Virology+I.....pdf

VIROLOGY Virology the study of viruses Virus A submicroscopic, parasitic, filterable agent consisting of nucleic acid surrounded by a protein coat Obligatory intracellular parasites ▪ Require living host cells to multiply These sound like bacteria...? General characteristics...

VIROLOGY Virology the study of viruses Virus A submicroscopic, parasitic, filterable agent consisting of nucleic acid surrounded by a protein coat Obligatory intracellular parasites ▪ Require living host cells to multiply These sound like bacteria...? General characteristics of Viruses Specific to Viruses Multiply inside living cells through synthesis of the cell o Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other cells. Contain single type of nucleic acid o DNA or RNA Contain a protein coat that surrounds nucleic acid o Can be enclosed by an envelope No ribosomes No ATP-generating mechanism Must take over host cell to use their metabolic machinery Host Range The spectrum of host cells a virus can infect Cell Tropism Most viruses infect only specific types of cells in one host Determined by specific host attachment sites and cellular factors Can only be attached to hosts dues to required factors for replication Chemical interactions Specific receptor sites Bacteriophages—viruses that infect bacteria Receptor sites are part of the cell wall of the host Animal Cell hosts Receptor sites on plasma membrane Viral Size Range from 20 nm to 1000 nm in length Depends on electron microscopy Outer surface held together by two complementary components Hydrogen bonds Depends on size Ratios Viral Structure Virion—complete, fully developed viral particle Nucleic acid Capsid Envelope Spikes General Morphology Helical viruses Polyhedral viruses Enveloped viruses Complex viruses Nucleic Acid DNA or RNA can be single- or double- stranded linear or circular Nonenveloped Polyhedral Virus Capsid protect nucleic acid Polyhedral viruses Animal, plant, and bacterial viruses Many sided Capsid shaped like an icosahedron A regular polyhedron with 20 triangular faces 12 corners Capsid Protects nucleic acid protein coat made of capsomeres (subunits) Some have one type Some have different sized types Shapes exterior depending on virus Helical Virus Helical structure Nucleic acid inside hollow, cylindrical capsid Long rods Rigid or flexible Rabies and ebola Envelope and Spikes Envelope lipid, protein, and carbohydrate coating on some viruses from animal cells Envelope determined by nucleic acid and materials from host cell Spikes projections from outer surface Carbohydrate-protein compelxes Attaches to host cell for identification Enveloped Helical Virus Enveloped viruses Complex viruses Complicated structures Bacteriophage Some have capsids that have additional attachments Sheath: a protective covering that has a cylindrical structure that acts like a contractile spring. Core of the tail and responsible for injection of viral genetic material into host. Pin: a a stabilizing element Initial attachment of the bacteriophage to bacterial cell surface. Makes contact with receptors on bacterial surface Baseplate: is a structural component that anchors tail fibers and the pin Primary attachment site for bacteriophage to the bacterial cell. Also essential to the sheath as they are connected and infect host cells. Tail fiber: appendages that help attach a virus to a host cell. Binds itself to the cell wall. Poxviruses Taxonomy of Viruses Based on geonomics and structure Genus names end in “-virus” Family names end in “–viridae” Order names end in “–ales” Viral species: a group of viruses sharing the same genetic information and ecological niche (host) Descriptive common names are used for species Subspecies are designated by a number Families of Viruses That Affect Humans Families of Viruses That Affect Humans Isolation, cultivation, and identification of viruses Viruses cannot replicate without a host cell Finding a host can be complicated due to their detection, enumeration, and identification Easiest host is bacteriophages, easy to grow Growing bacteriophage in laboratory Viruses must be grown in living cells Bacteriophages are grown in bacteria Bacteriophages form plaques, which are clearings on a lawn of bacteria on the surface of agar Each plaque corresponds to a single virus; can be expressed as plaque- forming units (PFU) Growing animal viruses in laboratory In living animals In embryonated eggs Virus injected into the egg Viral growth is signaled by changes or death of the embryo Growing animal In cell cultures Tissues are treated with enzymes to separate cells viruses in Virally infected cells are detected via their deterioration, known as the cytopathic effect (CPE) laboratory Continuous cell lines are used Viral Identification Cytopathic effects Serological tests Western blotting—reaction of the virus with antibodies Nucleic acids Restriction Fragment Length Polymorphisms (RFLPs) Polymerase Chain Reaction (PCR) VIROLOGY