What Are Viruses?

How Do Viruses & Bacteria Differ?

Living Or Non-Living? Acellular Lack metabolism, growth or development Exhibit parasitic tendencies due to need for host cell to assist with reproduction

Discovery of Viruses Adolf Mayer (1883) found that the disease causing agent involved with tobacco plants was too small to be filtered This agent also would not grow on nutrient agar Given the name “virus”, meaning “poison” in Latin

Structure of Viruses Wendell Stanley (1935)

PHAGE ANATOMY End Plate = matches with receptor proteins on host cell Tail = Facilitates movement of nucleic acids into host cell Capsule = protein shell containing nucleic acids

DNA-Containing Viruses PAPILLOMA ADENOVIRUS HERPES HEPATITIS B

RNA-Containing Viruses ROTAVIRUS RHINOVIRUS INFLUENZAPARAMYXOVIRUS

Bacteriophages Viruses that only infect certain strains of bacteria Because bacteria are prokaryotic, they are often more susceptible to viral transduction (i.e. insertion of viral nucleic acids)

LYTIC CYCLE Viral phage attaches to specific host cell, identified by surface proteins Viral genetic information is “injected” into the host cell The host cell’s ribosomes stop translating their normal proteins and translate the viral DNA, creating more viral phages The cell lyses (bursts) releasing more phages into the system, creating an exponential increase over a short number of generations

How Does Lysogeny Differ From The Lytic Phase of a Viral “lifecycle”?

LYSOGENY Phage transduces host cell by injecting nucleic acids (similar to lytic phages) However, the viral information is incorporated into the host cells genome without being expressed PROVIRUS The PROVIRUS is copied along with the rest of the host cell’s genome during the S subphase of mitosis Daughter cells receive PROVIRUS and copy it during their S subphases Eventually, the PROVIRUS information is expressed and the cell’s normal functions are altered as the LYTIC cycle begins

Retroviruses Viruses that transduce cells “in reverse” by injecting RNA into host cell RNA is written back into DNA via the enzyme reverse transcriptase The new viral “DNA” becomes a PROVIRUS and is copied during the S subphase of mitosis

How Does The Balance of The Viral Life Cycles Determine If A Disease Is An Epidemic, Endemic or Pandemic?

Epidemic Large number of afflicted hosts in a small area in a short period of time Epidemics are generally short-lived due to rapid virulence/effects and limited number of hosts EX: Ebola

Endemic A disease that is commonly present in a population, generally at low frequencies Often due to proximity of host to reservoir of pathogen Ex: Chicken Pox

PANDEMIC Disease that spreads worldwide, creating thousands of cases Generally, pandemic diseases require common vectors and long incubations or lysogenic modes Ex: Tuberculosis

Sudden Acute Respiratory Syndrome

HEPATITIS C