic and lytic cycles certain environmental conditions may cause prophage to become lytic

genome of a virus

*A typical virus contains either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), not both. however there are hybrid RNA-DNA viruses *The nucleic acid of a virus can be single-stranded or double-stranded. (ssDNA, dsDNA, ssRNA, dsRNA) *The number of base pairs in their genome depends on whether it is ss or ds. -single or double stranded RNA or DNA

lysogenic conversion

Bacterial cells carrying prophages are called lysogenic cells. Such bacterial cells may exhibit new properties. This type of change is called lysogenic conversion. An interesting example involves the bacterium Corynebacterium diphtheriae, which causes diphtheria. Two strains of this species exist, one that produces a toxin (and causes diphtheria) and one that does not. The only difference between these two strains is that the toxin-producing bacteria are infected by a specific temperate phage. The phage DNA codes for the powerful toxin that causes the symptoms of diphtheria. *in conclusion if a bacterium contains certain prophage DNA then it becomes a toxin.

bioterrorism

Bioterrorism is the intentional use of microorganisms or toxins derived from living organisms to cause death or disease in humans, animals, or plants on which humans depend. Terrorists could conceivably initiate epidemics of smallpox, anthrax, plague, yellow fever, Ebola hemorrhagic fever, and other potentially fatal diseases. anthrax and plague are caused by bacteria

vesicles

some viral particles travel together between cells in vesicles.

virus first hypothesis

states that viruses predate or coevolved with their current cellular hosts even before the life-forms assigned to the three domains diverged. One research team recently suggested that viruses initially existed in a pre-cellular world as self-replicating units. Gradually, these small units became more organized and more com- plex. Eventually they evolved to produce enzymes for the synthesis of membranes and cell walls, leading to the origin of the first cells. Evidence for this hypothesis comes from similarities found in the protein structures of some viral capsids and in genetic similarities between some viruses that infect archaea and some that infect bacteria. improbable that these similarities evolved independently. They hypothesize that viruses diverged very early—before archaea, bacteria, or eukaryotes. -viruses were first life forms or coevolved with early life forms

temperate phages

these do not immediately destroy their hosts. They alternate between a lytic and lysogenic cycle. *The temperate phage lambda is a model organism used to study temperate viruses. *Certain external conditions (such as ultraviolet light and X-rays) cause temperate viruses to revert to a lytic cycle and then destroy their host. Sometimes temperate viruses become lytic spontaneously.

prions

prions are protein particles prion, for "proteinaceous infectious particle." Sometimes, the PrP protein folds into a different shape, an insoluble form that can cause disease. This misfolded protein is the prion. Mutations in the gene that encodes the PrP protein increase the risk that the protein will misfold and become a prion. The prion then somehow induces other PrP molecules to misfold into the pathogenic form. Prions apparently can aggregate and accumulate in the brain and in certain other tissues and cause serious damage. Prions are found in the brains of patients with transmissible spongiform encephalopathies (TSEs). 1) Prion induces normal PrP to misfold, forming another prion. 2) Each prion can induce additional PrP proteins to misfold. 3) Proteins aggregate. The oldest known prion disease is scrapie in sheep and goats. When infected, animals lose coordination, become irritable, and itch so severely that they scrape off their wool or hair. Bovine spongiform encephalopathy (BSE) is a related prion disease popularly referred to as "mad cow disease" because some diseased cattle become aggressive. Transmissible from cow to human. The human disease is called vCJD because it is a variant of CJD, which is caused by the transformation of PrP proteins into prions. Human-to-human transmission of vCJD has been associated with tissue and organ transplants and with transfusion with contaminated blood. Chronic wasting disease, an illness related to mad cow disease, has spread among deer and elk populations in North America. Plants can act as a vector for prions when they attach to a plant and the plant is then consumed. Feline spongiform encephalopathy is a prion disease of domestic and captive felines. With this disease, large deposits of a misfolded protein accumulate in the liver and spleen. Prions are different from viruses in that they can arise spontaneously, mostly as a result of mutation. -are infectious proteins that become misfiled and cause other proteins to misfold. -cause Lethal diseases by aggregating and damaging tissue (especially in the brain) -Creutzfeldt-Jakob disease, Spongioform encephalopathies (mad cow), and chronic wasting disease.

