Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Middle East respiratory syndrome wikipedia , lookup
Ebola virus disease wikipedia , lookup
West Nile fever wikipedia , lookup
Human cytomegalovirus wikipedia , lookup
Marburg virus disease wikipedia , lookup
Orthohantavirus wikipedia , lookup
Hepatitis B wikipedia , lookup
Henipavirus wikipedia , lookup
Influenza A virus wikipedia , lookup
General Virology VIRUS STRUCTURE Virion vs virus • Virion is the infectious particle – composed of nucleic acid, protein capsid, +/- envelope – may be extracellular or intracellular • Virus is any stage of infection Capsid • Functions – Protection of NA – Attachment for naked viruses – Enzyme • Helical vs Icosahedral Symmetry • Tobacco mosaic virus is a ssRNA virus composed of 6000 nucleotides. The capsid is made of 2100 copies of a single protein subunit that contain 158 amino acids. Icosahedral symmetry • 20 identical equilateral triangles • Structural units on faces to give morphological capsomers Envelope • Attachment • Entry • Assembly- matrix proteins • Release • Proteins are viral • Lipids are host • If the membrane envelope is destroyed, the virus becomes noninfectious. Why? Genome - DNA or RNA • Sense: Positive-sense, Negative-sense, Ambi-sense Number: single, segmented • strandedness - (single) (double) • linear or circular, partial double stranded circle Proteins • structural proteins • non-structural virion proteins – transcriptase, – protease – integrase WHAT IS A VIRUS? Viruses : acellular organisms Genomes obligately replicate inside host cells using host metabolic machinery to different extents, to form a pool of components which assemble into particles called virions. F A virus differs from a cell in three fundamental ways: i A virus usually has only a single type of nucleic acid serving as its genetic material. This can be single or double stranded DNA or RNA; ii Viruses contain no enzymes for energy metabolism, thus cannot make ATP; iii Viruses do not encode sufficient enzymatic machinery to synthesize their component macromolecules, specifically, no protein synthesis machinery. ONE STEP GROWTH CURVE • • • • • 1939- Ellis and Delbruck: Infection with a high multiplicity of infection (MOI): ratio of virus to host cell – Simultaneous infection – Single replication cycle Sample at time intervals by plaque count for plaque-forming units (PFU), Identification of latent phase Determination of burst size/viral yield Detection F Viruses cannot be grown on sterile media, but require the presence of specific host cells. Virology and Human Disease Tuesday, August 12 Viruses • Infectious agents found in virtually all life forms including humans, animals, plants, insects, and bacteria • Virion Structure – genetic material (either DNA or RNA, double or single stranded, linear or circular) – Protein coat (capsid) surrounding genetic material • Rod (helical), polyheldral, or more complex – with or without a lipid envelope • Viruses are not free living – Unable to reproduce themselves outside of a living cell – Transmit their genetic information from one cell to another • Viruses often damage or kill the cells that they infect Figure 18.1 Size of a virus, a bacterium, and a eukaryotic cell Figure 18.2 Viral structure Rod-shaped Icosohedral Enveloped Complex Figure 18.3 A simplified viral reproductive cycle • Obligate intracellular parasites – Rely on the equipment of cell to replicate – Host range = viruses can only infect certain cell types • Dependant on recognition of host cell receptor • Entry, uncoating, replication, release, cell lysis, infection of new cells Bacterial Viruses • Bacteriophages – phages – Virulent phage • Reproduces only by a lytic life cycle – Lytic – results in the lysis of the host cell » Phage T4 (infects E. coli) – Temperate phage • Reproduce by either lytic or lysogenic life cycle – Lysogenic - replication does not destroy host » Phage λ (infects E. coli) Figure 18.4 The lytic cycle of phage T4 Figure 18.02x2 Phages Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a temperate phage Animal Viruses • Virus Classification – Genome • • • • DNA or RNA (+/- sense) Size (kb) Single or double stranded, linear or circular # segments, sequence – Morphology • Virion size and shape • Plus or minus envelope • Capsid symmetry and structure – Protein – Biological properties – Physical and chemical properties Figure 18.6 The reproductive cycle of an enveloped