Viruses are microscopic particles that invade and replicate inside living host cells. They contain either DNA or RNA surrounded by a protein coat. Viruses are not considered living because they cannot reproduce or metabolize outside of a host cell. In order to exist, a virus must infect a living host cell and use the cell's machinery to produce more viruses. There are two main types of viral infections - lytic infections immediately kill the host cell, while lysogenic infections integrate viral DNA into the host genome and lay dormant. Vaccines can provide protection against viruses that do not evolve rapidly.

1. Viruses
Chapter 18

2. What is a
virus?
•Microscopic particle
(microbe) that invades the
cells of living organisms.

3. Basic Structure Virus
• “Particles”
• Core- nucleic acid (DNA or RNA)
• Capsid- protein coat surrounds core

4. Are viruses living things?
Are viruses
living things?
Characteristic
Cells
Characteristic
Metabolism
Yes/No
Comments
Yes/No
Comments
No
particles – can not make
proteins
No
too simple to obtain/make
energy; only need it to
reproduce - steal it from
cells infect
Reproduce
Yes
Only inside host cell
Heredity
Yes
DNA or RNA
Homeostasis
No
No method for controlling
internal environment
Reproduce
Cells
Heredity
Metabolism
Homeostasis

5. Why should we study
viruses?

6. In order to
replicate,
In order to exist and
a virus
reproduce, must
invade a
a virus must invade
living cell.
a living cell.
This living
This living cell is referred to
cell is referred
to as a host. a host.
as

7. Viral Replication
General Steps:
• Recognition
• Attachment
• Injection
• Assembly
• Lysis

12. How do viruses infect your
cells?
Lytic Virus
•Examples:
Common Cold
Flu
•Replicates immediately and
kills the host cell
•You get sick right away.

13. How do viruses infect your
cells?
Lysogenic Virus
•Examples:
VIRAL DNA
HIV
Shingles
•Does not kill host cell
immediately
•Viral DNA injected into host
cell and then fuses with host
cell DNA
•Dormant period
•You don’t get sick right
away

14. Protection Against Viruses
• All viruses are pathogens.
• Animals, plants, fungi, protists, and bacteria can
all be infected by viruses.
• Antibiotics do not work.
• Antiviral medications
• Vaccines are used as prevention only.
• Vaccinations are ONLY useful against viruses
that do NOT evolve rapidly.
• Flu shot- many different types- flu