lytic reproductive cycles

*In a lytic cycle, the virus lyses (destroys) the host cell. *When the virus infects a susceptible host cell, it forces the host to use its metabolic machinery to replicate viral particles. *Viruses that have only a lytic cycle are described as virulent, which means that they cause disease and often death. *Once released the viruses infect other cells and the process begins again. The time for this to happen varies from less than 20 minutes to more than 1 hour. 5 steps are typical in lytic viral reproduction... 1. Attachment (or adsorption): Virus attaches to specific receptors on the host cell. This ensures that the virus infects only its specific host. virus attaches to specific receptors on host cell 2. Penetration: The virus penetrates the host plasma membrane and moves into the cytoplasm. Viruses that infect animal cells enter the host cell intact. some phages inject only their nucleic acid into the cytoplasm of the host cell were the capsid remains on the outside. virus penetrates host cells plasma membrane and moves into cytoplasm (entire virus or. just genetic material) 3. Replication and Synthesis: The viral genome contains all the information necessary to produce new viruses. Once inside a host cell, the virus degrades the host-cell nucleic acid. The virus then uses the molecular machinery of the host cell to replicate its own nucleic acid and produce viral proteins. Many antiviral drugs interfere with replication of viral nucleic acid. virus uses host cell enzymes and ribosomes to make copies of viral genetic material and proteins 4. Assembly: The newly synthesized viral components are assembled into new viruses. Newley made viral component are assembled into complete viruses 5. Release: Assembled viruses are released from the cell. Generally, lytic enzymes, produced by the phage late in the replication process, destroy the host plasma membrane. Phage release typically occurs all at once and results in rapid cell lysis. In contrast, animal viruses are often released slowly or bud off from the plasma membrane. Lytic enzymes destroy host plasma membrane and viral particles released all at once (rapid cell lysis) or gradually

Lysogenic cycles

*In lysogenic cycles, temperate viruses integrate into the host DNA *the viral genome becomes integrated into the host bacterial DNA. *The integrated virus is called a prophage, or provirus. When the bacterial DNA replicates, the prophage also replicates. *In this way, the prophage is passed on to new generations of the original infected cell. The viral genes that code for viral structural proteins may be repressed indefinitely. 1) Attachment. Phage attaches to cell surface of bacterium. 2) Penetration. Phage DNA enters bacterial cell. 3) Integration.Phage DNA integrates into bacterial DNA. 4) Replication. Integrated prophage replicates when bacterial DNA replicates. -after attachment and penetration virus INTEGRATES its genome not host genome indefinitely. -viral genes replicated ash sot genome replicated (viral genes inherited y host offspring) -PROVIRUS after it integrates

size of viruses

*Most known viruses are very small, ranging in size from 20 to 300 nm *The poliovirus is about 30 nm in diameter (about the size of a ribosome). *A poxvirus that causes smallpox can measure up to 300 nm long and 200 nm wide. *Pandoravirus salinas approaches 1 μm in length and is about 0.5 μm wide, larger than many bacteria and some eukaryotic cells. Only 7% of the genes from Pandoravirus salinas match with existing gene databases. These viruses infect amoebas.

combination of helical and polyhedral components

*Some viruses have both helical and polyhedral components. Viruses that infect bacteria are called bacteriophages, or simply phages *The T4 phage that infect the bacteria E.Coli consist of a polyhedral "head" attached to a helical "tail". Many phages have this shape some even have tail fibers that attach to the host cell.

The Baltimore classification system/ classification of viruses

*The Baltimore classification system classifies viruses based on the type of nucleic acid the virus contains, whether the nucleic acid is single-stranded or double-stranded, and how mRNA is pro- duced. Other traits considered in viral classification are the size and shape of the virus, the presence of an envelope, and the method by which the virus is transmitted from host to host.

classifying viruses with ICTV

*The International Committee on Taxonomy of Viruses (ICTV), a group of virologists, decides on specific criteria for classifying and naming viruses. Viruses can be classified based on their host range, the type of host species a specific virus (or other type of infectious agent) can infect. *They may be referred to as plant viruses, animal viruses, bacterial viruses, and so on. *Viruses are more formally classified into taxa from species to orders. *the type of nucleic acid is important in classifying viruses. *Virus family names include the suffix -viridae. (this classification system is not a traditional Linnaean system it does not assign viruses to domains, kingdoms or phyla. -when classifying viruses you need to know the type of host specie the virus can infect, genetic material, size and shape, presence or absence of envelope, transmission method.

