Intro to Viruses
What is a virus? Is it alive?
 A virus is a:
o sub-microscopic particle that
o cannot live, grow, or reproduce outside of a living cell.
What are viruses made of?
 Viruses are made up of
o genetic material, either RNA or DNA,
o surrounded by a protective protein coat called a capsid.
o
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How does the coat help the virus?
 The virus’ protein coat acts as a “key
o can only fit certain receptor sites (or locks) on specific cells.
o Each virus will only attach to a very specific target, or host, cell.
How do viruses reproduce?
 Viruses replicate through one of two cycles
o lytic cycle and the lysogenic cycle.
 Lytic cycle- host cell dies
1. Virus particle attaches to the host cell
2. Viral DNA invades the host cell
3. Host cell is forced to replicate the viral DNA components
a. nucleic acid (RNA or DNA)
b. protein coat (capsid)
4. Host cell is forced to assemble the viral particles into new viruses
5. Host cell bursts & dies, releasing the new viruses.

Lysogenic cycle- host cell is not killed.
o Viral DNA incorporates into the host cell’s DNA
o It’s passed onto to the daughter cells when the cell replicates.
o Later, the lytic cycle is triggered
 Causes cell’s death and the release of the new viruses.
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Can viruses be classified?
 Yes. Viruses are classified by the kind of nucleic acid they have for a core
o DNA viruses
o RNA viruses
What diseases are caused by viruses?
 Diseases caused by DNA viruses include
o chicken pox,
o measles, and
o mumps.
 Diseases caused by RNA viruses include
o influenza and
o some common colds.
How are viruses prevented or treated & cured?
 Viral specificity prevents most viruses from invading most cells.
 Only the immune system can cure a virus if you catch one!
o Viruses cannot be cured using antibiotics.
o Recently, some antivirals, like acyclovir, have been developed
 Some viruses can be prevented through vaccines.
What is a vaccine?
 A vaccine is a weakened form of virus given to “train” you immune system to remember,
identify, and kill a virus before it can cause illness.
This document is not for public usage or publication!
These notes developed from: Texas Education Service Center Curriculum Collaborative (©2011, TESCCC)