Download VIRUSES

VIRUSES
(Chapter 25)
1. Viruses are 1/2 - 1/100 the size of the smallest bacterium. Carried by air,
water, food, & body fluids they can be harmful to animals, plants, & bacteria.
(HIV/AIDS, common cold, flu, rabies, polio, 400 + plant viruses)
2. Most biologists consider them nonliving because they don’t exhibit all processes
of life. Viruses can only use living host cells to copy themselves.
3. Structural details of viruses:
a) protein outer coat called capsid determines its shape, type of cell it
infects, & how it attacks the cells
b) type of nucleic acid (either DNA or RNA) classifies the virus
c) larger viruses (flu) may have additional layer called an envelope
4. A bacteriophage is a virus that infects bacteria. To enter a host, virus must
recognize & attach itself to a receptor site. Then its nucleic acid is injected.
Once inside host cell, viral nucleic acid takes over all chemical reactions in the
bacterium & begins making copies of itself. This is called viral replication.
a) Lytic Cycle - new viruses are created & released to infect other host
cells
b) Lysogenic Cycle - viral nucleic acid combines with host’s nucleic acid. As
host grows & divides, viral nucleic acid is also copied & passed along. At
any time, virus can be activated & enter lytic cycle & kill host cell.
(herpes, chickenpox/shingles)