Viruses are obligate intracellular parasites that can only multiply within host cells. They contain either DNA or RNA, and have a protein coat called a capsid that may be surrounded by an envelope. To replicate, a virus must invade a host cell, take over its machinery, and use it to produce new viral particles, which then go on to infect more cells. While viruses use living cells to multiply, they are not themselves considered living things.

1. CHAPTER 13: VIRUSES, VIROIDS,
AND PRIONS
Exam 4 material

2. GENERAL CHARACTERISTICS OF VIRUSES
Virus is the Latin word for
“poison”
1935
Wendell Stanley isolated
the Tobacco Mosaic virus
for the first time

3. DEFINITION OF LIFE
 A complex set of processes resulting from the actions of proteins
specified by nucleic acids
 “Viruses are not living things. Viruses are complicated assemblies of
molecules, including proteins, nucleic acids, lipids, and
carbohydrates, but on their own they can do nothing until they enter
a living cell. Without cells, viruses would not be able to multiply.
Therefore, viruses are not living things.”

4. Viruses are “alive” (able to multiply) when they
enter a host organism
We will say that viruses are living, but they are not.

5. Obligate Intracellular parasite-must have a
living host cell in order to multiply
ALL VIRUSES ARE O.I.Ps

6. FEATURES OF VIRUSES
 Only one type of nucleic acid
 DNA or RNA (not both)
 A protein coat
 Made of lipids, carbohydrates, proteins

7. • Must have a host cell
• Uses the cells machinery to copy itself
• Makes specialized structures that can
transfer the viral information to other cells
• Lack personal enzymes
• http://www.youtube.com/watch?v=sHThHRV4uo

8. COMPARING PROKARYOTES & VIRUSES (P 387)
Prokaryotes
Viruses

9. HOST RANGE
Viruses can have a host range
A spectrum of host cells the virus can infect
Viruses that infect bacteria are bacteriophages
(phages)
Outer surface of virus must chemically interact with
specific receptor site on host cell

10. VIRAL SIZE
 Viruses are very small compared to bacterial cells
 Need an electron scanning microscope to view

11. VIRAL STRUCTURE
Virion = complete, fully developed, infectious viral particle
composed of nucleic acid and surrounded by a protein
coast that protects it from the environment

12. NUCLEIC ACID
Prokaryotes and Eukaryotes use RNA and DNA as
genetic information
Viruses use RNA or DNA-not both
It is single or double stranded

13. CAPSID AND ENVELOPE
Protein coat = capsid
Combination of lipid,
proteins, and carbohydrates
that convers the capsid =
envelope
Capsids without envelopes =
nonenveloped viruses

14. When a virus enters a host cell, the host antibodies are produced
An interaction between antibodies and virus’ should inactivate the
virus
Some viruses escape antibodies and are able to change their surface
proteins
Surface proteins are called spikes
This is why you can get the flu many different times

15. GENERAL MORPHOLOGY
Compare with prokaryotic morphology
 Helical : long rods, hollow and cylindrical spirals
 Polyhedral : many sided viruses
 Complex : complicated structure
 Many bacteriophages

16. TAXONOMY OF VIRUSES
A viral species is a
group of viruses sharing
the same genetic
information and
ecological niche.

17. GROWING VIRUSES
Nonliving: needs
a host cell
Difficult and
expensive to
maintain cultures
Usually learn
from
bacteriophages
plaque = clearing
within a media

19. VIRAL MULTIPLICATION
http://www.youtube.co
m/watch?v=Rpj0emEG
ShQ
A virus must be able to
invade a host
A virus must take over
a host’s machinery

20. BACTERIOPHAGES
LYTIC CYCLE
 1. attachment to receptor site
 2. injects DNA/RNA into cell
 3. host cell starts making viral
copies of nucleic acid
 4. bacteriophages are assembled
inside host cell
 5. host cell is lysed
LYSOGENIC CYLE
Process where cell is not
lysed and is able to stay
alive

21. FAMILIES OF VIRUSES
 Paroviridae
 Adenoviridae
 Papovaviridae
 Poxviridae
 Herpesviridae
 Hepadnaviridae
 Picornaviridae
•
•
•
•
•
•
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Rhabdoviridae
Filoviridae
Paramyxoviridae
Deltaviridae
Caliciviridae
Togaviridae
Flaviviridae
Coronaviridiae
PAGE 392-3
Orthomyxoviridae
Bunyaviridae
Arenaviridae
Retroviridae
Reoviridae

22. VIRUSES AND CANCER
Almost anything that can alter the genetic material of a
eukaryotic cell has the potential to make a normal cell
cancerous
HPV
Hepatitis B
Leukemia, Feline leukemia

23. VIRUSES AND INFECTIONS
Latent infection = infection capable of
staying dormant until stimulus is applied
Chronic viral infection = long lasting viral
infection that occurs over a gradual time
period (usually fatal)
HerpesFever blisters/cold sores
Chicken poxshingles

24. PRIONS
????????
 A prion is a proteinaceous infectious particle
 A protein that infects like a virus
 These diseases run in families; however, they can’t be totally inherited
 There is a lot that is not known about prions
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Mad Cow disease
Kuru
Creutzfeldt-Jakob disease
Fatal familial insomnia
Gerstmann-Straussler-Scheinker
syndrome

25. VIROIDS
Viroids are short pieces of RNA with no protein
coat that does not code for any protein
They cause plant diseases
Much is still to be learned about these particles