Biology Week 10: Viruses

Day 1 - 2 | Day 3 | Day 4 | Lab | Weekly Quiz | Quizstar

Virus Host cell Virulent Temperate Bacteriophage Envelope Capsid Tail fiber	Receptor site Lytic cycle Lysogenic cycle Prophage Nanometer HIV Inactivated Icosahedron Helix Baseplate	Attenuated Antiviral Antibiotic White blood cell Interferon Protein coat Sheath Lysis Transduction

Viruses

Can antibiotics be used to cure an illness caused by a virus?

Do viruses qualify as living things? They are much smaller and less complex than cells. They consist of either DNA or RNA enclosed in a protein coat called a capsid. Virons do not grow. They have no nucleus, cytoplasm, or membranes. They do not carry out cellular functions. Virons do not generate metabolic energy. They are obligate intracellular parasites, meaning that they require a host cell to reproduce. Viruses can be harmful. Virulent - disease causing. Temperate - not immediately disease causing. Viruses play a role in causing some cancers.

Viruses may be helpful.

• A temperate virus introduces nucleic acid from its former host into a new host, thus changing the genetic code of the host. By this process of transduction, viruses cause genetic variation within a host population.
• Certain varieties of flowers have been developed using viruses to alter the genetic code.

Characteristics used to classify viruses.

• Type of nucleic acid: viruses have either single-strand RNA or double-strand DNA, but never both.
• Physical structure: viruses are found in several general shapes.
• There are two basic shapes:
• Polyhedral/icosahedron (the capsid has 20 triangular faces)
• Helical/helix (the capsid resembles a coiled spring).
• Complex (example bacteriophage)

*Knowing the structure of a virus gives hints about how the virus interacts with a cell to initiate the infectious process and how the structure of the viral proteins allows interaction with the immune system. From this information, strategies to abort virus infection might be advised.

• The diagram at the top of the page is a virus that commonly infects bacteria cells. This virus is usually referred to as a bacteriophage.
• Two other common viruses:

• Envelope: some viruses use lipids from the cell to form a layer around the capsid during replication.
• Type of host cell: most viruses are very specific about the type of cell they invade. There are specific sites called receptor sites that the viruses attach to on the host cell's surface.

Compare Lytic and Lysogenic Cycles:
All viruses invade a host cell and reproduce through one of these cycles.

Phases of the Lytic Cycle of a Virulent Virus:

Take a look a this animation

1. Attachment: Virus attaches itself to the cell.
2. Entry: Enzymes weaken the cell wall and nucleic acid is injected into the cell, leaving the empty capsid outside the cell. Many viruses actually enter the host cell intact.
3. Replication: Viral DNA takes control of cell activity.
4. Assembly: All metabolic activity of the cell is directed to assemble new viruses.
5. Release: Enzymes disintegrate the cell in a process called lysis, releasing the new viruses.

The Lysogenic Cycle of a Temperate Virus:

• The virus attaches itself and injects its DNA into the cell.
• The viral DNA attaches itself to the host DNA, becoming a new set of cell genes called a prophage.
• When the host cell divides, this new gene is replicated and passed to new cells. This causes no harm to the cell, but may alter its traits.
• Now there are two possibilities:
• The prophage survives as a permanent part of the DNA of the host organism.
• Some external stimuli can cause the prophage to become active, using the cell to produce new viruses.

Some Diseases Caused by Viruses
Animals	Plants	Humans
Rabies	Tobacco mosaic disease	Common cold Flu Use this computer simulation to explore a flu epidemic.
Foot and mouth disease	Tomato bushy stunt	German measles Mumps
Newcastle disease	Maize dwarf	Chickenpox Smallpox
Distemper in dogs	Alfalfa mosaic disease	Mononucleosis
Cowpox	Sugar beet curly top	Cold sores Hepatitis Warts
Influenza in horses and birds	Dwarfism in rice	Herpes AIDS

Size Of Viruses:

Viruses are very small, ranging in size from 20 nanometers to 250 nanometers. A nanometer is equal to 0.00000004 inch (4X10^-8 in). The smallest of all bacteria is about the size of the largest virus. This picture shows round virus particles along with rod-shaped bacteria cells.

Controlling Viruses:

Viruses are usually easy to destroy while outside living organisms. Outside the body, the "AIDS" virus, HIV, can be destroyed with a solution of bleach that is almost weak enough for you to drink. But once inside a host, most substances that destroy the virus are also harmful to the host organism. For this reason, viral infections in animal cells can be extremely hard to cure. Viral infections in plant cells are almost impossible to cure.

