Intimate Strangers The Infectious Agents of Disease

Infectious Agents VirusesViruses BacteriaBacteria ProtozoaProtozoa

Viruses Sub microscopic “primitive life form”Sub microscopic “primitive life form” Can not self replicateCan not self replicate Obligate intracellular parasite (totally dependent on host cells for replication)Obligate intracellular parasite (totally dependent on host cells for replication) Unaffected by antibioticsUnaffected by antibiotics Made of a core of DNA or RNA surrounded by a protein coatMade of a core of DNA or RNA surrounded by a protein coat

Virus - From the Latin for ‘poison’ Viruses straddle the definition of life. They lie somewhere between molecular complexes and very simple biological entities. Viruses contain some of the structures and exhibit some of the activities that are common to organic life, but they are missing many of the others. In general, viruses are entirely composed of a single strand of genetic information encased within a protein capsule. Viruses lack most of the internal structure and machinery which characterize “life”. In order for a virus to replicate it must infect a suitable host cell.

Basic Structure of a Virus - protein coat containing DNA or RNA Capsid - protein coat containing DNA or RNA - similar to a membrane surrounding the capsid in some viruses; proteins, lipids, and glycoproteins make up the envelope Envelope - similar to a membrane surrounding the capsid in some viruses; proteins, lipids, and glycoproteins make up the envelope

Nucleic Acid-DNA or RNA? Viruses have either single-strand RNA or double-strand DNA, but never both.Viruses have either single-strand RNA or double-strand DNA, but never both. Those that contain RNA are termed retrovirusesThose that contain RNA are termed retroviruses Ex: HIVEx: HIV

Virus Structure Polyhedral (Capsid has 20 triangular faces and 12 corners) Ex: Influenza Virus Helical (Capsid is a coiled spring) Ex: Tobacco Mosaic Virus

Bacteriophage Virus that infects bacteria Structure is unique: polyhedral head on a helical tail with DNA inside the head Has “legs” that land on the surface of the bacteria to infect it

Viral Roles Viruses can be harmful. Viruses can be harmful.  Virulent - disease causing.  Temperate - not immediately disease causing.  Viruses play a role in causing some cancers. Viruses may be helpful. Viruses may be helpful.  A temperate virus can be used to cause genetic variation within a host population. (tulips)  Can be used to introduce genes into defective DNA “biologic vectors” (genetic engineering)

Viral Infection Lytic InfectionLytic Infection A. Virus injects DNA into host cell and forms a circleA. Virus injects DNA into host cell and forms a circle B. Host begins making viral proteins (can’t tell the difference b/t viral and its own DNA)B. Host begins making viral proteins (can’t tell the difference b/t viral and its own DNA) C. Assembles new virusesC. Assembles new viruses D. Lyses (bursts) the cell releasing new virusesD. Lyses (bursts) the cell releasing new viruses Ex: T4 BacteriophageEx: T4 Bacteriophage

Viral Infection Lysogenic InfectionLysogenic Infection A. Virus injects DNA into host cellA. Virus injects DNA into host cell B. DNA forms a prophage (literally embeds itself into the hosts’ DNA)B. DNA forms a prophage (literally embeds itself into the hosts’ DNA) C. Remains dormant for a period of time until environmental factors activate it into the lytic cycle (chemicals,radiation,illness, stress, etc)C. Remains dormant for a period of time until environmental factors activate it into the lytic cycle (chemicals,radiation,illness, stress, etc) D. Goes directly into the lytic cycleD. Goes directly into the lytic cycle Ex: Herpes Simplex VirusEx: Herpes Simplex Virus

Virus Life Cycle Animations 1. aids.edu/hiv_lifecycle/hivcycle_txt.html aids.edu/hiv_lifecycle/hivcycle_txt.htmlhttp:// aids.edu/hiv_lifecycle/hivcycle_txt.html 2. orsbiology/lifecyclehiv.html orsbiology/lifecyclehiv.htmlhttp:// orsbiology/lifecyclehiv.html 3. viruses/adlyt.html viruses/adlyt.htmlhttp://faculty.ccbcmd.edu/courses/bio141/lecguide/unit3/ viruses/adlyt.html 4. /viruses/lysosum01.html /viruses/lysosum01.htmlhttp://student.ccbcmd.edu/courses/bio141/lecguide/unit3 /viruses/lysosum01.html 1= HIV animation w/drug info 2= HIV narrated 3= Lytic Cycle 4= Lysogenic Cycle

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Some Diseases Caused by Viruses Plants Animals Humans Tobacco mosaic Foot and Mouth Common cold Tomato bushy Rabies German measles Alfalfa mosaic Distemper influenza Cold Sores Sugar beet curly top Cowpox Polio Dwarfism in rice Herpes, AIDS, Hepatitis Smallpox