Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 19 Viruses

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Structure of Viruses Viruses: – not cells – very small infectious particles: nucleic acid (DNA or RNA) protein coat (caspid) (in some cases) membranous envelope

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Capsids and Envelopes Capsid- protein shell that encloses the viral genome – can have various structures

Fig RNA Capsomere of capsid DNA Glycoprotein 18  250 nm 70–90 nm (diameter) Glycoproteins 80–200 nm (diameter) 80  225 nm Membranous envelope RNA Capsid Head DNA Tail sheath Tail fiber 50 nm 20 nm (a) Tobacco mosaic virus -Helical capsid (b) Adenoviruses -Icoshedral caspid (c) Influenza viruses -8 RNA molecules Wrapped in Helical Caspid (d) Bacteriophage T4 -Icosahedral head and Tail apparatus

Fig. 19-3d Head DNA Tail sheath Tail fiber 80  225 nm 50 nm (d) Bacteriophage T4

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig µm

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Viruses that infect bacteria Most complex capsids found among viruses Bacteriophages (phages)- Elongated capsid head that encloses their DNA Protein tail piece attaches the phage to the host and injects the phage DNA inside

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Reproductive Cycles of Phages Phages- best understood of all viruses – Ex: Phage Phages have two reproductive mechanisms: – Lytic cycle – Lysogenic cycle

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Lytic Cycle Lytic cycle- phage reproductive cycle that culminates in the death of the host cell produces new phages and digests the host’s cell wall, releasing the progeny viruses Animation: Phage T4 Lytic Cycle Animation: Phage T4 Lytic Cycle

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Lytic cycle 1. The phage injects its DNA 2. Phage DNA circularizes 1. 2.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Lytic Cycle Environmental cues = LYTIC cycle is induced 3. New phage DNA and proteins are synthesized and assembled into phages. 4. The cell lyses, releasing phages LYTIC

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Lysogenic Cycle Lysogenic cycle replicates the phage genome without destroying the host Viral DNA molecule incorporated into the host cell’s chromosome – Integrated viral DNA = prophage Every time the host divides, it copies the phage DNA and passes the copies to daughter cells Animation: Phage Lambda Lysogenic and Lytic Cycles Animation: Phage Lambda Lysogenic and Lytic Cycles

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 1. The phage injects its DNA 2. Phage DNA circularizes 1. 2.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Environmental cue = Lysogenic cycle induced 3. Phage DNA integrates into the bacterial chromosome, becoming a Prophage Prophage Lysogenic

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Environmental cue = Lysogenic cycle induced 4. The bacterium reproduces, copying the prophage and transmitting it to daughter cells Prophage Lysogenic 4. Daughter cell with prophage

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Environmental cue = Lysogenic cycle induced 5. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle Prophage Lysogenic 4. Daughter cell with prophage 5.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig Phage DNA Phage The phage injects its DNA. Bacterial chromosome Phage DNA circularizes. Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce population of bacteria infected with the prophage. The cell lyses, releasing phages. Lytic cycle is induced or Lysogenic cycle is entered Lysogenic cycle Prophage The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Reproductive Cycles of Animal Viruses Many viruses that infect animals have: RNA genome Membranous envelope Glycoproteins

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Reproductive Cycles of Animal Viruses 1. Glycoproteins on viral envelope bind to specific receptors on the host cell  entry 2. Caspid and viral genome enter the cell, caspid digested by host cell enzymes 3. Viral genome serves as template for complementary RNA strands by a viral enzyme 4. New copies of viral RNA genome are made using the complementary strands of RNA by host enzyme 5. Complementary RNA strands also serve as mRNA which is translated into caspid proteins and glycoproteins for the new viral envelopes. 6. Vesicles transport glycoproteins to the plasma membrane of host cell 7. A caspid assembles around viral genome molecule 8. Each new virus buds from the host cell with viral glycoproteins embedded in membrane derived from the host cell.

Fig Capsid RNA Envelope (with glycoproteins) Capsid and viral genome enter the cell HOST CELL Viral genome (RNA) Template mRNA ER Glyco- proteins Capsid proteins Copy of genome (RNA) New virus

Fig. 19-8b HIV Membrane of white blood cell HIV entering a cell 0.25 µm New HIV leaving a cell

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1.Describe virus structure 2.Describe and Distinguish between the lytic and lysogenic reproductive cycles 3.Explain why viruses are obligate intracellular parasites 4.Describe the generalized reproductive cycle of an animal virus with a membranous envelope

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Phage replication: Lytic vs. Lysogenic