Transcript Viruses
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?