Ch 23 Viruses AP Biology

Virus is one type of Pathogen Noncellular

Tobacco Mosaic Disease Spotted appearance on leaves Plant sap Ivanowsky Agent passed through filters, SO not bacteria Beijerinck Found that agent could only reproduce inside living cell

RESULTS Extracted sap from tobacco plant with tobacco mosaic disease Fig. 19-2 RESULTS 1 Extracted sap from tobacco plant with tobacco mosaic disease 2 Passed sap through a porcelain filter known to trap bacteria 3 Rubbed filtered sap on healthy tobacco plants Figure 19.2 What causes tobacco mosaic disease? 4 Healthy plants became infected

Infectious particle (NOT CELLS) Obligate intracellular parasite Viruses are NOT LIVING Infectious particle (NOT CELLS) Obligate intracellular parasite Nucleic acid + protein coat (capsid) Some have outer membrane DNA OR RNA Shape

Virus SHAPE Determined by proteins that make capsid Usually Helical Ex: Tobacco mosaic Polyhedral Ex: Adenovirus Combo of both helical and polyhedral Ex: T4

RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath Fig. 19-3 RNA DNA Membranous envelope Head RNA Capsomere DNA Capsid Tail sheath Capsomere of capsid Tail fiber Glycoprotein Glycoproteins 18  250 nm 70–90 nm (diameter) 80–200 nm (diameter) 80  225 nm Figure 19.3 Viral structure 20 nm 50 nm 50 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

On the Origin of Viruses OLD hypothesis “escaped gene hypothesis” Species-specific NEW Hypothesis Viruses arose before 3 domains Evidence = similarities in proteins of capsids to Archaea and Eubacteria

Bacteriophages “bacteria eaters” DNA (usually) inside Polyhedral Tail fibers – attachment Treat bacterial infections Reproduce by lytic and lysogenic cycles

2 types of Bacteriophage lifecycles: Lytic Cycle Virulent Viruses Virus destroys host cell Virulent = virus with only a lytic cycle

Steps of the Lytic cycle 1. Attachment (Absorption) Virus attaches to receptors on host cell wall 2. Penetration Viral nucleic acid enters Capsid remains outside host

Steps of the Lytic cycle- continued 3. Replication Copy viral nucleic acid and proteins 4. Assembly New virus parts assembled 5. Release Phage makes lytic enzymes to destroy host New viruses released

Fig. 19-5-1 1 Attachment Figure 19.5 The lytic cycle of phage T4, a virulent phage

Attachment Entry of phage DNA and degradation of host DNA 1 2 Fig. 19-5-2 1 Attachment 2 Entry of phage DNA and degradation of host DNA Figure 19.5 The lytic cycle of phage T4, a virulent phage

Attachment Entry of phage DNA and degradation of host DNA Fig. 19-5-3 1 Attachment 2 Entry of phage DNA and degradation of host DNA Figure 19.5 The lytic cycle of phage T4, a virulent phage 3 Synthesis of viral genomes and proteins

Attachment Entry of phage DNA and degradation of host DNA Fig. 19-5-4 1 Attachment 2 Entry of phage DNA and degradation of host DNA Phage assembly Figure 19.5 The lytic cycle of phage T4, a virulent phage 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers

Attachment Entry of phage DNA and degradation of host DNA Release Fig. 19-5-5 1 Attachment 2 Entry of phage DNA and degradation of host DNA 5 Release Phage assembly Figure 19.5 The lytic cycle of phage T4, a virulent phage 4 Assembly 3 Synthesis of viral genomes and proteins Head Tail Tail fibers

2 types of Bacteriophage lifecycles: Lysogenic Cycle Temperate Viruses Do not always destroy host Lysogenic – viral and host genomes become ONE  prophage (Lysogenic cell) Bacteria replicates all genetic material

The phage injects its DNA. Fig. 19-6 Daughter cell with prophage Phage DNA The phage injects its DNA. Cell divisions produce population of bacteria infected with the prophage. Phage DNA circularizes. Phage Bacterial chromosome Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Lytic cycle Lysogenic cycle The bacterium reproduces, copying the prophage and transmitting it to daughter cells. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Figure 19.6 The lytic and lysogenic cycles of phage λ, a temperate phage Prophage New phage DNA and proteins are synthesized and assembled into phages. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