II Viral Virion contains only a few genes needed for synthesis Components Supplied by Host: Protein synthesis machinery Multiplication Viral Genes: Encode structural Ribosomes tRNA components (e.g., capsid proteins) and enzymes for the Energy production viral life cycle. Virion Size: Smallest non- Enzyme Function: enveloped viruses lack enzymes. Synthesized and active only Larger virions may contain within the host cell. enzymes or mRNA. Resource Utilization: Viruses Enzyme Functions: Aid in: rely on host cell resources for Penetrating host cell replication. Replicating viral nucleic acid Viral Enzymes: Focused on Initiating protein synthesis replicating viral nucleic acid. Viral Multiplication For a virus to multiply: It must invade a host cell It must take over the host’s metabolic machinery One viron can replicate to several or even thousands of similar viruses in a single host. One-step growth curve The data are obtained by infecting every cell in a culture and then testing the culture medium and cells for virions and viral proteins and nucleic acids Eclipse period: the virus is not detectable in the extracellular environment because it is inside the host cell, where it uncoats and begins to replicate its genetic material and produce proteins. Multiplication of Bacteriophages Lytic cycle Phage causes lysis and death of the host cell Lysogenic cycle Phage DNA is incorporated in the host DNA Phage conversion Specialized transduction Can revert to Lytic cycle T-even bacteriophages, such as T2, T4, and T6, are a group of well-studied viruses that specifically infect and lyse Escherichia coli bacteria. Lytic Cycle 1. Attachment: phage attaches by the tail fibers to the host cell 2. Penetration: phage lysozyme opens the cell wall; tail sheath contracts to force the tail core and DNA into the cell 3. Biosynthesis: production of phage DNA and proteins 4. Maturation: assembly of phage particles 5. Release: phage lysozyme breaks the cell wall  Lysogenic cycle Lysogenic phages (temperate phages) Can proceed to lytic cycle Or can incorporate their DNA into host cell DNA to begin lysogenic cycle Lysogeny The phage remains latent (inactive) Bacterial host cells are known as the lysogenic cells Lysogenic cycle 1. Penetration 2. Linear phage DNA forms a circle 3. Circle multiply and transcribed Alternatively, the circle can recombine with and become part of the circular bacterial DNA , inserted phage DNA is now called prophage Prophage gene repressed by x2 repressor proteins, products of phage genes Repressors stop transcription by binding to operators Phage genes direct synthesis Release new virions that are turned off 4. Production of new phage, lysis cell (lytic cycle) Replicates the prophage DNA Prophage remains latent within the progeny cells. 5. Excision (popping-out) of the phage DNA, initiation of the lytic cycle Important results of lysogeny 1. Lysogenic cells are immune to reinfection by the same Phage 2. Phage conversion, the host cell may exhibit new properties 3. Makes specialized transduction  Specialized transduction Initiated during lysogenic stage Specific bacterial genes transferred to another bacterium via a phage Changes genetic properties of the bacteria Example: gal-gene Galactose-positive host passes to galactose-negative cell  Generalized transduction is a process by which RECOMBINATION bacterial DNA is transferred from one bacterium to another via a bacteriophage (virus). Phage protein coat Phage DNA Process: Bacterial chromosome Infection: A bacteriophage infects a donor bacterial cell. Donor A phage infects the cell donor bacterial cell. Lytic Cycle: The phage replicates and assembles within the host, often incorporating fragments of bacterial DNA. Phage DNA and proteins are made, Bacterial DNA Packaging: During phage and the bacterial chromosome is assembly, random pieces of the host's DNA can Generalized broken into pieces. be mistakenly packaged into new phage particles. Occasionally during phage assembly, pieces of bacterial DNA are pack- transduction Release: The infected cell lyses, releasing new aged in a phage capsid. Then the phages that contain both phage and bacterial DNA. Phage donor cell lyses and releases phage DNA Transduction: Bacterial particles containing bacterial DNA. A phage carrying DNA The phages can infect a new recipient bacterial Recipient bacterial DNA infects cell. cell a new host cell, the recipient cell. During infection, the bacterial DNA from the Donor Recipient donor is injected into the recipient. bacterial bacterial Recombination can DNA DNA Recombination: The injected bacterial DNA Recombinant occur, producing a may recombine with the recipient's cell reproduces recombinant cell with chromosome, leading to genetic variation. normally a genotype different from both the donor Significance: Contributes to genetic diversity and horizontal gene transfer in bacteria. Many cell and recipient cells. divisions Bacteriophage and Animal Viral Multiplication