Viruses • Viruses infect every type of cell, including bacteria, algae, fungi, protozoa, plants, and animals
Viruses
• Seawater can contain 100 million viruses per milliliter • For many years, the cause of viral infections was unknown • Louis Pasteur postulated that a “living thing” smaller than bacteria caused these diseases
H1N1 influenza Virus
• also proposed the term virus, which is Latin for “poison.”
General Characteristics of Viruses
Viruses • Ivanovski and Beijerinck showed that a disease in tobacco was caused by a virus
• Are viruses alive? • better described as active or inactive
• Loeffler and Frosch discovered an animal virus causes foot‐and‐mouth disease in cattle • Filterable virus • These early researchers found that when fluids from host organisms passed through porcelain filters designed to trap bacteria, the filtrate remained infectious • This result proved that a cell‐free fluid could contain agents that could cause infection Tobacco mosaic virus
Role of Viruses in Evolution
• Obligatory intracellular parasites • Not cells: no cell wall, cytoplasm, or organelles • Require living host cells in order to multiply • Very small • Submicroscopic ‐ visible only with Electron Microscope
Smallpox virus
How small are viruses? VERY!!
• 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
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Distinctive Features of Viruses
Distinctive Features of Viruses
• Viruses bear no resemblance to cells and lack any of the protein‐ synthesizing machinery found in cells
Capsid Covering Envelope (not found in all viruses)
• Viral structure is composed of regular, repeating subunits that give rise to their crystalline appearance
Virus particle
• Contain only those parts needed to invade and control a host cell – external coating – core containing one or more nucleic acid strains of DNA or RNA – sometimes one or two enzymes
Nucleic acid molecule(s) (DNA or RNA) Central core Matrix proteins Enzymes (not found in all viruses)
Poliovirus
Viral Structure – Nucleic acid core • DNA or RNA, single or double stranded
– Capsid • protein coat It’s amazing that a particle made of just a nucleic acid core and a protein coat (capsid) can kill higher level organisms
Capsid and Envelope Functions • Protect the nucleic acid from the host’s acid‐ and protein digesting enzymes
Viral Structure • Envelope • Some viruses have an envelope, some don’t • Where does the envelope come from? The virus “steals” some of the host plasma membrane when it leaves the cell!
• Spikes • Some viruses have carbo‐protein complexes projecting from surface • Just like organisms, viruses are very diverse!
General Morphology of Viruses • Viral capsids come in many shapes: – Helical (“like a slinky toy”)
• Assist in binding and penetrating host cell • Stimulate the host’s immune system—good for us!
– Icosahedral – Complex (Remember they can also be enveloped or not)
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Helical capsid
Icosahedral capsid • Three‐dimensional, 20‐sided with 12 evenly spaced corners (geometry in action!) • Variation in capsomer number
• Naked helical virus – Ex. Tobacco mosaic virus – Nucleocapsid is rigid and tightly wound into a cylinder‐shaped package
– Polio virus 32 capsomers – Adenovirus (cold virus) 240 capsomers
• Enveloped helical virus – Ex. Influenza, measles, rabies – Nucleocapsid is more flexible
Comparison between (a) naked helical plant virus and an (b) enveloped helical human virus.
Icosahedral viruses can be naked or enveloped.
• Don’t worry about memorizing the different numbers of sides or capsomers. • Just be amazed at how geometric in shape viruses are
Complex viruses • Structure is more complex than helical and icosahedral viruses • Pox virus • Several layers of lipoproteins • Causes smallpox, cowpox, chickenpox
• Bacteriophage • Virus that attacks bacteria • Polyhedral head with tail fibers • Looks like a spacecraft
Comparison of the morphology of a naked virus, enveloped virus and a complex virus. (Viruses are DIVERSE!)
