1. Viruses
Biology 4(C)

2. Learning objectives Viruses 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, be able to compare viruses to cells, understand viral reproduction, and understand the role of viruses in diseases

3. What is a Virus? Do viruses fulfill the characteristics of life?
Viruses cannot reproduce on their own
Need host machinery (ribosomes)
Viruses cannot metabolize energy
Need host energy
No, not considered biotic (a living thing)
Many scientists do not categorize viruses as living. To understand why, remember that living things fulfill all of the characteristics of life.
Viruses cannot reproduce on their own. They must use ribosomes located within the host to make more genetic material and proteins to form more viruses.
Viruses cannot metabolize energy. When a host creates new enzymes, energy located within the host is used.
Because they do not fulfill the characteristics of life, they are not considered a living thing.

4. Viral Structure - Bacteriophage
Capsid – protein shell that protects genetic information of virus
Viruses have a very different structure than cells. They are much smaller and contain fewer specialized structures. Here is an illustration of a bacteriophage. Bacteriophages, or phages for short, infect bacteria.
The structure at the top is called a capsid. It is a protein shell that encloses the viral genetic material.

5. Viral Structure - Bacteriophage
Genetic information – strands of DNA or RNA used to make viral proteins inside infected hosts
The genetic information of the phage is protected inside the capsid. It can be stored as either DNA or RNA. When it enters the host cell, it is used by host machinery to create viral proteins.

6. Viral Structure - Bacteriophage
Tail – protein shaft that contracts to inject viral genetic information into host
The tail makes up the middle of phages. When the phage is attached to the host, it contracts to inject the genetic material through the host’s membrane.

7. Viral Structure - Bacteriophage
Tail fibers – proteins that attach to the outside of a host
At the bottom of the phage are tail fibers. These proteins attach to the outside of a host before injection of genetic material.

8. Viral Structure – Other Types
Membranous envelope – lipids that attach and fuse to hosts
Viruses that infect animal cells often have membranous envelopes. This allows the virus to attach and fuse to the host’s cellular membrane.

9. Viruses vs. Cellular Life
DNA or RNA
Need a host to replicate
Never contain organelles
Do not convert energy
Cellular Life
Only DNA
Can reproduce independently
Eukaryotes contain organelles
Convert energy to perform tasks
How are viruses and cells different? For starters, some viral genetic information is packaged as RNA. In cells, genetic information is only stored as DNA.
Viruses need a host to replicate within. Cells are capable of reproducing on their own, given nutrients from their environment.
Viruses do not contain organelles like mitochondria or ribosomes, but these organelles are found in their eukaryotic hosts.
Viruses do not convert energy, but cells can convert glucose to ATP molecules for use in cellular tasks like molecular transport.

10. Viral Reproduction Viruses need a host to reproduce Two methods
Goal: create more copies of their genetic material
Two methods
Lytic Cycle
Lysogenic Cycle
Even though viruses are not alive, they still reproduce. While cells can do this on their own, viruses need a host cell. The goal is the same, to create more copies of their genetic material.
There are two ways to accomplish this goal. Viruses can use the lytic or lysogenic cycle.

11. Viral Reproduction – Lytic Cycle
Attaches to host
Injects genetic material into host
Cellular machinery duplicates genetic material and creates viral proteins (capsids, tail fibers)
New viruses are assembled
New viruses exit the cell by bursting the cellular membrane
End result – more viruses made, cell dies
During the lytic cycle, a virus attaches to a host and injects its genetic material.
Next, cellular machinery duplicates genetic material and creates viral proteins like capsids and tail fibers. These proteins are assembled into new viruses. Last, the viruses exits the cell by bursting a hole in the host’s cellular membrane.
At the end of the lytic cycle, there are many viruses made and the host cell usually dies from rupture.

12. Viral Reproduction – Lytic Cycle
Attachment
Lytic Cycle
Release
Entry
Here we can see this cycle visually.
Attachment - Attaches to host
Entry - Injects genetic material into host
Replication - Cellular machinery duplicates genetic material and creates viral proteins (capsids, tail fibers)
Assembly - New viruses are assembled
Release - New viruses exit the cell by bursting the cellular membrane
Assembly
Replication

13. Viral Reproduction – Lysogenic Cycle
Attaches to host
Injects genetic material into host
Viral genetic material is inserted into host genome
Viral genetic material lies dormant
When cell reproduces, new copies have viral genetic information
End result – more viral genome made, cell lives
Another method of producing more viral genetic material is the lysogenic cycle. It begins the same, the virus attached to a host and injects its genetic material.
Instead of producing proteins, however, the viral genetic material is inserted into the host genome and lies dormant.
When the host cell reproduces, both daughter cells have a copy of the viral genetic information inside their own genome.
At the last stage of the lysogenic cycle, the cell is still alive, but it and all of it’s daughter cells have viral genetic material lying dormant within their cells.

