Viruses Chapter 27 Nature of Viruses All viruses

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Viruses Chapter 27

Viruses Chapter 27

Nature of Viruses All viruses have same basic structure -Nucleic acid core surrounded by

Nature of Viruses All viruses have same basic structure -Nucleic acid core surrounded by capsid Nucleic acid can be DNA or RNA; Circular or linear; Single- or double-stranded Some viruses store specialized enzymes inside their capsids Many animal viruses have an envelope 2

Nature of Viruses 3

Nature of Viruses 3

Nature of Viruses 4

Nature of Viruses 4

Nature of Viruses 5

Nature of Viruses 5

Nature of Viruses 6

Nature of Viruses 6

Nature of Viruses are obligate intracellular parasites -Host range = Types of organisms infected

Nature of Viruses are obligate intracellular parasites -Host range = Types of organisms infected -Tissue tropism = Types of cells infected Viruses can remain dormant or latent for years More kinds of viruses exist than organisms 7

Viral Replication Viruses can only reproduce inside cells -Outside, they are metabolically inert virions

Viral Replication Viruses can only reproduce inside cells -Outside, they are metabolically inert virions Virus hijacks the cell’s transcription and translation machineries to express: -Early genes -Middle genes -Late genes End result is assembly and release of viruses 8

Viral Shapes Most viruses come in two simple shapes -Helical -Icosahedral Some viruses are

Viral Shapes Most viruses come in two simple shapes -Helical -Icosahedral Some viruses are complex -T-even phages = Binal symmetry -Poxviruses = Multilayered capsid Enveloped viruses are polymorphic 9

Viral Shapes 10

Viral Shapes 10

Viral Shapes 11

Viral Shapes 11

Viruses vary in size, as well as in shape 12

Viruses vary in size, as well as in shape 12

Viral Genomes Vary greatly Most RNA viruses are single-stranded -Replicated in the host cell’s

Viral Genomes Vary greatly Most RNA viruses are single-stranded -Replicated in the host cell’s cytoplasm -Retroviruses (HIV) employ reverse transcriptase -Most DNA viruses are double-stranded -Replicated in nucleus of eukaryotic host cell 13

Bacteriophage Are viruses that infect bacteria Viruses have also been found in archaea -Have

Bacteriophage Are viruses that infect bacteria Viruses have also been found in archaea -Have complex symmetry E. coli-infecting viruses are the best studied -Include the “T” series (T 1, T 2, etc. ) 14

Bacteriophage Exhibit two reproductive cycles -Lytic cycle = Virus kills the host cell -Lysogenic

Bacteriophage Exhibit two reproductive cycles -Lytic cycle = Virus kills the host cell -Lysogenic cycle = Virus incorporates into the cell’s genome Lytic phage are called virulent Lysogenic phage are called temperate 15

Bacteriophage Lytic cycle -Adsorption = Viral attachment -Penetration = Viral entry -Synthesis = Viral

Bacteriophage Lytic cycle -Adsorption = Viral attachment -Penetration = Viral entry -Synthesis = Viral components are made -Assembly = Components are put together -Release = Viral exit 16

Bacteriophage Lysogenic cycle -Virus integrates into cellular genome as a prophage -Propagates along with

Bacteriophage Lysogenic cycle -Virus integrates into cellular genome as a prophage -Propagates along with host genome -Resulting cell is called a lysogen Phage conversion occurs when the prophage alters the bacterial phenotype -Vibrio cholerae toxin is viral-encoded 17

Induction = Switch from the lysogenic to the lytic cycle 18

Induction = Switch from the lysogenic to the lytic cycle 18

Human Immunodeficiency Virus HIV causes acquired immune deficiency syndrome (AIDS) AIDS was first reported

Human Immunodeficiency Virus HIV causes acquired immune deficiency syndrome (AIDS) AIDS was first reported in the US in 1981 Some people are resistant to HIV infection -Have mutation in the CCR 5 gene -Encodes a receptor for HIV -Also for the smallpox virus 19

Human Immunodeficiency Virus HIV targets CD 4+ cells, mainly helper T cells -Without these

Human Immunodeficiency Virus HIV targets CD 4+ cells, mainly helper T cells -Without these cells, the body cannot mount an effective immune response -Host may ultimately die from a variety of opportunistic infections Tests for HIV detect anti-HIV antibodies -Not circulating viruses 20

