Viruses (Ch. 19-3) And Koch’s Postulate

I.What is a Virus? A.Virus – particle of nucleic acid, protein, & sometimes lipids that only reproduce by infecting living cells B.Typical Virus = DNA or RNA surrounded by a protein coat 1.Some are more complex with membranes

II.Viral Infection A.Specificity – viruses use protein receptors to bind to specific cells to infect 1.Plant viruses do not affect animals 2.Animal viruses only infect certain species of animals

B.Lytic Infection 1.Viral DNA enters cell, makes copies of itself, & causes cell to burst C.Lysogenic Infection 1.Virus inserts its DNA into host’s DNA, which is then replicated along with host’s DNA

D.Viruses & Disease 1.Oncogenic Viruses – cause cancer in animals (HPV virus causes cervical cancer) 2.Retroviruses – RNA is genetic material vs. DNA, which is later copied to DNA

III.Are Viruses Alive? A.Most scientists say no! 1.Are not made of cells and do not have all characteristics of life B.Evolution of Viruses 1.Most likely evolved after living cells 2.May have evolved from the genetic material of living cells video

Germ Theory – infectious diseases are caused by microorganisms, or germs Koch’s Postulate 1.Pathogen should be found in sick organism 2.Pathogen should be isolated & grown in pure culture 3.Purified pathogens are placed in a new, healthy organism and cause disease in new organism 4.Injected pathogen should be reisolated from second host & be identical to original pathogen