14. Viral Reproduction – Lysogenic Cycle
Attachment
Entry
Separation
Here we can see this cycle visually.
Attachment - Attaches to host
Entry - Injects genetic material into host
Insertion – Viral genetic material is inserted into the host’s genome
Reproduction - New daughter cells contain viral genetic material
Separation – Viral genome sometimes separates from host genome.
Insertion
Reproduction

15. Viral Reproduction Lytic Cycle Lysogenic Cycle New viruses made
Cellular host dies
Lysogenic Cycle
Genome copies made
Cellular host lives
Viruses can use both cycles
Infect many cells with lysogenic
Create many viruses at once with lytic
Here are the two cycles compared. In the lytic, new viruses are made and the cellular host dies. In the lysogenic, the viral genome is copied through host replication. The host lives, but it is infected.
Some viruses use both cycles to reproduce efficiently. First, they can infect many cells with the lysogenic cycle. Lying dormant, many copies of their genetic material will spread as hosts replicate. When the hosts receive an environmental cue, the lytic cycle begins. Virus proteins are made and burst from their hosts.

16. Viral Reproduction Lytic & Lysogenic Cycles
This image shows both cycles at once. Remember that a virus can remain in the lysogenic cycle to build dormant viral genomes before shifting to the lytic cycle.

17. Images by Matt Gonda [Public domain]
Viral Reproduction in Animals
Some animal viruses exit without lysing the host
Envelopes that fuse to the host cell’s plasma membrane
Images by Matt Gonda [Public domain]
Some viruses that reproduce in animals exit without lysing the host. These viruses have envelopes that fuse to the host cell’s plasma membrane.
Here you can see an image of an animal virus budding out of it’s host.

18. Viral Diseases - AIDS AIDS – acquired immune deficiency syndrome
Describes loss of immune system because of HIV
Caused by HIV - human immunodeficiency virus
makes helper T cells useless
Prevention
No vaccine
Limit transmission (use condoms)
Avoid transmission (use clean needles)
The disease AIDS, or acquired immune deficiency syndrome, is characterized by the loss of the immune system. It’s caused by HIV, the human immunodeficiency virus, which makes helper T cells useless.
There is currently no vaccine for the HIV virus. Infection can be prevented by limiting and avoiding transmission.

19. Viral Diseases – AIDS – HIV Replication
Here is an illustration of HIV replication during the lytic cycle. HIV packages special proteins with its RNA-based genome to ensure replication. When the lytic cycle is occurring, these special proteins are made by hosts and packaged into new viruses.

20. Graoh by Jurema Oliveira [GFDL]
Viral Diseases - AIDS
This graph shows the effect of HIV infection over time. The blue line traces the number of helper T cells. The red line traces the amount of viral genome present.
Here you can the effects of both lytic and lysogenic cycles. After initial infection, the viral genome enters a lytic period and creates many copies while destroying helper T cells.
Graoh by Jurema Oliveira [GFDL]

21. Viral Diseases - AIDS Initial infection Clinical latency AIDS Death
Helper T cells rapidly decline
Viral genome rapidly increases
Clinical latency
Viral genomes lay mostly dormant in infected cells
AIDS
Rate of viral creation outweighs helper T cell creation
Death
Immune system too weak to fight common pathogens
In initial infection, the number of helper T cells drops and a large amount of HIV genome is produced.
During clinical latency, the viral genomes lay mostly dormant.
After a period of time, the rate of new viruses is higher than the rate of new helper T cells. 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.
People do not die directly from HIV. Instead, HIV creates a condition in which their body cannot fight off common pathogens.

22. Image by NIAD [Public Domain]
Viral Diseases - Influenza
Influenza (the flu)
Causes fever, fatigue, and respiratory infections
More severe than common cold, can be deadly
Caused by a variety of influenza viruses
Change often, new vaccines yearly
Can blend with bird and swine strains to produce new viruses
Prevention
Seasonal vaccine
Limit transmission (wash hands)
Influenza, commonly known as the flu, causes fever, fatigue, and respiratory infections. The flu is more severe than the common cold and can be deadly.
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.
The flu can be prevented by receiving the seasonal vaccine and limiting transmission of the virus by washing your hands.
Image by NIAD [Public Domain]

23. Viral Diseases – Common Cold
The Common Cold
Causes fever, fatigue, and respiratory infections
Less severe than influenza
Caused by a variety rhinoviruses
Over 200 different virus strains
Prevention
No vaccine
Limit transmission (wash hands)
The common cold can cause fever, fatigue, and respiratory infections. It is less severe than influenza, rarely causing death.
Because over 200 different rhinovirus strains cause a cold, there is no vaccine. You can limit transmission by washing your hands often.
Image by Robin S [GNU]

24. Image by The CDC [Public Domain]
Viral Diseases – Hepatitis A
Hepatitis A
Causes inflammation of liver, jaundice appearance
Rarely results in liver failure
Caused by a hepatitis A virus
Carried through infected food or water
Prevention
Vaccine
Limit transmission (wash hands, food)
Hepatitis A causes inflammation of the liver, which produces a jaundice, or yellow, appearance. Rarely, it can cause liver failure.
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.
Image by The CDC [Public Domain]