Viruses come in a variety of shapes and sizes and are composed of either RNA or DNA surrounded by a protein capsid. They can only replicate by infecting living host cells. Viruses enter cells through different methods like endocytosis, fusing with membranes, or piercing the cell. Viral infection can result in lysis of the host cell or lysogeny where the viral DNA remains dormant within the host DNA. Common viral diseases are spread through routes like respiratory droplets, contaminated objects or fluids, mosquitoes, and food. Prevention methods include vaccines, good hygiene, and disinfectants.

1. 18.2 Viral Structure and Reproduction
Viruses exist in a variety of shapes and sizes.

2. 18.2 Viral Structure and Reproduction
– Simple and small pathogen
– Made of Capsid and RNA or DNA
– Non living (must infect living cells)

3. 18.2 Viral Structure and Reproduction
Shapes
capsid nucleic acid
lipid
envelope
surface
proteins
Surface proteins
capsid
nucleic acid
lipid envelope
surface capsid
proteins
nucleic acid
(rabies)
(foot-and-mouth
disease)
(influenza)
1. Enveloped
2. Helical
3. Polyhedral

4. 18.2 Viral Structure and Reproduction
4. Bacteriophages- viruses that infect bacteria.
capsid
DNA
tail sheath
tail fiber

5. 18.2 Viral Structure and Reproduction
Viruses enter cells in various ways.
– bacteriophages pierce host cells
colored SEM; magnifications:
large photo 25,000; inset 38,000x

6. 18.2 Viral Structure and Reproduction
- viruses of eukaryotes enter by endocytosis and also
fuse with membrane

7. 18.2 Viral Structure and Reproduction
host bacterium
The bacterophage attaches
and injects it DNA into a host
bacterium.
The host bacterium breaks apart,
or lyses. Bacteriophages are able
to infect new host cells.
The viral DNA directs the host
cell to produce new viral parts.
The parts assemble into new
bacteriophages.
The viral DNA
forms a circle.
Viruses cause two types of infections.
• A lytic infection causes the host cell to burst.
The virus may enter the
lysogenic cycle, in which the
host cell is not destroyed.

8. 18.2 Viral Structure and Reproduction
• A lysogenic infection does no immediate harm.
The viral DNA is called a prophage
when it combines with
the host cell’s DNA.
Although the prophage is not
active, it replicates along with
the host cell’s DNA.
The prophage may leave the
host’s DNA and enter the
lytic cycle.
Many cell divisions produce a
colony of bacteria infected
with prophage.

9. 18.2 Viral Structure and Reproduction
Viral Diseases
• Common Cold – contaminated objects
• Influenza – respiratory droplets (sneeze)
• HIV/AIDS – body fluids
• Chicken Pox – respiratory droplets (sneeze)
• Hepatitis B – contaminated blood/ body fluids
• West Nile Virus - mosquitoes
• HPV – sexual contact (causes cancer)
• Small Pox – respiratory droplets
• Mumps – respiratory droplets
• Measles – respiratory droplets
• Food Poisoning – eating bad food

10. 18.2 Viral Structure and Reproduction
Virus Prevention
• Vaccines
• Wash hands/ Good hygiene
• Cough/ Sneeze in tissue/ sleeve
• Disinfectants (breaks apart capsid/RNA)