HIV Infection Cycle Attachment = Viral gp 120 attaches to CD 4 and a

HIV Infection Cycle Attachment = Viral gp 120 attaches to CD 4 and a co-receptor on host cells Entry = By endocytosis Replication = Reverse transcriptase converts RNA to double-stranded DNA -DNA is incorporated into host genome Assembly = Occurs after a variable period of dormancy Release = New viruses exit by budding 21

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HIV Infection Cycle During an infection, HIV is constantly replicating and mutating -Initially, gp

HIV Infection Cycle During an infection, HIV is constantly replicating and mutating -Initially, gp 120 uses CCR 5 as a coreceptor -Later, CXCR 4 is used -Thus, the mutated virus can infect a broader range of cells 23

Treatments for HIV Research is currently under way in the following five areas: -1.

Treatments for HIV Research is currently under way in the following five areas: -1. Combination drug therapy -Highly active antiretroviral therapy (HAART) -AZT and protease inhibitors -2. Vaccine therapy -Using a harmless strain of HIV 24

Treatments for HIV Research is currently under way in the following five areas: -3.

Treatments for HIV Research is currently under way in the following five areas: -3. Blocking receptors -Use chemokines to block CCR 5/CXCR 4 -4. Disabling receptors -Through mutations in the genes -5. Blocking replication -Use of CAF (CD 8+ cell antiviral factor) 25

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Influenza Flu viruses are enveloped and have a segmented RNA genome -Type A =

Influenza Flu viruses are enveloped and have a segmented RNA genome -Type A = Serious epidemics in humans and other animals -Type B and C = Mild human infections -Subtypes differ in their protein spikes -Hemagglutinin (H) = Aids in viral entry -Neuraminidase (N) = Aids in viral exit 30

Influenza H and N proteins are constantly changing -Thus we have yearly flu shots,

Influenza H and N proteins are constantly changing -Thus we have yearly flu shots, and not a single vaccine -Type A viruses are classified into 13 distinct H subtypes and 9 distinct N subtypes 31

Influenza Flu viruses can also undergo genetic recombination when 2 subtypes infect a cell

Influenza Flu viruses can also undergo genetic recombination when 2 subtypes infect a cell -This creates novel combinations of spikes unrecognizable by human antibodies -Antigenic shifts have caused pandemics -Spanish flu of 1918, A(H 1 N 1) -Asian flu of 1957, A(H 2 N 2) -Hong Kong flu of 1968, A(H 3 N 2) 32

Influenza New strains of flu originate in the Far East -Virus hosts are ducks,

Influenza New strains of flu originate in the Far East -Virus hosts are ducks, chicken and pigs -In 1997, avian influenza A(H 5 N 1) was discovered -“Bird flu” has killed over 100 people -However, it does not appear to spread between humans 33

Emerging Viruses Are viruses that extend their host range -Considerable threats in the aviation

Emerging Viruses Are viruses that extend their host range -Considerable threats in the aviation age Hantavirus -Causes deadly pneumonia -Natural host is deer mice Ebola virus -Causes severe hemorrhagic fever -Host is unknown 34

Emerging Viruses SARS -Severe acute respiratory syndrome -Caused by a coronavirus -Host is civets

Emerging Viruses SARS -Severe acute respiratory syndrome -Caused by a coronavirus -Host is civets -RNA genome contains six main genes -SARS vaccines currently being developed 35

Viruses and Cancer Viruses may contribute to about 15% of all human cancers Viruses

Viruses and Cancer Viruses may contribute to about 15% of all human cancers Viruses can cause cancer by altering the growth properties of human cells -1. Triggering expression of oncogenes -2. Disrupting tumor-suppressor genes In June 2006, the FDA approved the use of a new HPV vaccine to prevent cervical cancer 36

Prions “Proteinaceous infectious particles” Cause transmissible spongiform encephalopathies (TSEs) -Mad cow disease -Scrapie in

Prions “Proteinaceous infectious particles” Cause transmissible spongiform encephalopathies (TSEs) -Mad cow disease -Scrapie in sheep -Creutzfeldt-Jacob disease in humans Animals have normal prion proteins (Pr. Pc) -Misfolded proteins (Pr. Psc) cause disease 37

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Viroids Tiny naked molecules of circular RNA Cause diseases in plants -Coconuts It is

Viroids Tiny naked molecules of circular RNA Cause diseases in plants -Coconuts It is unclear how they cause disease 39