Plate 31 Introduction to Viruses

Smallpox Chickenpox Influenza (“the flu”) Herpes Polio Rabies Ebola Human Papillomavirus (HPV) Human Immunodeficiency Virus Diseases Caused by Viruses

Virus Size Viruses can only seen by electron microscopes Average size is measured in nanometers (nm) Bacteria & human cells are measured in micrometers (1000 times larger than nm)

Size Comparison E. coli is 3000 nm in size Chlamydia trachomatis is 300 nm in size Rabies virus is 200 nm in size

Two General Shapes Icosahedron Helix Viruses can come in a wide variety of shapes, but generally they are either helical or icosahedral

Viral Components Nucleic Acid – either DNA or RNA that controls viral replication Capsid – outer protein coat for protection (made of capsomeres) Lipid envelope – flexible outer membrane, from host cell

Viral Genetic Material DNA RNA (retroviruses)

Lipoprotein Coat Cell membrane from killed host cells Can protect against host immune system “Cloak of invisibility”

Obligate Intracellular Parasites Viruses are sometimes referred to as “obligate intracellular parasites” Obligate – needed / necessary Intra – within Cellular – relating to cells Parasite – one organism harms another in a long-term interaction