steps to how viruses replicate

*The reproductive cycle of viruses begins with a virus coming into contact with a host cell. *The virus typically attaches to the surface of the host cell. *The viral nucleic acid must enter the host cell and synthesize the components it needs to reproduce itself. *Then viral components are assembled, and viruses are released from the cell, ready to invade other cells. Two types of viral reproductive cycles are lytic and lysogenic cycles.

enveloped viruses

*have an outer mem- branous envelope that surrounds the capsid. Typically, the virus acquires the envelope from the host cell's plasma membrane as it leaves the host cell *inside the host cell, the virus directs the synthesis of certain proteins, which are then inserted into the host's plasma membrane. Thus, the viral envelope consists of phospholipids and proteins of the host's plasma membrane as well as proteins encoded by the virus itself. Some viruses possess envelope glycoproteins that extend out from the envelope as spikes. *The human immunodeficiency virus (HIV) that causes AIDS is an enveloped virus.

helical viruses

*helical viruses include- tobacco mosaic virus, and they appear as long rods or threads. *The capsid is a hollow cylinder made up of proteins that form a groove into which RNA fits

viral replication

*viruses reproduce, but only within the complex environment of the living host cells they infect. *Viruses use their genetic information to force their host cells to replicate their viral nucleic acid and make the proteins they need. *They take over the transcriptional and translational mechanisms of the host cell.

retroviruses use of reverse transcriptase

1) HIV attaches to host-cell plasma membrane. 2) HIV enters host-cell cytoplasm. 3) Capsid is removed by enzymes. Reverse transcriptase catalyzes synthesis of single-stranded (ss) DNA that is complementary to viral RNA. 4) The ssDNA then serves as template for synthesis of second DNA strand, resulting in double-stranded (ds) DNA. 5) dsDNA is transferred to host nucleus and enzyme integrase integrates DNA into host chromosome. 6) When activated, viral DNA useshost enzymes to transcribe viral RNA. 7) Viral RNA leaves nucleus,viral proteins are synthesized on host ribosomes, and virus is assembled. 8) Virus buds from host cell, using host-cell plasma membrane to make viral envelope. HIV infects T helper cells, specialized cells of the host's immune system. The virus attaches to protein receptors, known as CD4, on the plasma membrane of T helper cells. HIV has two identical single-stranded RNA molecules. shorter 1) upon entry into host cell, viral RNA converted to DNA 2) Viral DNA intermediate integrated into host DNA 3) When activated, viral DNA uses host RNA polymer to make viral RNA from viral DNA 4) Viral RNA used to make viral Proteins 5) New viral RNA + viral proteins assembled into new retroviruses that exit host cell dna 3)

steps of a viral infection in animal cells

1) Virus attaches to specific receptors on plasma membrane of host cell. 2) Membrane fusion.Viral envelope fuses with plasma membrane. 3) Virus is released into host-cell cytoplasm. 4) Viral nucleic acid separates from its capsid. 5) Viral nucleic acid enters host- cell nucleus and replicates. 6) Viral nucleic acid is transcribed into mRNA 7) Host ribosomes are directed by mRNA to synthesize viral proteins. 8) Vesicles transport glycoproteins to host-cell plasma membrane. 9) New viruses are assembled and enveloped by host-cell plasma membrane. 10) Viruses are released from host cell.

virus

A virus is a very small infective agent that consists of a core of nucleic acid and is dependent on a living host. *non- living particles because they are not composed of cells and they cannot carry on metabolic activities or reproduce on their own. *They do not have the components necessary to carry on cellular respiration or to synthesize proteins and other molecules. *contain the nucleic acids necessary to make copies of themselves *They replicate by invading living cells and commandeering their metabolic machinery. To multiply, a virus must infect a cell in which it can replicate. Thus, viruses are obligate intracellular parasites; they survive only by using the resources of a host cell, the cell the virus invades. *Viruses infect all types of organisms, including bacteria, archaea, protists, plants, fungi, and animals. Some viruses even infect other viruses. Interestingly, viruses evolve by natural selection. *is sub cellular/ acellular *cannot reproduce without host cell, obligate intracellular parasites *cannot make their own proteins *cannot independently perform metabolic processes *viruses evolve by natural selection -are not within the 3 domains of life

