1. Viruses
Viruses

2. Viruses Learning Objectives Know the structure of viruses
Compare viruses to cells
Understand viral reproduction
Understand the role of viruses in diseases
After this lesson you will know the structure of viruses and be able to compare viruses to cells, understand viral reproduction, and understand the role of viruses in diseases.

3. Gypsy Moth Virus Factory
A healthy gypsy moth caterpillar forages on high leaves during the night and hides down below during the day. A caterpillar infected with a baculovirus, however, will remain in the trees. As the virus reproduces, the caterpillar’s body slowly becomes a pool of viruses. Eventually, the body melts and releases viruses down onto other caterpillars on the ground. This behavioral shift is an adaptation of the virus to increase its infection rate. For a virus, the number one goal is reproducing.

4. Characteristics of Viruses
Virus – a small infectious agent of genetic material encased in protein
Not made of cells
Cannot reproduce on their own
Cannot metabolize their own energy
Uses the host cell’s energy and machinery to copy itself
Non-living, not considered biotic
Not considered microorganisms
A virus is a small infectious agent of genetic material encased in protein. Unlike all living organisms, viruses are not made up of cells, they cannot reproduce on their own, and they cannot metabolize their own energy. Using the host cell’s energy and machinery, such as ribosomes, a virus can make millions of copies of itself. Because of these factors, a virus is generally not considered biotic, a living thing. Therefore, viruses are not really microorganisms.

5. Characteristics of Viruses
Different structure than cells
Smaller than cells
Contain few specialized structures
Genetic material can be DNA or RNA
Capsid – protein container that surrounds genetic material
Viruses have a very different structure than cells. They are much smaller and contain fewer specialized structures. Their genetic material, which can be DNA or RNA, is contained in a protein container called a capsid.

6. Characteristics of Viruses
Helical
Tobacco mosaic
Polyhedral
Adenovirus
Enveloped
HIV
Bacteriophages
Organized by shape and capsid, there are four primary types of viruses: helical, polyhedral, enveloped, and bacteriophages. Helical viruses, such as tobacco mosaic viruses, consist of genetic material surrounded by a capsid that is a hollow, twisted cylinder. Adenoviruses are polyhedral viruses that consist of genetic material surrounded by a many-sided capsid. Some viruses, especially those that infect animals, have an extra outer, protective layer of lipids called an envelope. Envelopes are formed from host cellular membrane. Because of this, the envelopes have special molecules studded on the surface that enable the virus to identify and attach to the host cell membrane.

7. Characteristics of Viruses
Bacteriophage – a virus that infects bacteria
Complex virus
Structure: protein tail with fibers
Irregularly shaped
Contain double-stranded DNA
A bacteriophage, which is a virus that infects bacteria, is an example of a complex virus. It has a protein tail with fibers that allow the virus to attach to the bacterium in order to inject its genetic material into the cell. These viruses are irregularly shaped rather than helical or polyhedral and generally contain double-stranded DNA.

8. Reproduction of Viruses
Parts of a virus enter the host cell
Metabolic machinery is hijacked
Viral DNA or RNA instructs the host cell to make copies
of the virus
Viral proteins and genetic material are reproduced
Two pathways for viral reproduction
To reproduce, parts of a virus enter the host cell and hijack metabolic machinery. Viral DNA or RNA serves as information that instructs the host cell to make copies of the viral proteins and genetic material. Once the viral genetic information has entered, there are two main pathways for reproduction.

9. Reproduction of Viruses
Lytic pathway – form of viral reproduction in which viral proteins and genetic material self-assemble and form new viruses
New viruses leave host cell to infect other nearby cells
Lysis – viruses burst the cell membrane as they leave
Results in death of host cell
In the lytic pathway, the viral proteins and genetic material self-assemble and form new viruses. The new viruses then leave the host cell to infect other nearby cells. As the viruses leave, they burst the cell membrane in a process called lysis. The lytic pathway results in the death of the host cell and the release of the newly formed viruses.

10. Reproduction of Viruses
Lytic pathway
The lytic pathway includes viral attachment, entry, replication, self-assembly of new viruses, and release of viruses from the host cell.

11. Reproduction of Viruses
Lysogenic pathway – form of viral reproduction in which the viral genetic material integrates within the host’s chromosomes
Host cell passively becomes genetic viral factory
Viral genes copied each time cell replicates via mitosis
Some viruses do not kill the host cell right away. Instead, they use the lysogenic pathway. During this pathway, the viral genetic material integrates within the host’s chromosomes. The host cell then passively becomes a genetic viral factory, copying the viral genes each time it replicates via mitosis.

12. Reproduction of Viruses
Lysogenic pathway
During the lysogenic pathway, viral genetic material is inserted into the host’s genome. The resulting daughter cells contain viral genetic material.

