Viruses

1. Chapter 23 Viruses

2. Pathogens • Agents that cause disease • Many microorganisms: bacteria, fungi, protozoa • Bacteria are prokaryotes, but only a few are pathogens; most are essential for life on Earth • Viruses are all pathogens – they all cause disease in something, although they don’t all cause disease in us

3. Bacteriophage Influenza virus Ebola virus

4. Characteristics of Viruses • A nucleic acid core (either DNA or RNA but not both) • A protein coat – capsid • Generally extremely small – much smaller than bacteria • Acellular: • No metabolic activities: cannot perform cellular respiration or protein synthesis • Cannot reproduce without the help of the host cell they infect

5. Taxonomy of Viruses • Most scientists do not consider them living things • They are not classified in any domain or kingdom • They are usually grouped according to the type of nucleic acid and the presence or absence of a capsid

6. Evolution of viruses • Escaped gene hypothesis: • Most widely held idea • They escaped from living cells – this explains why each virus is only able to infect one particular type of cell • They evolved early in the history of life on Earth, before the three domains separated • This hypothesis has no formal name • Viruses probably did not exist before their host cells arose

7. Reproductive cycles of viruses • Lytic cycle – the virus lyses the host cell by forcing it to replicate viral particles • Virulent– viruses that only reproduce this way • Five steps: • Attachment – attaches to specific receptors on the host cell • Penetration – nucleic acid passes into the host cell • Replication and synthesis – host cell’s DNA is degraded and the cell replicates viral nucleic acids and proteins • Assembly – newly synthesized viruses are put together • Release – lytic enzymes destroy the host cell’s membrane and the new viruses are released

8. The lytic cycle

9. Reproductive cycles of viruses… • Lysogenic cycle – the viral genome becomes integrated into the host DNA • Temperate – these cells are not killed directly, but instead replicate the viral nucleic acid as they reproduce; these cells often exhibit new properties • Four steps: • Attachment – attaches to specific receptors • Penetration – nucleic acid passes into host cell • Integration – viral nucleic acid is integrated • Replication – all nucleic acid is copied

10. The lysogenic cycle

11. Viroids and Prions • Viroid– very short strand of RNA without any protein coat • Cause a variety of plant diseases • Hard to eradicate • Prion– an infectious agent that is only protein - consists of 208 amino acids • ‘mad cow disease’