Compared Multiplication of Animal Viruses 1. Attachment: viruses attach to the cell membrane 2. Entry by receptor-mediated endocytosis or fusion 3. Uncoating by viral or host enzymes 4. Biosynthesis: production of nucleic acid and proteins 5. Maturation: nucleic acid and capsid proteins assemble 6. Release by budding (enveloped viruses) or rupture Attachment Attaches to complementary receptor sites Receptor cites are made of proteins and glycoproteins Spike attaches to receptors Entry Uncoating Follows attachment Receptor-mediated endocytosis Separation of viral nucleic acid from protein coat Plasma membrane fold inwards to form vesicles Animal cells uncoat by lysosomal enzymes, which degrade the protein of Enveloped viruses enter through fusion the viral capsid Biosynthesis Adenoviridae Double-stranded DNA, nonenveloped Respiratory infections in humans Conjunctival infection Gastroenteritis Tumors in animals Poxviridae Double-stranded DNA, enveloped Cause skin lesions Vaccinia and smallpox viruses (Orthopoxvirus)  Herpesviridae Double-stranded DNA, enveloped HHV-1 and HHV-2—Simplexvirus; cause cold sores HHV-3—Varicellovirus; causes chickenpox HHV-4—Lymphocryptovirus; causes mononucleosis HHV-5—Cytomegalovirus HHV-6 and HHV-7—Roseolovirus HHV-8—Rhadinovirus; causes Kaposi’s sarcoma Papovaviridae Double-stranded DNA, nonenveloped Papillomavirus Causes warts Can transform cells and cause cancer Hepadnaviridae Double-stranded DNA, enveloped Hepatitis B virus Use reverse transcriptase to make DNA from RNA Viral Identification Cytopathic effects Serological tests Western blotting—reaction of the virus with antibodies Nucleic acids Restriction Fragment Length Polymorphisms (RFLPs) Polymerase Chain Reaction (PCR) VIROLOGY III Biosynthesis of RNA Essentially same as DNA viruses RNA multiply in the host cell’s cytoplasm 4 types of nucleic acid RNA viruses 1. ssRNA (+ sense strand): Viral RNA serves as mRNA for protein synthesis 2. ssRNA (-antisense strand): Viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis 3. dsRNA (double stranded RNA) 4. Retrovirus Differences are in how mRNA and viral RNA are produced RNA-dependent RNA polymerase Enzyme isn’t encoded in any cell’s genome Viral genes are made by enzyme in host cell Catalyzes the synthesis of another strand of RNA Complementary strand base sequence to original infecting strand After viral RNA and proteins have been synthesized, follows maturation ssRNA (+ sense strand) Viral RNA serves as mRNA for protein synthesis Picornaviridae Single-stranded RNA, + strand, nonenveloped Enterovirus Poliovirus and coxsackievirus Rhinovirus Common cold Hepatitis A virus Togaviridae Single-stranded RNA, + strand, enveloped Alphavirus Transmitted by arthropods; includes chikungunya Rubivirus Rubella ssRNA (-antisense strand) Viral RNA is transcribed to a + strand to serve as mRNA for protein synthesis Rhabdoviridae Single-stranded RNA, − strand, one RNA strand Lyssavirus Rabies Numerous animal diseases dsRNA (double stranded RNA) Reoviridae Double-stranded RNA, nonenveloped Reovirus (respiratory enteric orphan) Rotavirus (mild respiratory infections and gastroenteritis) Multiplication and Inheritance Processes of the Retroviridae Biosynthesis of RNA Viruses That Use DNA Single-stranded RNA, produce DNA Use reverse transcriptase to produce DNA from the viral genome Viral DNA integrates into the host chromosome as a provirus Retroviridae Lentivirus (HIV) Oncoviruses Viruses and Cancer Several types of cancer are caused by viruses Viruses infect but don’t induce cancer May develop long after a viral infection Cancers caused by viruses are not contagious Sarcoma: cancer of connective tissue Adenocarcinomas: cancers of glandular epithelial tissue Transformation of normal cells into tumor cells Oncogenes transform normal cells into cancerous cells Oncogenic viruses become integrated into the host cell’s DNA and induce tumors A transformed cell harbors a tumor-specific transplantation antigen (TSTA) on the surface and a T antigen in the nucleus DNA Oncogenic viruses Adenoviridae Herpesviridae Poxviridae Papovaviridae Hepadnaviridae Epstein-Barr Human Hepatitis B virus papillomavirus virus RNA oncogenic viruses Retroviridae Viral RNA is transcribed to DNA (using reverse transcriptase), which can integrate into host DNA HTLV-1 and HTLV-2 cause adult T cell leukemia and lymphoma Viruses to treat cancer Latent viral infections Latent virus remains in asymptomatic host cell for long periods May reactivate due to changes in immunity Cold sores, shingles Persistent viral infections A persistent viral infection occurs gradually over a long period; is generally fatal Subacute sclerosing panencephalitis (measles virus) Prions: Overview Prions: Characteristics Prions: Diseases Viral Identification Cytopathic effects Serological tests Western blotting—reaction of the virus with antibodies Nucleic acids Restriction Fragment Length Polymorphisms (RFLPs) Polymerase Chain Reaction (PCR)