virus • Glycoproteins on envelope recognize receptor on host cell • Viral envelope fuses with cell membrane • Genome and capsid enter the cell • Genome is copied – New RNA genomes – mRNA translated into capsid proteins and glycoproteins for envelope • Capsid assembly • Virus buds from cell Viral Entry: receptors and fusion • Initial attachment – binding to host cell – Viral surface protein recognizes receptor • Carbohydrates • Lipids • Proteins – transmembrane • Entry – virion conformational change in response to receptor or pH – Location of entry • Plasma membrane (neutral pH) • Endosomal membrane (acidic pH) – Type of entry • Fusion – enveloped viruses • Penetration – nonenveloped viruses Modification of Host Cell Function • Effects on cellular translation – Viruses activate PKR (cellular kinase) which suppresses cellular translation • Effects through receptor binding – Viral receptor binding may mimic the effects of the natural ligand • Induction of cell proliferation – Viruses need replication machinery and induce cells to enter the cell cycle upon infection • Effects on cellular RNA processing – Inhibit cellular transcription – Degrade cellular mRNAs – Alter RNA processing or export Viral Replication • DNA viruses – Replicated and transcribed similar to host cell – Examples = adenovirus, herpes virus • RNA viruses – Plus stranded • Translated directly (therefore RNA is infectious) • Virally encoded RNA-dependant RNA polymerase (RdRp) synthesizes more genomic RNA and mRNA for proteins • Example = poliovirus – Minus stranded • Must be transcribed by RdRp for replication and transcription of viral mRNA • Enzyme is carried by the virus into the cell during infection • Example = influenza, measles • Retroviruses – HIV Figure 18.02x1 Adenovirus Virus infects the upper respiratory tract (common Figure 18.x6 Herpes • 8 herpesviruses infect humans • Human diseases – – – – – Oropharyngeal and genital lesions – herpes simplex Chickenpox – varicella zoster virus Congenital microcephaly – CMV (growth retardation) Burkitt’s lymphoma – EBV (childhood tumor) (mononucleosis) Kaposi’s sarcoma – KSHV Viral Replication • DNA viruses – Replicated and transcribed similar to host cell – Examples = adenovirus, herpes virus • RNA viruses – Plus stranded • Translated directly (therefore RNA is infectious) • Virally encoded RNA-dependant RNA polymerase (RdRp) synthesizes more genomic RNA and mRNA for proteins • Example = poliovirus – Minus stranded • Must be transcribed by RdRp for replication and transcription of viral mRNA • Enzyme is carried by the virus into the cell during infection • Example = influenza, measles • Retroviruses – HIV Figure 18.x3 Paralytic Polio • Summer time epidemic disease – summer of 1885 in England • More than 20,000 cases each year in the U.S. • Infection spreads to brain and CNS and replicates in muscle cells, spreading to motor neurons and causing paralysis • Poliovirus was cultured – Robbins, Enders, and Weller received the Nobel Prize in 1954 • First vaccine developed in 1955 by Salk was inactivated virus • Live attenuated vaccine (Sabin) was approved in 1961 and eliminated virus from the Americas Viral Replication • DNA viruses – Replicated and transcribed similar to host cell – Examples = adenovirus, herpes virus • RNA viruses – Plus stranded • Translated directly (therefore RNA is infectious) • Virally encoded RNA-dependant RNA polymerase (RdRp) synthesizes more genomic RNA and mRNA for proteins • Example = poliovirus – Minus stranded • Must be transcribed by RdRp for replication and transcription of viral mRNA • Enzyme is carried by the virus into the cell during infection • Example = influenza, measles • Retroviruses – HIV Simpler infectious agents - Prions • Infectious proteins – causing degenerative brain diseases – Scrapie in sheep – Mad cow disease in cows – Creutzfeldt-Jakob disease in humans • Prions are misfolded forms of normal proteins in the brain • Prions induce normal proteins to convert to prion form triggering a chain reaction that increases their numbers Figure 18.10 A hypothesis to explain how prions propagate What are viruses? • Small obligate intracellular parasites • Virion – Complete virus particle : nucleic acid + protein coat, which may be surrounded by an envelope – It is the form in which the virus moves between cells or hosts • Viral Genome – EITHER RNA or DNA genome surrounded by a protective virus-coded protein coat (Capsid) • Propagation