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Viruses
Dead or alive?
Viral structure

Viruses are not
cells.
 Basic structure:
Nucleic Acid
– Protein capsule
– Nucleic acid core
(RNA or DNA)
tail fibers

Parasites – need
a living host
Protein Capsule
sheath
Virus Categories

DNA viruses – stable, do not mutate
rapidly
– Smallpox, Hepatitis B, Herpes

RNA viruses – mutate rapidly, unstable
– HIV, Rhinovirus, SARS, Polio, West Nile,
common cold
Are viruses alive?
Only 3 characteristics of life:
reproduction, evolution, and genetic
code (DNA/RNA)
 Can only reproduce inside a host
cell!
 Process or reproduction = lytic cycle

Lytic Cycle
Virus attaches to host cell’s membrane
and injects its nucleic acid into the host
cell.
 The viral nucleic acid takes over protein
synthesis, creating new viruses.
 The host cell bursts, lyses, releasing the
newly formed viruses.

HIV

HIV doesn’t target just any cell, it goes right for the cells that
want to kill it. “Helper" T cells are HIV's primary target. These
cells help direct the immune system's response to various
pathogens.

HIV undermines the body's ability to
protect against disease by depleting T
cells thus destroying the immune system.

The virus can infect 10 billion cells a day,
yet only 1.8 billion can be replaced daily.

After many years of a constant battle, the body
has insufficient numbers of T-Cells to mount an
immune response against infections. At the point
when the body is unable to fight off infections, a
person is said to have the disease AIDS.

It is not the virus or the disease that ultimately
kills a person; it is the inability to fight off
something as minor as the common cold.
Chapter 18 Questions

Use the textbook to answer the following 3
questions:
– What are the 4 main viral structures?
– What is the difference between the lytic and
lysogenic cycles?

Critical Thinking:
– How do vaccines work to prevent viruses? Why
don’t antibiotics work on viruses?
– How do you think viruses can be used for good?