Chapter 23

 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

Influenza virus Ebola virus Bacteriophage

 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

 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

 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

 Lytic cycle – the virus lyses the host cell by forcing it to replicate viral particles  Virulent – viruses that only reproduce this way  Five steps: 1. Attachment – attaches to specific receptors on the host cell 2. Penetration – nucleic acid passes into the host cell 3. Replication and synthesis – host cell’s DNA is degraded and the cell replicates viral nucleic acids and proteins 4. Assembly – newly synthesized viruses are put together 5. Release – lytic enzymes destroy the host cell’s membrane and the new viruses are released

 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: 1. Attachment – attaches to specific receptors 2. Penetration – nucleic acid passes into host cell 3. Integration – viral nucleic acid is integrated 4. Replication – all nucleic acid is copied

 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’