depends on specialized host cells supplying the machinery for replication, metabolism and biosynthesis • The DNA or RNA genome may be : – ss – single stranded or – ds – double stranded • Genomes may be either: – (+) sense: Positive-sense viral RNA is identical to viral mRNA and thus can be immediately translated into protein by the host cell. OR – (-) sense: Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA polymerase before translation. Definitions • Bacteriophage – Virus that infects prokaryotic (bacterial) cells. • Nucleocapsid: – viral nucleic acid + the protein coat that encloses it. – Represents the packaged form of the viral genome. Viral Structure - Overview Nucleic acid Capsid Nucleocapsid Envelope protein Viral Membrane envelope** Spike protein protein Viral Structure • Varies in size, shape and symmetry • VIP for classification • 3 types of capsid symmetry: – Cubic (icosahedral) • Has 20 faces, each an equilateral triangle. Eg. adenovirus – Helical • Protein binds around DNA/RNA in a helical fashion eg. Coronavirus – Complex • Is neither cubic nor helical eg. poxvirus http://micro.magnet.fsu.edu/cells/virus.html Viral Structure Figure 1 An array of viruses. (a) The helical virus of rabies. (b) The segmented helical virus of influenza. (c) A bacteriophage with an icosahedral head and helical tail. (d) An enveloped icosahedral herpes simplex virus. (e) The unenveloped polio virus. (f) The icosahedral HIV with spikes on its envelope. Viral Replication • When a virus infects a cell, nucleic acid must be uncoated and gain access to metabolic machinery of cell. • Virus life cycle is characterized by: – attachment – penetration, with entry of nucleic acid into cell – early expression of virus genes (either directly by translation, if virus contains "+" RNA, or indirectly after transcription and then translation) – replication of virus nucleic acid – synthesis of new virion components – packaging and assembly of new virions – exit from cell • Attachment – specific binding of a virion protein (the anti-receptor) to a constituent of the cell surface (the receptor) • e.g. hemagglutinin of influenza virus • some complex viruses (HSV) may have more than one species of anti-receptor molecule • Penetration – energy-dependent step – occurs almost instantaneously after attachment • After the virus attaches to the host cell, it can enter the cell by several mechanisms: – Transfer of the entire viral particle across the cell membrane by endocytosis – Transfer of only the viral genome through the cell membrane – Fusion of the viral envelope with the host cell membrane • Uncoating – at same time as penetration or shortly after – separation of viral nucleic acid (n.a.) from outer structural components • Released as: – free n.a. (picornaviruses) – as nucleocapsid (reoviruses) = may need acidic pH in endosome – viruses only infectious agent for which dissolution of infecting agent obligatory step in replicative pathway • Expression of viral genome and synthesis of viral components • After the viral nucleic acid is released inside the host cell: – The transcription and translation processes of the host cell are redirected for the production of viral proteins and nucleic acids – The different types of nucleic acid genomes are expressed and replicated in several ways: • DNA genomes undergo replication-using processes similar to cellular replication • RNA genomes may be +ssRNA; Can be read directly as an mRNA or reverse transcribed by reverse transcriptase into DNA • RNA genomes may also be -ssRNA; The RNA must first be used as a template to form +mRNAs Assembly and Release • Components of capsid synthesis directed by late genes • Assembly of enveloped viruses needs interaction with plasma membrane which has been modified • Final stage of infection • Enveloped viruses released gradually by budding or exocytosis • Naked viruses accumulate in cytoplasm and released during lysis Prions • Prions – – – – – Infectious particles that are entirely protein. No nucleic acid Highly heat resistant Animal disease that affects nervous tissue Affects nervous tissue and results in • Bovine spongiform encepahltits (BSE) “mad cow disease”, • scrapie in sheep • kuru & Creutzfeld-Jakob Disease (CJD) in humans • Vaccination Edward Jenner developed the first vaccine in 1798 for smallpox Edward Jenner Microorganisms as the Medical Enemy • the other side of the picture – influenza: 1918-1919 • killed 20 million people Micrograph of Influenza virus Medical Microbiology Successes • smallpox – last known disease in the world was documented in 1977 – believed at one time prior to eradication, that 80% of the world’s population would be affected by smallpox Smallpox virus Clinical Manifestation of Smallp Prions • prions – contain only protein – causative agent for some neurodegenerative diseases in humans and animals Prion What is a Virus? • Definition: “Poison, venomous secretion” • Obligate intracellular parasites – Dependent on other “HOST” organisms – Produced from assembly of pre-formed components rather than “growing” or division • Viruses are composed of nucleic acid (DNA or RNA) surrounded by a protective protein coat (Capsid) • There are viruses that can infect every type of living cells (animal, plant, bacteria, etc.) • Are they ALIVE?? History of Virology • 1796 - Edward Jenner vaccinated a boy, James Phipps, with material from a cowpox lesion on the hand of a milkmaid. The boy became immune to small pox! • 1885 - Louis Pasteur experiments with rabies vaccination. He originates the terms "virus" and "vaccination" and develops the scientific basis for Jenner's experimental approach to vaccination Louis Pasteur (1822-1895) History of Virology • 1892 –Dmitri Iwanowski shows that extracts from diseased tobacco plants can transmit disease to other plants after passage through filters fine enough to retain the smallest known bacteria. – This is generally recognized as the beginning of Virology! But, nobody understood the significance until… Dmitri Iwanowski (1864-1920) • 1898 – Beijernick made the same discovery, but suggested that the pathogen is a distinct agent, not just really small bacteria Martinus Beijerinick (1851-1931) History of Virology • 1911 - Francis Peyton Rous demonstrated that a virus (Rous sarcoma virus) can cause cancer in chickens. Rous is the first person to show that a virus could cause cancer in animals. • 1935 - Wendell Stanley crystallizes tobacco mosaic virus and shows that it remains infectious. Polio Francis Peyton Rous (1879-1970) TMV History of Virology • 1937- The first to propagate yellow fever virus in chick embryos and successfully produced an attenuated vaccine. • 1940 - Helmuth Ruska used an electron microscope to take the first pictures of virus particles. • 1949 - John Enders, Thomas Weller and Frederick Robbins were able to grow poliovirus in vitro using human tissue culture. Max Theiler (1899-1972) History of Virology • 1955 -The Salk vaccine against polio is introduced into general use. Jonah Salk • 1979 - The W.H.O. officially declared smallpox to be completely eradicated! – First microbial disease ever to be completely eliminated Smallpox EM History of Virology • 1981 – First recorded cases of AIDS, mostly in homosexual populations • 1983 - the discovery of human immunodeficiency virus (HIV), the causative agent of AIDS. Virus Classification • A universal system for classifying viruses, and a unified taxonomy, has been established by the International Committee on Taxonomy of Viruses (ICTV) since 1966. The system makes use of a series of ranked taxons: • Family (-viridae) (eg. Retroviridae) – Subfamily (-virinae) (e.g. Orthoretrovirinae) • Genus (-virus) (e.g. Lentivirus) • Currently there are 30,000 - 40,000 known viruses • BUT – there are 106 virus particles per milliliter of seawater – most are unknown! Virus Size • Most viruses vary from 30-200nm in length Viral Composition • All virions (extracellular virus) are composed of: – Nucleic acid genome core - the genetic material! – Capsid composed of viral proteins to protect genome Viral Capsids • Capsids are composed of repeating units of Viral proteins, and serve to protect the viral genome • Generally, capsids are formed by very regular, symmetric patterns • Most capsids are either HELICAL or ICOSAHEDRAL Capsid structure: Helical • Repeating structure of proteins surrounding viral genome Example: Tobacco Mosaic Virus (TMV) Viral Genome (DNA or RNA) Capsid Proteins Capsid Structure: Icosahedral • An Icosahedron is a 20sided structure • Viral genome at the core Example: Adenovirus Capsid Proteins (Genome in the center) Viral Structure: Envelope Some Viruses also contain • Envelope: – Lipid bilayer (membrane) derived from host membranes, surrounding capsid – Contains Viral Glycoprotein “spikes” • Matrix: – Protein layer between Capsid and Envelope Viral Structure: Envelope Matrix Examples: Influenza virus HSV (herpes simplex) Glycoproteins Capsid The Basics • Entry – The virus must find/enter into a cell • Making copies of virus parts – Replication of viral genome – Gene expression of viral genes • Exit – Assembly of viral components – Leave cell; go on to infect new cells Entry - Steps • Attachment – Tethering the virus to the cell • Penetration – Crossing the plasma membrane • Uncoating – Releasing the genome Entry - Attachment • (Glyco)proteins on the surface of the virus bind to receptor proteins on cell surface • “Tethers” the virus to cell surface Entry - Penetration • Crossing the PM • Several different strategies: – TRANSLOCATION of the entire virion across the cell membrane; eg. Poliovirus – ENDOCYTOSIS of the virus into intracellular vacuoles; eventually into the cytoplasm. Eg. Adenovirus (unenveloped), Influenza virus (enveloped) – FUSION of the viral envelope with the cell membrane. Requires the presence of a viral fusion protein in the virus envelope; eg. Retroviruses, Herpesviruses Entry - Uncoating • Release of the GENOME by (partial) removal of capsid proteins surrounding genome • Depending on site of replication, genome may be released into cytoplasm or translocated into the nucleus Entry – Different Strategies http://www.mssm.edu/micro/vintro3.gif Replication/Gene Expression After the genome has entered the host cell, two simultaneous steps occur: • Replication of the genome • Transcription and translation of viral genes (gene expression) How? Why are these steps necessary? DNA Replication - Overview http://library.thinkquest.org/04apr/00217/images/content/74-Summary-DNA-Replication.jpg Gene Expression - Overview http://stemcells.nih.gov/StaticResources/info/scireport/images/figurea6.jpg RNA processing/splicing - Overview • Transcribed RNA is modified to make mature mRNA: 5’ cap 3’ poly-A tail Replication/Gene Expression Two things to consider in replication (dependent on type of viral genome) • Location – Nucleus – cytplasm • Type of enzymes used for replication and/or gene expression – – – – DNA polymerase DNA-dependent RNA Polymerase RNA-dependent RNA Polymerase Reverse transcriptase (RNA-dependent DNA polymerase) SOME VIRUSES NEED TO BRING THE NECESSARY ENZYMES WITH THEM! Gene Expression • Gene expression is tightly regulated in viruses in order to control viral replication • Viruses need to use cell machinery (i.e. ribosomes, tRNAs, etc.) to synthesize proteins • Viral mRNAs contain “normal” cellular control signals – ribosome-binding sites – splice signals – polyadenylation signals Gene Expression • Usually, viruses first express low levels of regulatory proteins (enzymes, etc.) • Later in infection, viruses express tons and tons of structural proteins – i.e. proteins that get included in virus (e.g. capsid protein, glycoproteins, etc.) Gene expression • Most viral proteins get made/assembled in cytoplasm • Envelope proteins get made into ER and “travel” through endomembrane system - Glycoprotein Assembly • Involves the assembly of all the components necessary for the formation of the mature virion • Assembly may be spontaneous, or driven with the help of cellular/viral proteins that are not components of the mature virions • The site of assembly varies for different viruses: – Picornaviruses, Poxviruses, Reoviruses - In the cytoplasm. – Adenoviruses, Herpesviruses, Parvoviruses - In the nucleus. – Retroviruses - On the inner surface of the cell membrane. Egress Non-enveloped Viruses: • Cell lysis - releasing the newly made virions in the extracellular matrix Enveloped Viruses: • Budding – acquiring a viral envelope – This process is generally coupled with egress, eg. Capsid budding into the PM Egress - Budding HIV: Putting it all together Typical lifecycle of Unenveloped virus: Putting it all together Typical lifecycle of Enveloped virus: Lytic vs. Lysogenic Many Transcription Strategies Don’t worry about virus names (in red). Introduction to Virology • The cultivation of viruses is complex and includes three common methods – Chicken egg culture – Cell culture – Animal inoculation ATTACHMENT Click after each step to view process PENETRATION UNCOATING HOST FUNCTIONS Transcription Translation REPLICATION VIRAL LIFE CYCLE ASSEMBLY (MATURATION) RELEASE 93 MULTIPLICATION 1. Protomers 2. Capsomers 4. MatureC apsid 3. ProCapsid PRION Protein Normal Folding Pattern of PrP Alternate Folding Pattern of PrPSc