13. Reproduction of Viruses
Combined pathways
These copied viral genes can remain dormant until they are activated by various factors, such as stress. When the viral genes are activated, the virus enters the lytic pathway. Viral components are produced, assembled, and released from the cell as lysis occurs. Viruses can use the lysogenic pathway to first infect many cells and then use the lytic pathway later to create many viruses at once.

14. Diseases Caused by Viruses
Helper T cells – type of white blood cells that play an important role in the immune system
HIV (human immunodeficiency virus) – viral disease which targets helper T cells
Helper T cells become useless in fighting disease
Viruses cause several major diseases in humans. Helper T cells are white blood cells that play an important role in the immune system. When attacked by HIV (human immunodeficiency virus), these helper T cells become useless in fighting disease.

15. Diseases Caused by Viruses
AIDS (acquired immune deficiency syndrome) – a syndrome characterized by an extremely weak or absent immune system; caused by HIV
Die from secondary infections
No vaccine
Limiting and avoiding transmission of virus can prevent infection
After years of attack, patients lose their immune system and are diagnosed with the disease called AIDS (acquired immune deficiency syndrome). Patients with AIDS generally die from secondary infections. Currently, there is no vaccine for HIV, thus limiting and avoiding transmission can prevent infection.

16. Diseases Caused by Viruses
HIV replication
HIV has a devastating replication strategy. Once the virus enters the body, spikes of viral proteins extend from the HIV envelope and attach to the membrane of human white blood cells. The viral genome then enters a lytic period. During this stage, the RNA-based genome serves as instructions for special proteins, such as reverse transcriptase, to be made by hosts and packaged into new viruses. An RNA genome orchestrates the production and assembly of HIV viruses within host cells.

17. Diseases Caused by Viruses
HIV exits by budding from the host cell
Envelope of virus fuses to plasma membrane of host cell
Virus exit cell wrapped in host cell membrane
Like many other viruses that infect animals, HIV exits by budding from the host cell. The envelopes of these viruses fuse to the plasma membrane of the host cell. The virus then exits the cell wrapped in host cell membrane. This process enables the virus to bud from the cell rather than lysing it.

18. Diseases Caused by Viruses
Initial infection: number of helper T cells drops
Large amount of HIV genome produced
Clinical latency: virus enters lysogenic pathway
Viral genomes lay mostly dormant
Lytic cycle resumes
Steady decrease in helper T cells
More HIV viruses are produced
More immune cells infected
When enough helper T cells are destroyed, the patient is diagnosed with AIDS
At initial infection, the number of helper T cells drops and a large amount of HIV genome is produced. During clinical latency, the virus enters the lysogenic pathway and viral genomes lay mostly dormant. After a period of time, the lytic cycle resumes. There is a steady decrease in helper T cells and more and more HIV viruses are produced and infect more immune cells. Eventually, enough helper T cells are destroyed that the patient is diagnosed with AIDS.  

19. Diseases Caused by Viruses
This graph shows the effect of HIV infection over time. Notice that as the number of viruses increases, the number of helper T cells decreases.

20. Diseases Caused by Viruses
Influenza – a viral disease commonly known as “the flu”
Symptoms of fever, fatigue, respiratory infections
May cause death
Caused by a variety of influenza viruses
Strains mutate often
Receiving seasonal vaccine can help prevent
Washing hands frequently and avoiding contact with those infected can reduce transmission
Influenza, commonly known as the flu, has symptoms of fever, fatigue, respiratory infections, and even death. The flu is caused by a variety of influenza viruses. Because these strains mutate and change often, new vaccines are needed yearly. Strains of the viruses can also blend together to produce new strains. Receiving the seasonal vaccine can help prevent the flu. Additionally, washing hands and avoiding contact with those who are infected can also reduce transmission.

21. Diseases Caused by Viruses
Common cold – an infection caused by a variety of viruses, most commonly rhinoviruses
Symptoms similar to the flu but less intense
Currently no vaccine
Reduce transmission by washing hands frequently and avoid contact with those infected
The common cold is caused by a variety of viruses, most commonly types of rhinoviruses. Symptoms are similar to those of the flu, but they are generally less intense. Since the common cold is caused by so many different strains of virus, there is currently no vaccine. Washing hands and avoiding contact with those who are infected can reduce transmission of the common cold.

22. Diseases Caused by Viruses
Hepatitis A – a RNA virus which causes inflammation of the liver
Produces a jaundice, or yellow, appearance of skin and eyes
Virus is carried through infected food or water
Vaccine can prevent
Transmission can be limited by washing hands and food and using clean water
Hepatitis A, a RNA virus, causes inflammation of the liver, which produces a jaundice, or yellow, appearance. This disease is caused by the hepatitis A virus, which is carried through infected food or water. A vaccine can be administered to prevent hepatitis A. By washing hands and food and using clean water, transmission can be limited.

23. Viruses Learning Objectives Know the structure of viruses
Compare viruses to cells
Understand viral reproduction
Understand the role of viruses in diseases
You should now know the structure of viruses, and be able to compare viruses to cells, understand viral reproduction, and understand the role of viruses in diseases.