1. Chapter 15
Viruses

2. 15.1 Viruses are infectious particles of genetic information and protein
A. Viruses are smaller and simpler than cells
Examples of diseases caused by viruses: AIDS, smallpox, influenza, the common cold, rabies, polio, chicken pox
Viruses are small, they spread disease, they are genetic information enclosed in a protein coat.
Typical viruses are about a tenth of the size of a bacterium. They do not have organelles
All viruses have two common features:
1. genetic information- all viruses contain genetic material(DNA or RNA) that carries instructions to make more viruses
2. Protein coat- called a capsid- surrounds the genetic information. The capsid shape determines how the virus is classified.

3. Some viruses have a lipid-rich envelope outside the protein coat
Some viruses have a lipid-rich envelope outside the protein coat. This helps the virus invade the host cell. It is another criteria for classification. Examples of enveloped viruses- HIV and Influenza
The overall structure of a virus can be complex. Bacteriophages(viruses that infect bacteria) have parts that resemble tails, spikes and legs.
B. Viruses infect only certain organisms. This is called the host range. Virtually all living things get viral infections, but viruses can only infect cell that have a specific receptor.
A reservoir for a virus is a site where it exists in nature. A host animal that may or may not show symptoms of infection. Ex. Birds, rodents, mosquitoes, raccoons.

4. bacteriophage

5. C. Are viruses alive?
Features of living things that viruses have: genetic material that mutates, evolves
Features of living things that viruses do NOT have: they do not metabolize, respond to stimuli or reproduce on their own.
Viruses are not part of the taxonomic hierarchy that scientists use to classify life.
They are grouped into species, genera and families according to type of nucleic acid, structure, replication, and type of disease it causes.

6. Virus shapes

7. Virus types

8. 15. 2 Five stages of viral replication
1. Attachment- A virus attaches to a host cell. (only on a cell in which it can reproduce)
2. Penetration: ex. Endocytosis, through a hole in the cell wall,
3. synthesis- the host cell makes copies of viral DNA as well as proteins and all of the resources required to make new viruses.
4. assembly- the capsid assembles and DNA is packed inside. If the virus has an envelope it doesn’t fully form until the virus breaks off from the host acquiring its outer covering from the cell membrane of the host.
5. release- Some viruses break down the host cell wall, killing the host cell. Some bud from the host cell carrying off parts of the membrane.
The amount of time between infection and cell death varies. It can be hours or days or years.

9. Bacteriophage
Replication process

10. Replication of a virus
Notice the endocytosis for stage 2!

11. 15.3 cell death may be immediate or delayed
A. some viruses kill cells immediately- lytic infection- the bacteriophage immediately causes the cell to burst releasing other viruses to infect more cells unit all the bacteria are dead.
This may be a possible treatment for bacterial infections:
Advantages- specific to a certain type of bacteria, evolve with the host so resistance is unlikely
Disadvantages- it takes time to identify the exact strain of bacteria causing infection
B. viral DNA can “Hide” in a cell- lysogenic infection- the host cell is not immediately destroyed. Some viruses combine their DNA with the host DNA. A prophage is the DNA of a lysogenic bacteriophage that is inserted into the host chromosome. So the viral genes replicate every time the cell divides.
The virus stays in the lysogenic stage until some signal causes it to enter the lytic stage where the cell is killed and the new viruses are released.