Compared to the number of vaccines developed to treat bacterial diseases, there are very few vaccines for viral infections. Virus vaccines are made with either inactivated or attenuated viruses. Inactivated viruses do not replicated in a host cell. Attenuated viruses have been genetically altered so they are not able to cause disease. The first viral vaccinations were for measles, mumps, and rubella. There are now vaccines for hepatitis A and B, chickenpox, smallpox, and rabies.

Virologist

Antiviral drugs interfere with the synthesis of viral nucleic acid or with the formation of viral capsids during replication. Antibiotics specifically attack the metabolism of a bacterial cell. Since viruses use only the reproductive machinery of a cell, antibiotics are of no use in destroying viruses.

Your body does have some natural ability to inhibit viral infections. There are two limited ways the body fights viruses:

• White blood cells engulf viruses in the blood and "digest" them.
• Interferon is a protein produced by cells when exposed to a virus. This protein binds to the cell membranes of neighboring cells and "interferes" with the ability of a virus to enter the cell.

Day 1-2 Assignment - Viruses
This assignment must be turned in by the end of class tomorrow to receive credit.

How quickly can a virus replicate in a host organism? Complete the following activity to determine how many viruses an organism might be infected with after a half hour, after 2 hours, after 4 hours.

1. Complete this virus replication worksheet & graph.

• Do you have the correct rubric, "paper rubric" for paper graphs or "Excel rubric" for Excel graphs?
  
• Remember, the rubric always goes on top, then graph, then worksheet.

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Day 3

How are viral infections in plants usually stopped?

Day 3 Assignment - Viruses
This assignment must be turned in by the beginning of class tomorrow to receive credit.

Part A: View part 1 of this Understanding Viruses

1. As you view part 1 of this video write down and answer the four discussion questions at the beginning of this part.

Part B: Use this weeks information about viruses to answer the following questions.

1. Are viruses considered living? Explain your answer.
2. Describe how viruses are classified?
3. List the names of 3 different types of viruses.
4. What is the difference between a temperate and virulent virus.
5. What is the lytic cycle?
6. How is the lysogenic cycle different from the lytic cycle?
7. Name 3 diseases caused by viruses and give a brief summary about each.
8. Why is it hard to cure viral infection?
9. Describe different ways we try to control viruses?
10. Describe how your body might fight Viruses?

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Day 4

What is the Flu?

The flu is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death. While most healthy people recover from the flu without complications, some people, such as older people, young children, and people with certain health conditions, are at high risk for serious complications from the flu.

Be Aware of Common Flu Symptoms

The flu usually starts suddenly and may include these symptoms:

Fever (usually high) Headache Tiredness (can be extreme) Cough Sore throat

Runny or stuffy nose Body aches Diarrhea and vomiting also can occur but are more common in children.

These symptoms are referred to as "flu-like symptoms." A lot of different illnesses, including the common cold, can have similar symptoms.

Know the Risk from the Flu

Some of the complications caused by the flu include bacterial pneumonia, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes. Children and adults may develop sinus problems and ear infections.

Know How the Flu Spreads

The flu spreads in respiratory droplets from coughing and sneezing. It usually spreads from person to person, though occasionally a person may become infected by touching something with virus on it and then touching their mouth or nose.

Adults may be able to infect others 1 day before getting symptoms and up to 7 days after getting sick. So it is possible to give someone the flu before you know you’re sick as well as while you are sick. See this site for more information on the flu

Day 4 Assignment - Viruses
This assignment must be turned in by the end of class day after tomorrow to receive credit.

Part A: View part 2 of this the same video from day 3 above. Understanding viruses

1. As you view part 2 of this video write down and answer the four discussion questions at the beginning of this part.

Part B: Pick one specific virus and research the following:

1. What is the name of the virus?

2. Describe the virus, name its parts. (can include a diagram)

3. Is it a DNA or RNA type virus?

4. Is it harmful?

5. What type of organism does it infect and what part?

6. What symptoms does the virus cause?

7. How does the virus replicate itself?

8. It what geographical region has it been found? Any area of origin?

9. What is used to fight back against this type of virus?

Research Links:

• Virus Databases On-Line - Australian National University
• All the Virology on The Web - Extensive Links provided by Tulane University
• The Big Picture Book of Viruses - Garry Lab at Tulane University School of Medicine
• Microbiology 109 - Dr. Alan Cann, of the University of Leicester
• Virology 335 - Dr. Alan Cann, of the University of Leicester
• Clinical Virology - University of Cape Town, South Africa
• Virus of the Month - Institute for Molecular Virology
• Plant Virus Pictures - Institute of Arable Crops Research
• Virus Structure Images - University of Cape Town, South Africa

Antibiotics have no effect on viral infections.

Viral infections in plants are usually stopped by burning the plants to prevent the spread of the virus.