DNA VIRUSES WITH NO ENVELOPE

Adenoviruses: Respiratory tract disorders (e.g., sore throat, tonsillitis), conjunctivitis, and gastrointestinal disorders are caused by more than 40 types of adenoviruses in humans; other varieties infect other animals dsDNA; replicate in the host nucleus Papovaviruses: Human warts and some degenerative brain diseases; some cancers, including cervical cancer‡‡ dsDNA Parvoviruses: Infections in dogs, swine, arthropods, rodents; gastroenteritis in humans (transmitted by consumption of infected shellfish) ssDNA; some require a helper virus to multiply

Emerging virus

An emerging virus is one that is new to a population or that is rapidly increasing in incidence. The best-known example is HIV, the virus that causes AIDS. Other emerging viruses include the coronavirus that causes severe acute respiratory syndrome (SARS); chikungunya, a viral disease transmitted by mosquitos; Ebola virus that causes hemorrhagic fever (see chapter opening photo- graph); and Zika virus, which causes serious birth defects, including microcephaly (small brain). Many new, continual, or reemerging pathogens appear suddenly and can strike globally.

viral infections in ANIMALS

Animal viruses cause hog cholera, foot-and-mouth disease, canine distemper, avian influenza, and certain types of cancer (such as feline leukemia and cervical cancer). In humans, viruses cause herpes simplex (one type causes genital herpes), mumps, rubella (German measles), rubeola (measles), warts, infectious mononucleosis, influenza, viral hepatitis, Ebola hemorrhagic fever, and AIDS Norwalk virus is transmitted directly from person to person and indirectly by fecal contamination of water and food. The virus is also spread when infected people sneeze or cough, sending particles into the air; that is, virus particles become aerosolized. Infected individuals can shed viral particles for many weeks after symptoms abate. diverse strains of the Norwalk virus species make up the noroviruses. Human activity, including social factors such as urban- ization, global travel, immigration, and war, contributes to epidemics of infectious disease. Climate change increasingly impacts human health as mosquitoes and other vectors of viral infection expand their range into warmer areas. Human travel facilitates greater movement, introduction and reintroduction of viruses and their carriers increases. Living conditions, includ- ing sanitation, nutrition, physi- cal stress, quality of health care, intravenous drug use, and sexual practices, are important factors in the spread of disease. -like most viruses these are host and sometimes tissue specific: like flu virus can only attach cell lining the respiratory tract of certain vertebrates and poliovirus binds to motor neurons of CNS -animal host cells actively ENVELOPE viruses by ENDOCYTOSIS (virus surrounded by extra membrane from host cell.

RNA VIRUSES WITH ENVELOPE

Bunyaviruses: St. Louis encephalitis; hantavirus pulmonary syndrome (caused by Sin Nombre virus, a hantavirus) ssRNA Coronaviruses: Upper respiratory infections; SARS ssRNA; largest known RNA virus Filoviruses: Hemorrhagic fever, including that caused by the Ebola virus ssRNA Flaviviruses: Yellow fever; West Nile infection; Zika virus infection; hepatitis C (the most common reason for liver transplants in the United States) ssRNA Orthomyxoviruses: Influenza (flu) in humans and other animals ssRNA; serve as a template for mRNA synthesis; medium-sized viruses that often exhibit projecting glycoprotein spikes Paramyxoviruses: Rubeola (measles) and mumps in humans; distemper in dogs ssRNA; resemble orthomyxoviruses but somewhat larger Retroviruses: AIDS; some types of cancer ssRNA viruses that contain reverse transcriptase for transcribing the RNA genome into DNA; two identical molecules of ssRNA Rhabdoviruses: Rabies ssRNA Togaviruses: Rubella (German measles) ssRNA; can serve as mRNA; large diverse group of medium- sized enveloped viruses; many transmitted by arthropods

pandemic

Global endemic Flu pandemics also occurred in 1957 and 1968. Each year new strains of influenza virus evolve and become infectious. If the new combination of viral genes is unfamiliar to the human immune system, the virus may spread easily, resulting in a pandemic. Viruses mutate rapidly and exchange genetic material with other viral strains: these new viruses may be able to infect other host and spread rapidly; can cause pandemics -human activity and climate change could cause new epidemics/ pandemics -climate change has allowed disease vectors like mosquitoes to expand their range.

