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 InfectionA. Specificity – viruses use protein receptors

to bind to specific cells to infect1. Plant viruses do not affect animals

2. Animal viruses only infect certain species of animals

B. Lytic Infection1. Viral DNA enters cell, makes copies of

itself, & causes cell to burst

C. Lysogenic Infection1. Virus inserts its DNA into host’s DNA,

which is then replicated along with host’s DNA

D.Viruses & Disease1. 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 Viruses1. Most likely evolved after living cells

2. May have evolved from the genetic material of living cells

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Germ Theory – infectious diseases are caused by microorganisms, or germs

Koch’s Postulate1. 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