Icosahedral virus ‐ Hepatitis B
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Filamentous Helical Virus ‐ Ebola
Complex Virus ‐ Pox virus
Viral Nucleic Acid
Enzymes in the Virus Particle
• Viruses contain either DNA or RNA (but not both) • The Nucleic Acid can be in different forms: • • • •
Single stranded (ss) DNA Double stranded (ds) DNA (like us) ssRNA dsRNA
– Possess only the genes to invade and regulate the metabolic activity of host cells – Ex. Hepatitis B (4 genes) and herpes viruses (100 genes) compare to E. coli (4000) and humans (40,000) – No viral metabolic genes because the virus uses the host’s metabolic resources
• Enzymes for specific operations within their host cell – polymerases 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
HIV Reverse Transcriptase
Classifying Viruses • We 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
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Medically Relevant Virus Groups
Viral Multiplication Cycle
Viruses
• The viral life cycle (multiplication cycle) depends on the type of virus and species infected. • We’ll consider the following: – Animal viruses – Retroviruses – Bacteriophages
Viral Multiplication Cycle: Animal Viruses
Viral Multiplication Cycle: Animal Viruses
Phase
1. Adsorption/Attachment to the host cell
Short description of each phase in viral multiplication
Description
1.Adsorption/ Attachment
Viral particle (virion) attaches to specific receptor on outside of host cell
2.Penetration
Virion penetrates the host cell through the cell membrane and/or cell wall to enter the cytoplasm
3. Uncoating
Removable of the capsid and envelope, if present, exposes nucleic acid of the virus
If you can stop just ONE of these steps you can stop 4. Synthesis the multiplication of viruses! 5. Assembly 6. Release
Production of virion parts (nucleic acid, capsid, spikes…) Parts made in step 4. synthesis are put together like a toy at Christmas Virion exits the cell by lysing the cell or exocytosis and may steal some of the host membrane for its envelope
Viral Multiplication Cycle: Animal Viruses 2. Penetration of the animal virus into the cell occurs by endocytosis or fusion between the viral envelope and the host cell membrane. 3. Uncoating of the capsid and envelope, if present, exposes the nucleic acid of the virus
Naked Virus
• 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
Viral Multiplication Cycle: Animal Viruses 4. Synthesis of the viral nucleic acid and proteins occurs 5. Assembly of the viral proteins and nucleic acid into it’s capsid and envelope, if present. Making a new virus.
Enveloped Virus
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Viral Multiplication Cycle: Animal Viruses 6. Release: The mature virus leaves the cell. At this point it can obtain an envelope by budding off the host cell membrane.
Animal Viruses: Cytopathic effects(CPEs • Cytopathic effects (CPEs) is damage that occurs to the host cell due to a viral infection. It can include: – Inclusion bodies • Compacted masses of viruses
– Syncytia • Many cells fused into a large clump cell
– Chronic latent state • Virus lays in wait until it’s reactivated
– Transformation • Cancer: increased rate of growth, alterations in DNA, continuous cell division, loss of contact inhibition
Viral Multiplication Cycle : Retroviruses • Retroviruses are unique since they carry their own polymerase (reverse transcriptase) and transcribe RNA‐‐>DNA (1) Attachment/Attachment (2) Penetration (3) Uncoating HIV is the most famous (4) Synthesis‐by RT retrovirus that can integrate (5) Integration its DNA into our DNA and lay latent for years! (6) Synthesis (7) Assembly (8) Release
Viral Multiplication Cycle : Retroviruses • After uncoating retroviruses use their reverse transcriptase to make DNA from their RNA. • This DNA is then integrated into the genome of the host • From here the virus can lay latent for many years until it goes through the 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 host’s life – can remain latent in the cytoplasm
Persistent Infections • Provirus – A persistent infection in which the viral DNA is incorporated into the DNA of the host
• Can be an inactive viral infections or a retrovirus. – In inactive viral infections the virus will not replicate itself but through replication of its host cell. – Endogenous retroviruses are always in the state of a provirus.
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Persistent Infections • Chronic latent state – Viruses go into a period of inactivation in cells – May emerge under the influence of various stimuli
• Herpes simplex cold sore • Chickenpox Shingles (10‐20% of the time) • HIV AIDS
Viruses and Cancer • 15‐20% of cancers are virus‐induced • Several types of cancer are caused by viruses • 1908‐virologists transferred chicken leukemia by infecting healthy chickens with cell‐free filtrates from diseased chickens • 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
Viruses and Cancer
Viruses and Cancer
VIRAL CANCER: prostate cancer specimen, the brown‐ stained cells are the malignant cells that express virus proteins.
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.