DNA VIRUSES WITH ENVELOPE

Herpesviruses: Cold sores (herpes simplex virus type 1); genital herpes, a sexu- ally transmitted disease (herpes simplex virus type 2); chicken- pox and shingles (herpes varicella-zoster virus); infectious mononucleosis and Burkitt's lymphoma (Epstein-Barr virus) dsDNA; medium to large, enveloped viruses; replicate in the host nucleus* Poxviruses: Smallpox, cowpox,† monkeypox, and economically important diseases of domestic fowl dsDNA; large, complex viruses; replicate in the cytoplasm of the host cell

Virion

Microbiologists use the term virion to refer to the complete virus particle that is outside a host cell in a dormant state. The virion is the form in which the virus moves from the cell in which it was produced to a new host cell in which it can replicate its genome.

animal viruses life cycles

Most viruses cannot survive very long outside a living host cell. Their survival depends on being transmitted from one animal to another. However, their host range may be quite limited because attachment to a host cell is very specific. The type of attachment proteins on the surface of a virus determines what type of cell it can infect. Receptors typically vary with each species and sometimes with each type of tissue. Some viruses, such as the adenoviruses, have fibers that project from the capsid and adhere to complementary receptors on the host cell. Other viruses, such as those that cause herpes and rabies, are surrounded by a lipoprotein envelope with projecting glycoprotein spikes that attach to a host cell. Viruses have several ways to penetrate animal cells After attachment to a host-cell receptor, some enveloped viruses fuse with the animal cell's plasma membrane. The entire virus, including the capsid and nucleic acid, is released into the animal cell. Other viruses enter the host cell by endocytosis. In this process the plasma membrane of the animal cell invaginates, forming a membrane-enclosed vesicle that contains the virus. Endocytosis is advantageous to the virus because the endocytotic vesicle delivers the virus deep into the cytosol. In DNA animal viruses, replication of viral DNA and protein synthesis are similar to the processes by which the host cell would normally carry out its own DNA replication and protein synthesis. In RNA viruses, RNA synthesis takes place with the help of an RNA-dependent RNA polymerase. After viral genes are transcribed, the viral structural proteins are synthesized. The capsid is produced, and then assembly of new virus particles takes place. Finally, release occurs. Viruses that do not have an outer envelope exit by cell lysis The plasma membrane ruptures, releasing many new viral particles. Enveloped viruses obtain their lipoprotein envelopes by picking up a fragment of the host plasma membrane as they leave the infected cell.

RNA VIRUSES WITH NO ENVELOPE

Noroviruses: (made up of strains of Norwalk virus) Most common cause of viral gastroenteritis in humans ssRNA Picornaviruses: Polio (poliovirus); hepatitis A (hepatitis A virus); intestinal disorders (enteroviruses); common cold (rhinoviruses); aseptic meningitis (coxsackievirus, echovirus) ssRNA that can serve as mRNA; diverse group of small viruses Reoviruses: Vomiting and diarrhea (rotaviruses); encephalitis (coltiviruses, orbiviruses) dsRNA

ways bacteriophages can help...

Phages are being engineered to specifically destroy particular bacteria due to their ability to target specific host species. Scientists are also genetically engineering phages so that bacteria will be slower to evolve resistance to them. *blood culture test uses bacteriophages to identify and determine if Staphylococcus aureus bacteria are present and if they have resistance to methicillin. *Phage therapy also has applications in dentistry, veterinary medicine, agriculture, and food science. For example, certain phages can kill deadly strains of E. coli in cattle. *commonly used to identify and eradicate target bacteria present in food products and food preparation facilities. Sprays containing bacteriophages are being used to destroy surface bacteria in clinical settings and to prevent bacterial growth on medical instruments, catheters, and surgical materials.

polyhedral viruses

Polyhedral viruses, such as the adenoviruses (which cause a number of human illnesses, including some respiratory infections), appear somewhat spherical. Its capsomers are organized in equilateral triangles. *The capsid of a large virus can consist of several hundred capsomers. *the most common polyhedral structure is as icosahedron (structure with 20 identical surface faces, where each face is a triangle.