How do temperate viruses cause symptoms? STRESS UV light, X-rays Spontaneous NOW lytic cycle – destroy host

Lysogenic Conversion Lysogenic cells exhibit new properties Ex: Clostridium botulinum Usually harmless Produce toxin when infected with specific phage

Viruses that Infect Animals Similar to a bacteriophage Methods may differ

Table 19-1a Table 1

Table 19-1b Table 1

Viruses that infect Animal Cells Attachment Species specific, cell specific Ex: Herpes, influenza, rabies – lipoprotein envelope with projecting glycoprotein spikes for attachment Measles, Pox viruses – many cell types Polioviruses – digestive tract, motor neurons

A Couple Ways to Penetrate Animal Cells 1. Membrane Fusion 2. Endocytosis Release of Animal Viruses 1. Budding 2. Lysis

Capsid and viral genome enter the cell Capsid Fig. 19-7 Capsid and viral genome enter the cell Capsid RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Copy of genome (RNA) Glyco- proteins Figure 19.7 The reproductive cycle of an enveloped RNA virus New virus

DNA OR RNA????? DNA Viruses Making viral DNA/proteins easy because process similar to host’s RNA Viruses RNA replication and transcription need RNA-dependent RNA polymerase

Retroviruses RNA Reverse transcriptase: RNADNA intermediate (cDNA) cDNA  dsDNA dsDNA into host genome RNA polymerases transcribe incorporated DNA into copies of viral RNA Viral proteins made Capsid produced New viruses assembled EX: HIV

Viral envelope Glycoprotein Capsid RNA (two identical strands) Reverse Fig. 19-8a Glycoprotein Viral envelope Capsid RNA (two identical strands) Reverse transcriptase HOST CELL HIV Reverse transcriptase Viral RNA RNA-DNA hybrid DNA NUCLEUS Provirus Chromosomal DNA RNA genome for the next viral generation Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS mRNA New virus

Membrane of white blood cell HIV HIV entering a cell Fig. 19-8b Membrane of white blood cell HIV Figure 19.8 The reproductive cycle of HIV, the retrovirus that causes AIDS 0.25 µm HIV entering a cell New HIV leaving a cell

harmless derivatives of pathogenic microbes Viral Treatment No antibiotics Antiviral drugs Vaccines – Prevention harmless derivatives of pathogenic microbes stimulate the immune system

New or previously uncommon SARS –China, AIDS, West Nile Emerging Viruses New or previously uncommon SARS –China, AIDS, West Nile Outbreaks - why? Extend range High mutation rates

(a) The 1918 flu pandemic Fig. 19-9a Figure 19.9 Influenza in humans and other animals (a) The 1918 flu pandemic

(b) Influenza A H5N1 virus 0.5 µm Fig. 19-9b Figure 19.9 Influenza in humans and other animals (b) Influenza A H5N1 virus

(c) Vaccinating ducks Fig. 19-9c Figure 19.9 Influenza in humans and other animals (c) Vaccinating ducks

H1N1

Viruses can Infect Plant Cells Spread – insects (aphids, leafhoppers) Entrance – damaged cells; infected seeds/asexual propagation Once infected – virus passes between cells by plasmodesmata Most RNA Symptoms: Reduced size; spots/streaks/mottled patterns Prevention – virus resistant strains

Fig. 19-10 Figure 19.10 Viral infection of plants

Smaller than Viruses Viroids Prions Naked RNA RNA polymerases – copy Viroid = template Infect nuclei of plants/animals Interfere with gene regulation Protein-like No nucleic acid Linked to TSEs Transmissable spongiform encephalopathies

TSEs Brain becomes spongelike Radiation useless No RNA/DNA Mammals – have gene to encode prion protein Normally harmless GE mice – lack prion protein gene – immune to TSE infection Creutzfeldt-Jakob disease

Scrapie (a TSE) Sheep/goats Lose coordination Irritable Severely itch

Bovine Spongiform Encephalopathy (BSE) “mad cow” UK 1990s epidemic Cattle ate feed mixed with sheep's brains+; (contained prions) Cow  human Similar to CJD

You should now be able to: Explain how envelopes are formed Distinguish between the lytic and lysogenic reproductive cycles Explain why viruses are obligate intracellular parasites Describe the reproductive cycle of an HIV retrovirus Describe viroids and prions