Viruses

Evolution of Virus

• Alive or Not– Biologically inert– Unable to replicate genes– Unable to regenerate ATP– probably evolved as bits of

cellular nucleic acid

Structural Component of Virus

• small infectious particles • nucleic acid– Double or single stranded DNA– Double or single stranded RNA– Single circular molecule or linear

• Capsid– Protein shell

• some cases, a membranous envelope– Made from membrane of host cell

Virus structure

Obligate intracellular parasites• Replicate only in host cells

Identification of a host cell

• Lock and key– Viral surface proteins and specific receptor

molecules of the host

Bacteriophage

• Virus that infects bacteria• Bacterial Defenses– Natural selection– Restriction enzymes– Lysogeny• Co-exist

Lytic vs Lysogenic

Retrovirus

• Equipped with RNA transcriptase• RNA DNA• DNA can enter the nucleus into the host

genome• HIV

Viral Infections in Animals

• Damage or kill cells• Produce toxins• Defense tools– Vaccines– Anti-viral drugs

Viral infections in Plants

• Horizontal transmission– entering through damaged cell walls

• Vertical transmission– inheriting the virus from a parent

Viroids and Prions

• Viroids – circular RNA molecules that infect plants and

disrupt their growth• Prions – slow-acting– virtually indestructible infectious proteins – brain diseases in mammals – mad cow– propagate by converting normal proteins into the

prion version