Viruses

1. Explain why a virus is somewhere between living and non-living.

• A virus does not perform many characteristics of living things and must rely on another organism in order to replicate itself.

• However, viruses do contain genetic material and can reproduce if a host is available.

2. Describe how small a virus is.

• Viruses are very small, between 20 and 400nm.

• They are so small that 5000 influenza viruses could fit on the head of a pin!

3. What is a pathogen? List three examples of viral pathogens.

• A pathogen is a disease-causing agent.

• Many viruses are pathogens including the AIDS virus, the influenza virus, and the common cold virus.

4. What is the host range of a bacteriophage, a plant virus, the rabies virus, a human cold virus and

the AIDS virus?

• The host range (number of organisms, cells, or tissue a pathogen can infect) for a bacteriophage is quite limited – they can only infect specific types of bacterial species.

VIRUS HOST RANGE

Plant virus Wide - many species of plants

Rabies Fairly limited - many mammals (rodents, dogs, humans)

Human cold Very limited - cells of the human respiratory tract

HIV Very limited - white blood cells

5. List and describe the 4 stages of viral replication. What is this process called?

• Viral replication is known as the lytic cycle.

• Viruses rely on a host cell to reproduce more viruses.

• The lytic cycle consists of four stages including attachment, synthesis, assembly and release.

Attachment

• There is chemical recognition and the virus attaches to the host cell.

• The virus invades the cell by injecting its DNA or RNA material into the cell.

• Sometimes the entire virus will enter the host cell’s cytoplasm.

Synthesis

• The DNA or RNA takes over the host cell.

• It begins directing the host cell to replicate the viral components such as proteins, nucleic acids, enzymes, and capsid proteins.

Assembly

• The viral enzymes, proteins and nucleic acids are assembled to form new viruses.

Release

• The host cell lysis (breaks open) and the new viruses are released.

• The host cell dies.

6. Draw an annotated diagram of viral replication (fig 3).

7. How is the lytic cycle of viral replication different from the lysogenic cycle?

Lysogenic cycle Lytic cycle

Does not kill host cell outright Kills host cell within minutes of infection

Viral genetic material integrates with host’s genetic material and replicates as host cell reproduces

Viral genetic material is replicated by hijacking host cell.

Can lie dormant for years causing no apparent symptoms in host organism

Causes symptoms in host organism as a result of lysis

For example: a temperate phage replicates this way

For example: a virulent phage replicates this way

For example: certain cancer-causing viruses replicate this way

For example: the common cold virus replicates this way

8. Can human viral diseases be treated? If so, how?

• Some viral disease can be prevented with vaccines, but many are difficult if impossible to treat.

• Vaccines work by boosting our own immune defenses by stimulating the production of antibodies before the body actually comes into contact with the pathogenic virus.

9. What is the most commonly held theory as to where viruses originally came from?

• Many scientists believe viruses originated as fragments of genetic material which somehow detached from a cellular organism.

10. What are viral vectors? How can they be

useful to humans?

• Viral vectors are used as a form of gene therapy. Essentially, viruses are used for their ability to infect another cell with their genetic material.

• Scientists can insert “good” genetic material into a vector and have it infect a target cell.

• For example, a child with ADA deficiency may have a vector, containing the normal ADA gene, insert its genetic material into the immune cells of the child.

11. Explain why viral vectors do not work as a treatment for human diseases such as Alzheimer’s and Parkinson’s.

• Unfortunately viral vectors are not useful as a treatment for diseases in which the affected cells do not multiply like neurons (cells of the spinal cord and brain).

• This includes disease such as Alzheimer’s and Parkinson’s Disease as they are neurological disorders.

12. How might a bacteriophage be useful to humans in terms of a treatment for certain diseases?

• Since virulent bacteriophages kill their host cell, could we use these viruses as a treatment against pathogenic bacteria that infect human cells?