Biology Sylvia S. Mader Michael Windelspecht Chapter 20 Viruses Modified by D. Herder Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. See separate FlexArt PowerPoint slides for all figures and tables pre-inserted into PowerPoint without notes. 1

20.1 Viruses, Viroids, & Prions  Associated with a number of plant & animal diseases  noncellular  May have a DNA or RNA genome.  First seen by the electron microscope 2

Viral Diseases in Humans 3

Viruses 4 Covering Inner core Capsid (protein) Envelope (not found in all viruses) Virus particle Nucleic acid molecule (DNA or RNA) Various proteins (enzymes) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Viruses 5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. TEM 90,000XTEM 80,000X Adenovirus: DNA virus with a polyhedral capsid and a fiber at each corner. capsid a. DNA protein unit fiber fiber protein capsid DNA tail sheath tail fiber base plate pins neck b. T-even bacteriophage: DNA virus with a polyhedral head and a helical tail. a: © Dr. Hans Gelderblom/Visuals Unlimited; b: © Eye of Science/Photo Researchers, Inc.

6 Viruses 20nm Influenza virus: RNA virus with a spherical capsid surrounded by an envelope with spikes. capsid envelope c.d. RNA spikes Tobacco mosaic virus: RNA virus with a helical capsid. TEM 500,000X capsid RNA c: © Dr. O. Bradfute/Peter Arnold/Photolibrary; d: © K.G. Murti/Visuals Unlimited Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Viruses  Are obligate intracellular parasites Cannot reproduce outside a living cell  Can be cultured only inside living cells Chicken egg Tissue culture  Herpes & HIV Viruses lack metabolism; thus, antibiotics have no effect 7

Culturing Viruses 8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Ed Degginger/Color Pic Inc.

Replication of Viruses  Gain entry into specific host cell Capsid (or spikes of the envelope) adhere to specific receptor sites on the host cell surface.  Viral nucleic acid then enters a cell Viral genome codes for production of protein units in the capsid.  Relies on host cell enzymes, ribosomes, transfer RNA (tRNA), and ATP for its own replication 9

Viruses Bacteriophages – Viruses that infect bacterial cells There are two types of bacteriophage life cycles  The lytic cycle Viral reproduction occurs The host cell undergoes lysis  The lysogenic cycle Viral reproduction does not occur immediately but may occur in the future 10

Lytic Cycle Five stages:  Attachment  Penetration  Biosynthesis  Maturation  Release 11

Lysogenic Cycle Phage becomes a prophage  Becomes integrated into the host genome  Becomes latent  May later reenter the lytic cycle 12

Lytic and Lysogenic Cycles in Prokaryotes 13 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. capsid nucleic acid bacterial cell wall bacterial DNA

Lytic and Lysogenic Cycles in Prokaryotes capsid nucleic acid 1. ATTACHMENT Capsid combines with receptor. bacterial cell wall bacterial DNA 2. PENETRATION Viral DNA enters host. viral DNA 3. BIOSYNTHESIS Viral components are synthesized. LYTIC CYCLE 4. MATURATION Assembly of viral components. 5. RELEASE New viruses leave host cell. INTEGRATION Viral DNA is integrated into bacterial DNA and then is passed on when bacteria reproduce. viral DNA LYSOGENIC CYCLE prophage daughter cells 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Lytic and Lysogenic Cycles in Prokaryotes © Eye of Science/Photo Research, Inc. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 15

Virus Reproduction  Animal virus enters the host cell  Uncoating releases viral DNA or RNA Budding: –Viral particles released in a bud –Acquires a membranous envelope Retroviruses (AIDS) –Contain reverse transcriptase –Carries out RNA  cDNA reverse transcription –cDNA becomes integrated into host DNA »Replicated as host DNA replicates –Viral DNA is transcribed; new viruses are produced 16

Reproduction of the Retrovirus HIV 17 ER ribosome viral RNA reverse transcriptase viral RNA capsid spike envelope receptor Integration host DNA provirus viral enzyme viral mRNA capsid protein nuclear pore cDNA 3. Reverse transcription 2. Entry 4. Biosynthesis 1. Attachment 5. Maturation 6. Release Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Flu Pandemic A flu virus has an H (hemagglutinin) spike and an N (neuraminidase) spike  H spike allows the virus to bind to the receptor 16 different types  N spike attacks host plasma membranes Allows mature viruses to exit the cell 9 different types Each type of spike can occur in different varieties Our immune system only recognizes H spikes and N spikes it has been exposed to 18

Spikes of Bird Flu Virus 19 capsid RNA genome envelope a. Viral genetic mutations occur in a bird host b. Combination of viral genes occurs in human host Human flu virus Bird Flu virus In host cell combination mutation 1mutation 2 N (neuraminidase) spike H (hemagglutinin) spike Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Viroids and Prions  Viroids Naked strands of RNA Many crop diseases  Prions Protein molecules with contagious tertiary structure Some human and other animal diseases - Mad cow disease 20