H1N1

Reassortment of genetic material can also occur inside a human host, producing a viral strain that can spread from person to person (see figure). H1N1 influenza evolved in this way. 1) Influenza virus may mutate to forms that can infect human cells. 2) Influenza viruses from two different strains infect human cell by releasing their RNA into cell. Neither of these viruses can spread from human to human. 3) RNA from both viruses is duplicated in nucleus (duplicated RNA strands not shown). 4) New viruses assembled with reassorted RNA; new viruses have RNA from both strains. 5) Viruses of new strain leave host cell and now are highly infectious to humans.

polydnaviruses

Recent studies on the origin of polydnaviruses contribute to our understanding of viral evolution. Polydnaviruses are particles that consist of multiple circles of dsDNA encased in capsid proteins and an envelope. Each circle of DNA contains part of the virus genome. These viruses are found in ovary cells of many species of parasitic wasps. Genes needed for these viruses to replicate are found only in the wasp genome; the viruses can replicate only in the wasp ovary cells. The wasp injects polydnaviruses along with her eggs into certain caterpillars. The polydnaviruses express toxins that interferer with the caterpillars immune defenses and development. The wasp eggs hatch and develop inside the caterpillar. The young wasps feed on the caterpillar. ] The research team concluded that the polydnavirus evolved from a nudivirus that infected wasps millions of years ago. In time, the virus genome became incorporated into the wasp genome. Proteins needed for viral replication are now part of the wasp's DNA, and the virus can replicate only in the wasp's ovaries. a mutualistic relationship has evolved between the wasp and the polydnavirus. The investigators suggested that the polydnavirus acts as a gene vector, transporting large chunks of DNA to the caterpillar.

Retroviruses

Retroviruses are RNA viruses that have a DNA polymerase called reverse transcriptase, which transcribes the RNA genome into a DNA intermediate This DNA becomes integrated into the host DNA by an enzyme also carried by the virus. Copies of the viral RNA are synthesized as the incorporated DNA is transcribed by host RNA polymerases. The human immunodeficiency virus (HIV) that causes AIDS is a retrovirus. Some retroviruses can cause cancer directly or by interacting with other cancer-causing viruses. -Rna viruses that use reverse transcriptase (special DNA polymerase) to convert their RNA genome into DNA intermediate -Example HIV and certain cancer causing viruses

capsid

The nucleic acid core of the virus is surrounded by a protein coat called a capsid. *The capsid is a protective protein coat *The capsid consists of protein subunits called capsomers. The capsomers determine the shape of the virus. Capsids generally are helical, polyhedral, or a combination of both shapes. -core of nucleic acid surrounded by capsid -protein coat usually helical or polyhedral in shape

virology

The study of viruses is called virology, and biologists who study viruses are virologists.

Antiviral drugs

These drugs are currently used to treat many diseases, including HIV, hepatitis B and C, herpes, and influenza A and B. Antiviral drugs do not typically destroy viruses. Rather, they inhibit the development, or replication, of many types of RNA and DNA viruses. however there is resistance For example, amantadine, which inhibits penetration or uncoating of viral nucleic acids, had been effective in treating patients with influenza. viral strains are becoming resistant to Tamiflu. A single mutation that occurs spontaneously is apparently responsible for this resistance. Some inhibit viral attachment to host cells, and others interfere with replication of viral nucleic acid. Viruses are being genetically modified to treat diseases such as cancer. Viruses can be reprogrammed to enter, reproduce, and destroy cancer cells. Researchers are developing strategies using RNA-targeted CRISPR to target and inhibit the hepatitis C virus (a ssRNA virus) in cells. Ongoing research suggests that these strategies may be effective against several other viruses, including HIV. -viruses quickly evolve resistance to antivirals -these drugs typically inhibit life cycle enzymes

