1. Lesson 7-3 What are Viruses?
Lesson 3 Reading Guide

2. Characteristics of Viruses
A virus is a strand of DNA surrounded by a layer of protein that can infect and replicate in a host cell.
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3. Viruses contain proteins, nucleic acids & can reproduce
Characteristics of Viruses (cont.)
A virus:
is smaller than the smallest bacteria!
Viruses contain proteins, nucleic acids & can reproduce
But…..
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4. Virus characteristics
Characteristics of Viruses (cont.)
Viruses
Don’t eat, grow or breathe
Do not have any organelles, or a cell membrane.
They are NOT cells!
Based on this info, do you think viruses are living organisms????
No!
A virus is not a living thing because it does not have all the characteristics of a living organism.
Virus characteristics

5. Characteristics of Viruses (cont.)
Viruses can have different shapes, such as crystal, cylinder, sphere, and bacteriophage shapes.
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6. Characteristics of Viruses (cont.)
Bacteriophage viruses only infect bacteria
Protein coat
Bacteriophage virus

7. Characteristics of Viruses (cont.)
Viruses must use organisms to carry on the processes that we usually associate with a living cell.
Viruses must be inside a cell, called a host cell, to replicate.
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8. Characteristics of Viruses (cont.)
When a virus enters a cell, it can either be active or latent.
Latent viruses go through an inactive stage and their genetic material becomes part of the host cell’s genetic material.
Once it becomes active, a virus takes control of the host cell and replicates.
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9. Lytic Cycle
Even though viruses are not truly a living organism, they can reproduce.
How?
The Lytic Cycle
A 4-step process
Viral reproduction

10. Lytic Cycle Step 1- attachment Step 2 - injection Step 3- Replication
Step 4 - Escape

11. Lytic Cycle Virus attaches to a host cell
Virus injects DNA into the host cell
Host cell is then directed to make more viral DNA, and new protein coats (replication)
The host cell is often destroyed and the viruses escape and move on to infect other cells
Lytic Cycle

12. Characteristics of Viruses (cont.)
As viruses replicate, their DNA or RNA frequently mutates, enabling them to adjust to changes in their host cell.
Viruses cause many human diseases, such as chicken pox, influenza, HIV, and the common cold. Viruses can also affect other animals and plants.
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13. Lesson 3

14. Treating and Preventing Viral Diseases
Since viruses are constantly changing, viral diseases can be difficult to treat.
Antibiotics work only against bacteria, not viruses.
One of the best ways to prevent a viral infection is to limit contact with an infected human or animal.
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15. Treating and Preventing Viral Diseases (cont.)
When a virus infects a person, their body begins to make antibodies.
An antibody is a protein that prevents an infection in your body.
Antibodies bind to viruses and other pathogens and prevent them from attaching to a host cell.
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16. Treating and Preventing Viral Diseases (cont.)
Natural immunity develops when a mother passes antibodies on to her unborn baby.
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17. Treating and Preventing Viral Diseases (cont.)
A vaccine is a mixture containing material from one or more deactivated pathogens, such as viruses.
When an organism is given a vaccine for a viral disease, the vaccine triggers the production of antibodies.
After being vaccinated against a particular pathogen, the organism will not get as sick if exposed to the same pathogen again.
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18. Research with Viruses
Scientists are researching new ways to treat and prevent viral diseases in humans, animals, and plants.
Viruses have been used by some scientists to treat genetic disorders and cancer using gene transfer.
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