The Infectious Agents of Disease Intimate Strangers The Infectious Agents of Disease

Infectious Agents Viruses Bacteria Protozoa

Viruses Sub microscopic “primitive life form” Can not self replicate Obligate intracellular parasite (totally dependent on host cells for replication) Unaffected by antibiotics Made 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 Capsid- protein coat containing DNA or RNA, and enzymes Envelope- similar to a membrane surrounding the capsid in some viruses; (proteins, lipids, and glycoproteins make up the envelope)

Nucleic Acid-DNA or RNA? DNA Viruses: Smallpox, Chickenpox, Herpes, HPV, Adenovirus (can cause the cold), and Hep B RNA Viruses: Measles, mumps, rabies, ebola, flu, polio, rhinovirus **Retrovirus: contains reverse transcriptase to incorporate RNA DNA, then into host DNA Example: HIV *Both types can replicate in both lifecycles. Some can lie dormant (latent) Ex: HIV (in the host DNA), Chicken Pox and Herpes (in neurons only)

They may look similar….

Virus Structure Polyhedral Helical (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 may be helpful. Virulent - disease causing. Temperate - not immediately disease causing. Viruses play a role in causing some cancers. 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 A. Virus injects DNA/RNA into host cell Lytic Infection A. Virus injects DNA/RNA into host cell B. Host begins making viral proteins (can’t tell the difference b/t viral and its own nucleic acid) C. Assembles new viruses D. Lyses (bursts) the cell releasing new viruses, some can “bud” off the surface Ex: T4 Bacteriophage, flu, cold

Viral Infection A. Virus injects DNA/RNA into host cell Lysogenic Infection A. Virus injects DNA/RNA into host cell B. Once DNA is present, 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) D. Goes directly into the lytic cycle Ex: Herpes Simplex Virus

                                                                                                              

Virus Life Cycle Animations 1= HIV narrated 2= Lytic vs Lysogenic 3= Flu Attack

Just how big is a virus? http://www.cellsalive.com/howbig.htm

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