viral infections on PLANTS

Viral infections do not usually kill plants, but they do stunt their growth; cause changes in the shape of the foliage; and may cause spots, streaks, or mottled patterns on leaves, flowers, or fruits Most plant viruses have capsids, but not envelopes. The genome of 75% of plant viruses consists of ssRNA Plant viruses are typically named according to the type of host plant they infect and their effects on the plant. -Tobacco mosaic virus, described at the beginning of this chapter, was the first virus ever discovered. It was transmissible (could be transferred from one plant to another infected sap. It was tiny (infective agent wasn't removed from infected sap by filter designed to remove bacteria, and it could not be seen with a light microscope so it required an electron microscope). It also required a host cell (would only grow in culture if living cells present) -Insects are important vectors of plant disease. As they feed on plant tissues, aphids, leaf-hoppers, and many other insects spread viral diseases among plants. some plant viruses can replicate within certain insects as well as in the host plant. The virus first existed in insects and evolved to replicate in plants. nematodes and parasitic protozoa are vectors and transmit the virus through the roots. Viruses cannot penetrate plant cells unless they are damaged because plant cells have thick cell walls. Sap from virus-infected plants can be transmitted to healthy plants by injury from hands or agricultural equipment. Natural predation by animals can also transfer contaminated sap. plant viruses can be transmitted from one generation to the next through infected seeds or pollen. Natural and artificial asexual propagation can also pass plant viruses to succeeding generations. once infected the virus spreads through the plant body by passing through plasmodesmata (cytoplasmic channels that connect adjacent plant cells.) some defenses include growing callus-like tissue, which covers the injured area, through RNA silencing, using small interfering RNAs (siRNAs), which are stimulated in the presence of viral dsRNA. There are no known cures so infected plants are commonly burned. Vectors of plant virus transmission are controlled or eradicated following chemical or biological protocols. -typically don't kill plants (adversely affect growth and or appearance) -often spread by insect vectors

pathogens

Viruses that are pathogens are responsible for some very serious diseases, including Ebola hemorrhagic fever, rabies, influenza (flu), hepatitis, and AIDS. However, although viruses must infect cells to reproduce, most known viruses do not cause disease. Most viruses, including those that normally inhabit the human body, are harmless or are symbionts. Interestingly, viral genes from retroviruses comprise 8% of the human genome; some of these genes code for proteins essential to human life.

zoonotic disease

When an animal disease crosses the species barrier and infects humans, it is called a zoonotic disease. New strains of influenza are thought to infect one species, such as a species of birds. At first, they are not able to spread among mammals. However, because viruses can evolve, the avian strain can mutate and become virulent. The avian virus can also exchange RNA with a virus that has the genetic material necessary to spread among swine or other mammals. Swine can be susceptible to both avian and human strains of the virus. Inside a swine's body, the viruses can exchange bits of RNA and develop the ability to infect humans. -viruses that infect humans that originated in animals; most human viruses

regressive hypothesis

also known as reduction hypothesis Asserts that viruses are remnants of cellular organisms and evolved from small cells that were parasites in larger cells. Genes that they did not need, like those for protein synthesis, were gradually lost through evolution. This hypothesis is supported by certain bacteria (chlamydia and rickettsia) that are able to reproduce only inside the cells of their hosts also supported by the discovery that some giant viruses have genes that code components for protein translation. Proponents of the regressive hypothesis suggest that these giant viruses evolved from a free-living, complex cellular ancestor. Later, they became parasitic and gradually lost some of the genes necessary for protein synthesis. This hypothesis explains how viruses could have existed before their present-day hosts evolved. -Viruses may be remnants of cellular organisms -evolved from small cells that were parasites of larger cells -gradually lost unneeded genes like those for protein synthesis

progressive hypothesis

also known as the escape hypothesis viruses may have originated as mobile genetic elements such as transposons or plasmids (small, circular DNA fragments). Such fragments could have escaped from one cell and entered another cell through damaged cell membranes. According to this hypothesis, the origin of viruses may be traced back to archaeal, bacterial, protist, plant, or animal cells. Their multiple origins may explain why many viruses are species specific; perhaps they infect only those species that are closely related to the organisms from which they originated. This hypothesis is supported by the genetic similarity between some viruses and their host cells—a closer similarity than exists between one type of virus and another. -Viruses may have originated as escape pieces of nucleic acid from prokaryotic and eukaryotic cells: escaped from one cell and entered another

Re-emerging viruses

are those that have been almost eradicated and then suddenly recur, causing an epidemic, sometimes in a new geographic area. Many familiar diseases, such as influenza, malaria, tuberculosis, and bacterial pneumonias, continue to infect large numbers of people and increasingly reappear in forms that are resistant to drug therapy. Drug-resistant pathogens are a major challenge to public health and other health care professionals.

archaea viruses

attack archaea in a manner similar to bacteriophages *most having dsDNA and a head and tail structure similar to bacteriophages. Some archaeal viruses, however, have unique forms different from any other viruses (making them more morphologically diverse than bacteriophages).

how do bacteria protect themselves from phage infection?

bacteria produce restriction enzymes, enzymes that cut up the foreign DNA of the phage. This action prevents the phage DNA from duplicating. The bacterial cell protects its own DNA by slightly modyfying it after replication so that the restriction enzyme does not recognize the sites it would cut.