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Bacteriophages
DNA & RNA Oncogenic Viruses in Humans • DNA Oncogenic Viruses • papillomavirus‐‐> cervical cancer • Epstein‐Barr virus‐‐>Burkitt’s lymphoma, nasopharyngeal carcinoma • Hepatitis B‐‐>liver cancer
• RNA Oncogenic Viruses • Human T‐cell leukemia virus • Human herpesvirus 8 (HHV8)Kaposi’s Sarcoma
Bacteriophage Multiplication Cycle Lytic Cycle (1) Absorption (2) Penetration (3) Biosynthesis (4) Maturation/Assembly (5) Release
Lysogenic Cycle (1) Absorption (2) Penetration (3) Integration of phage DNA into host DNA (4) Binary Fission (5) Occasionally, excision of phage DNA initiates lytic cycle at stage (3)
Temperate Phages can “switch” between the Lytic and Lysogenic Cycles Lysogenic Cycle (1) Absorption (2) Penetration (3) Integration of phage DNA into host DNA (4) Binary Fission (5) Occasionally, excision of phage DNA initiates lytic cycle at stage (3), biosynthesis
• From the Greek phage meaning “eating” • Every bacterial species is parasitized by various specific bacteriophages • Often make the bacteria they infect more pathogenic
The lytic cycle of Viral Infection by Bacteriophages Lytic Cycle (1) Absorption (2) Penetration (3) Biosynthesis (4) Maturation/Assembly (5) Release
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 from its temperate phage – Corynebacterium diphtheriae – diphtheria toxin – Vibrio cholerae – cholera toxin – Clostridium botulinum – botulinum toxin
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Isolation & Cultivation of Viruses • Can you inoculate culture media with viruses and expect them to grow? (Like we do with bacteria?) • Viruses always need a living host to replicate. • Bacterial viruses • Grow bacteria, then infect with virus • Animal viruses • Infect living animals • Infect embryonated eggs • Infect animal cell cultures
Identifying a Virus • Similar to bacteria: • Immunological methods – Draw blood and analyze antibody‐antigen interactions (using a specific antibody, you can identify the specific virus that binds with it)
Isolation & Cultivation of Viruses • A monolayer of monkey kidney cells is a cell culture enabling the propagation of viruses. • Plaques: areas where virus‐ infected cells have been destroyed and show up as a clear, well‐ defined patches in the cell sheet
Viruses and Chemotherapy • Difficult in viruses • It is usually difficult to treat viral infections with drugs since the virus relies on the metabolic machinery (enzymes) of the host. In order to inhibit/kill the virus with a drug you may inhibit/kill the host cell in the process.
• Examine genetic sequences • Signs and symptoms
Treatment of Viral Infections • Antibiotics and sulfa drugs ineffective – Antibiotics attack membrane bound structures. Viruses lack membranes – Sulfa drugs inhibit metabolic pathways. Viruses lack these.
Treatment of Viral Infections • Methods – Inhibit viral penetration: Amantidine – Inhibit DNA synthesis – Inhibit viral protein synthesis: Acyclovir – Interfere with viral protein modification: Protease inhibitors (Saquinavir) – Inhibit viral enzymes: AZT – Prevent viral production: Interferon
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Prevention of Viral Infections Vaccination ( Smallpox, Mumps, Polio) • Interferon
•
• Cell protein produced by infected cells. Protects other cells by inhibiting viral replication. • Protection occurs naturally • Can be artificially produced
• Sanitation
Viruses and Human Health • Viruses with high mortality rates: rabies, AIDS, Ebola • Viruses that cause long‐ term debility: polio, neonatal rubella • Viruses with possible connection to chronic afflictions with an unknown cause: type 1 diabetes, multiple sclerosis, various cancers, Alzheimer’s disease, obesity Polio
Parasitic Particles: Prions • PRIONs (pree‐ons): proteinaceous infectious particle • 9 animal diseases caused by PRIONs: “mad‐cow” disease, scrapie‐ sheep, kuru, Creutzfeldt‐Jakob disease • Spongiform encephalitis: large vacuoles in brain • Fatal neurological degeneration, fibril deposits in brain, and loss of brain matter • Prions only destroyed by incineration or autoclaving in 1 N NaOH
Parasitic Particles: Prions • Cellular PrP protein • Made by all mammals • Normal structure with α‐helices called cellular PrP
• Prion PrP • Disease‐causing form with β‐pleated sheets called prion PrP
• Prion PrP changes shape of cellular PrP so it becomes prion PrP‐this is it’s form of replication!
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